Orographic precipitation occurs on the windward side of mountains and is caused by the rising air motion of a large-scale flow of moist air across the mountain ridge, resulting in adiabatic cooling and condensation. In mountainous parts of the world subjected to consistent winds (for example, the trade winds), a more moist climate usually prevails on the windward side of a mountain than on the leeward or downwind side. Moisture is removed by orographic lift, leaving drier air (see foehn wind) on the descending and generally warming, leeward side where a rain shadow is observed. In South America, the Andes mountain range blocks Pacific moisture that arrives in that continent, resulting in a desertlike climate just downwind across western Argentina.
Effect on precipitation
Prevailing winds
Prevailing winds | Effect on precipitation
Orographic precipitation occurs on the windward side of mountains and is caused by the rising air motion of a large-scale flow of moist air across the mountain ridge, resulting in adiabatic cooling and condensation. In mountainous parts of the world subjected to consistent winds (for example, the trade winds), a more moist climate usually prevails on the windward side of a mountain than on the leeward or downwind side. Moisture is removed by orographic lift, leaving drier air (see foehn wind) on the descending and generally warming, leeward side where a rain shadow is observed. Orographic precipitation occurs on the windward side of mountains and is caused by the rising air motion of a large-scale flow of moist air across the mountain ridge, resulting in adiabatic cooling and condensation. Orographic precipitation occurs on the windward side of mountains and is caused by the rising air motion of a large-scale flow of moist air across the mountain ridge, resulting in adiabatic cooling and condensation.
what are the major global wind systems on earth
user
polar easterlies, westerlies to name a few.
assistant
what is the first one about?
user
are the dry, cold prevailing winds that blow from the high-pressure areas towards the low-pressure areas.
assistant
how does it effect the precipitation?
user
Orographic precipitation occurs on the windward side of mountains and is caused by the rising air motion of a large-scale flow of moist air across the mountain ridge, resulting in adiabatic cooling and condensation. In mountainous parts of the world subjected to consistent winds (for example, the trade winds), a more moist climate usually prevails on the windward side of a mountain than on the leeward or downwind side. Moisture is removed by orographic lift, leaving drier air (see foehn wind) on the descending and generally warming, leeward side where a rain shadow is observed. In South America, the Andes mountain range blocks Pacific moisture that arrives in that continent, resulting in a desertlike climate just downwind across western Argentina.
Prevailing winds | Effect on precipitation
The Sierra Nevada range creates the same effect in North America forming the Great Basin and Mojave Deserts.
Prevailing winds | Effect on precipitation
The region in which the equatorward moving air masses converge and rise, is known as the intertropical convergence zone, or ITCZ. Within that zone develops a band of thunderstorms that produce high-precipitation. Having lost most of its water vapor to condensation and precipitation in the upward branch of the Hadley cell circulation, the descending air is dry. As the air descends, low relative humidities are produced as the air is warmed adiabatically by compression from the overlying air, producing a region of higher pressure. The subtropics are relatively free of the convection, or thunderstorms, that are common in the equatorial belt.
Hadley cell | Major impacts on precipitation by latitude
Snow falling in Canada has much less HO than rain in Florida; similarly, snow falling in the center of ice sheets has a lighter signature than that at its margins, since heavier O precipitates first. Changes in climate that alter global patterns of evaporation and precipitation therefore change the background ratio.
Δ18O | Mechanism
This trend reverses in the summer when strong heating of the land causes high pressure and tends to block moisture-rich air from the Pacific from reaching land. This explains why most of coastal North America in the highest latitude experiences dry summers, despite vast rainfall in the winter.
Prevailing winds | Westerlies and their impact
When winds blow over elevated terrain, air forced upwards expands and cools due to the decrease in pressure with height. Since colder air can hold less water vapour, moisture condenses to form clouds and precipitates as rain or snow on the mountain's upwind slopes. The change of state from vapour to liquid water releases latent heat energy which heats the air, partially countering the cooling that occurs as the air rises. The subsequent removal of moisture as precipitation renders this heat gain by the air irreversible, leading to the warm, dry, foehn conditions as the air descends in the mountain's lee.
Foehn wind | Condensation and precipitation
Higher temperatures lead to increased evaporation and surface drying. As the air warms, its water-holding capacity also increases, particularly over the oceans. In general the air can hold about 7% more moisture for every 1 °C of temperature rise. In the tropics, there's more than a 10% increase in precipitation for a 1 °C increase in temperature. Changes have already been observed in the amount, intensity, frequency, and type of precipitation. Widespread increases in heavy precipitation have occurred even in places where total rain amounts have decreased. Projections of future changes in precipitation show overall increases in the global average, but with substantial shifts in where and how precipitation falls.
Effects of climate change | Precipitation
The consequences of this include changes in cloud cover and rates of precipitation, and changes in surface pressure and atmospheric dynamics, as has been observed.
Brian Tinsley | Introduction
Storm systems affect the state with heavy rains between October and March. Local climates vary considerably on each island due to their topography, divisible into windward ("Koolau") and leeward ("Kona") regions based upon location relative to the higher mountains. Windward sides face the east to northeast trade winds and receive much more rainfall; leeward sides are drier and sunnier, with less rain and less cloud cover. In South America, the Andes mountain range blocks Pacific moisture that arrives in that continent, resulting in a desertlike climate just downwind across western Argentina. The Sierra Nevada range creates the same effect in North America forming the Great Basin and Mojave Deserts.
Precipitation | Orographic effects
This extra heat leads to greater upward motion, which can induce additional shower and thunderstorm activity. Rainfall rates downwind of cities are increased between 48% and 116%. Partly as a result of this warming, monthly rainfall is about 28% greater between downwind of cities, compared with upwind. Some cities induce a total precipitation increase of 51%. Increasing temperatures tend to increase evaporation which can lead to more precipitation. Precipitation generally increased over land north of 30°N from 1900 through 2005 but has declined over the tropics since the 1970s. Globally there has been no statistically significant overall trend in precipitation over the past century, although trends have varied widely by region and over time.
Rain | Human influence
The westerlies explain why coastal North America tends to be wet, especially from Northern Washington to Alaska, during the winter. Differential heating from the Sun between the land which is quite cool and the ocean which is relatively warm causes areas of low pressure to develop over land. This results in moisture-rich air flowing east from the Pacific Ocean, causing frequent rainstorms and wind on the coast. This moisture continues to flow eastward until orographic lift caused by the Coast Ranges, and the Cascade, Sierra Nevada, Columbia, and Rocky Mountains causes a rain shadow effect which limits further penetration of these systems and associated rainfall eastward.
Prevailing winds | Westerlies and their impact
The temperature decrease with height and cloud depth are directly affected by both the water temperature and the large-scale environment. The stronger the temperature decrease with height, the deeper the clouds get, and the greater the precipitation rate becomes. In mountainous areas, heavy snowfall accumulates when air is forced to ascend the mountains and squeeze out precipitation along their windward slopes, which in cold conditions, falls in the form of snow. Because of the ruggedness of terrain, forecasting the location of heavy snowfall remains a significant challenge.
Precipitation | Snow
Fronts are the principal cause of significant weather. "Convective precipitation" (showers, thundershowers, and related unstable weather) is caused by air being lifted and condensing into clouds by the movement of the cold front or cold occlusion under a mass of warmer, moist air. If the temperature differences of the two air masses involved are large and the turbulence is extreme because of wind shear and the presence of a strong jet stream, "roll clouds" and tornadoes may occur. In the warm season, lee troughs, breezes, outflow boundaries and occlusions can lead to convection if enough moisture is available. "Orographic precipitation" is precipitation created through the lifting action of air moving over terrain such as mountains and hills, which is most common behind cold fronts that move into mountainous areas.
Weather front | Precipitation produced
Orographic precipitation occurs on the windward (upwind) side of mountains and is caused by the rising air motion of a large-scale flow of moist air across the mountain ridge, resulting in adiabatic cooling and condensation. In mountainous parts of the world subjected to relatively consistent winds (for example, the trade winds), a more moist climate usually prevails on the windward side of a mountain than on the leeward or downwind side. Moisture is removed by orographic lift, leaving drier air (see katabatic wind) on the descending and generally warming, leeward side where a rain shadow is observed. In Hawaii, Mount Waiʻaleʻale, on the island of Kauai, is notable for its extreme rainfall, as it has the second-highest average annual rainfall on Earth, with .
Precipitation | Orographic effects
Eventually, the cloud droplets will grow large enough to form raindrops and descend toward the Earth where they will freeze on contact with exposed objects. Where relatively warm water bodies are present, for example due to water evaporation from lakes, lake-effect snowfall becomes a concern downwind of the warm lakes within the cold cyclonic flow around the backside of extratropical cyclones. Lake-effect snowfall can be locally heavy. Thundersnow is possible within a cyclone's comma head and within lake effect precipitation bands. In mountainous areas, heavy precipitation is possible where upslope flow is maximized within windward sides of the terrain at elevation.
Precipitation | Introduction
As the wind narrows and is compressed into the passes its velocity increases dramatically, often to near-gale force or above. At the same time, as the air descends from higher elevation to lower, the temperature and barometric pressure increase adiabatically, warming about 5 °F for each 1,000 feet it descends (1 °C for each 100 m). Relative humidity decreases with the increasing temperature. The air has already been dried by orographic lift before reaching the Great Basin, as well as by subsidence from the upper atmosphere, so this additional warming often causes relative humidity to fall below 10 percent. The end result is a strong, warm, and very dry wind blowing out of the bottom of mountain passes into the valleys and coastal plain.
Santa Ana winds | Meteorology
Frontal precipitation is the result of frontal systems surrounding extratropical cyclones or lows, which form when warm and tropical air meets cooler air. Frontal precipitation typically falls out of nimbostratus clouds. When masses of air with different densities (moisture and temperature characteristics) meet, the less dense warmer air overrides the more dense colder air. The warmer air is forced to rise and, if conditions are right, creates an effect of saturation, causing precipitation. In turn, precipitation can enhance the temperature and dewpoint contrast along a frontal boundary. Passing weather fronts often result in sudden changes in environmental temperature, and in turn the humidity and pressure in the air at ground level.
Precipitation types | Cyclonic
Precipitation occurs when local air becomes saturated with water vapor, and can no longer maintain the level of water vapor in gaseous form. This occurs when less dense moist air cools, usually when an air mass rises through the atmosphere. However, an air mass can also cool without a change in altitude (e.g. through radiative cooling, or ground contact with cold terrain). "Convective" precipitation occurs when air rises vertically through the (temporarily) self-sustaining mechanism of convection. "Stratiform" precipitation occurs when large air masses rise diagonally as larger-scale atmospheric dynamics force them to move over each other. "Orographic" precipitation is similar, except the upwards motion is forced when a moving air mass encounters the rising slope of a landform such as a mountain ridge.
Precipitation types | Mechanisms
In regions with minimal vegetation, such as coastal and desert areas, transverse sand dunes orient themselves perpendicular to the prevailing wind direction, while longitudinal dunes orient themselves parallel to the prevailing winds.
Prevailing winds | Effect on nature
In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravity from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. Precipitation occurs when a portion of the atmosphere becomes saturated with water vapor (reaching 100% relative humidity), so that the water condenses and "precipitates" or falls. Thus, fog and mist are not precipitation but colloids, because the water vapor does not condense sufficiently to precipitate. Two processes, possibly acting together, can lead to air becoming saturated: cooling the air or adding water vapor to the air.
Precipitation | Introduction
Projections suggest a reduction in rainfall in the subtropics, and an increase in precipitation in subpolar latitudes and some equatorial regions. In other words, regions which are dry at present will in general become even drier, while regions that are currently wet will in general become even wetter. Although increased rainfall will not occur everywhere, models suggest most of the world will have a 16-24% increase in heavy precipitation intensity by 2100.
Effects of climate change | Precipitation
Changes in winds and convection patterns over and around cities impacts precipitation. Contributing factors are believed to be urban heat island, heightened surface roughness, and increased aerosol concentration.
Urban climatology | Precipitation
Because of this temperature difference, warmth and moisture are transported upward, condensing into vertically oriented clouds (see satellite picture) which produce snow showers. The temperature decrease with height and cloud depth are directly affected by both the water temperature and the large-scale environment. The stronger the temperature decrease with height, the deeper the clouds get, and the greater the precipitation rate becomes.
Air mass | Modification
This happens mostly during the summer wet season, between December and March, when the ITCZ moves south and a summer monsoon is active over South America. Most precipitation is brought by easterly winds coming from the Amazon and the Atlantic Ocean, whereas the westerly winds that dominate during the dry season do not carry much moisture. Thus, humidity generally decreases in a westward direction. The amount of precipitation is modulated by the El Niño Southern Oscillation. During phases of El Niño the weather is drier, snow cover smaller and glacier retreat increases. Over longer timespans, precipitation in the region increases whenever iceberg discharge and cooling occur in the North Atlantic.
Coropuna | Precipitation
Extratropical cyclones can bring cold and dangerous conditions with heavy rain and snow with winds exceeding 119 km/h (74 mph), (sometimes referred to as windstorms in Europe). The band of precipitation that is associated with their warm front is often extensive, forced by weak upward vertical motion of air over the frontal boundary which condenses as it cools and produces precipitation within an elongated band, which is wide and stratiform, meaning falling out of nimbostratus clouds. When moist air tries to dislodge an arctic air mass, overrunning snow can result within the poleward side of the elongated precipitation band. In the Northern Hemisphere, poleward is towards the North Pole, or north.
Precipitation | Snow
This mechanism has become a popular textbook example of atmospheric thermodynamics. However, the common occurrence of 'dry' foehn events, where there is no precipitation, implies there must be other mechanisms.
Foehn wind | Condensation and precipitation
In areas with wet and dry seasons, soil nutrients diminish and erosion increases during the wet season. Animals have adaptation and survival strategies for the wetter regime. The previous dry season leads to food shortages into the wet season, as the crops have yet to mature. Developing countries have noted that their populations show seasonal weight fluctuations due to food shortages seen before the first harvest, which occurs late in the wet season.
Precipitation | Effect on agriculture
In parts of the world subjected to relatively consistent winds (for example the trade winds), a wetter climate prevails on the windward side of a mountain than on the leeward (downwind) side as moisture is removed by orographic precipitation. Drier air (see katabatic wind) is left on the descending, generally warming, leeward side where a rain shadow is formed. Terrain-induced precipitation is a major factor for meteorologists as they forecast the local weather. Orography can play a major role in the type, amount, intensity and duration of precipitation events. Researchers have discovered that barrier width, slope steepness and updraft speed are major contributors for the optimal amount and intensity of orographic precipitation.
Orography | Orographic precipitation
Precipitation is likely within 15 to 25 hours if winds are steady from NE E to S, or sooner if winds SE to S. Other winds bring an overcast sky.
Cirrus spissatus cloud | Introduction
Precipitation forms as smaller droplets coalesce via collision with other rain drops or ice crystals within a cloud. Short, intense periods of rain in scattered locations are called "showers." Moisture that is lifted or otherwise forced to rise over a layer of sub-freezing air at the surface may be condensed into clouds and rain. This process is typically active when freezing rain occurs. A stationary front is often present near the area of freezing rain and serves as the focus for forcing and rising air. Provided there is necessary and sufficient atmospheric moisture content, the moisture within the rising air will condense into clouds, namely nimbostratus and cumulonimbus if significant precipitation is involved.
Precipitation | Introduction
values for precipitation are always negative. The major influence on is the difference between ocean temperatures where the moisture evaporated and the place where the final precipitation occurred; since ocean temperatures are relatively stable the value mostly reflects the temperature where precipitation occurs. Taking into account that the precipitation forms above the inversion layer, we are left with a linear relation: which is empirically calibrated from measurements of temperature and as a = for Greenland and for East Antarctica. The calibration was initially done on the basis of "spatial" variations in temperature and it was assumed that this corresponded to "temporal" variations (Jouzel and Merlivat, 1984).
Paleothermometer | Common paleothermometers
Orographic precipitation, also known as relief precipitation, is precipitation generated by a forced upward movement of air upon encountering a physiographic upland (see anabatic wind). This lifting can be caused by two mechanisms: Upon ascent, the air that is being lifted will expand and cool. This adiabatic cooling of a rising moist air parcel may lower its temperature to its dew point, thus allowing for condensation of the water vapor contained within it, and hence the formation of a cloud. If enough water vapor condenses into cloud droplets, these droplets may become large enough to fall to the ground as precipitation.
Orography | Orographic precipitation
Prevailing winds can vary due to the uneven heating of the Earth.
Prevailing winds | Introduction
Similarly, in Asia, the Himalaya mountains create an obstacle to monsoons which leads to extremely high precipitation on the southern side and lower precipitation levels on the northern side.
Precipitation | Orographic effects
A minimum depth of is necessary, and an average depth of or more is generally accepted as sufficient. Wind shear is also a significant factor. Linear snow squall bands produce more thundersnow than clustered bands; thus a directional wind shear with a change of less than between the ground and in height must be in place. However, any change in direction greater than through that layer will tear the snow squall apart. A bare minimum fetch of is required so that the air passing over the lake or ocean water will become sufficiently saturated with moisture and will acquire thermal energy from the water.
Thundersnow | Lake effect precipitation
Within the Southern Hemisphere, poleward is towards the South Pole, or south. Southwest of extratropical cyclones, curved cyclonic flow bringing cold air across the relatively warm water bodies can lead to narrow lake-effect snow bands. Those bands bring strong localized snowfall which can be understood as follows: Large water bodies such as lakes efficiently store heat that results in significant temperature differences (larger than 13 °C or 23 °F) between the water surface and the air above. Because of this temperature difference, warmth and moisture are transported upward, condensing into vertically oriented clouds (see satellite picture) which produce snow showers.
Precipitation | Snow
The average snow to liquid ratio across the contiguous United States is 13:1, meaning of snow melts down to of water. The El Niño-Southern Oscillation affects the precipitation distribution, by altering rainfall patterns across the West, Midwest, the Southeast, and throughout the tropics. During the summer, the Southwest monsoon combined with Gulf of California and Gulf of Mexico moisture moving around the subtropical ridge in the Atlantic Ocean bring the promise of afternoon and evening thunderstorms to the southern tier of the country as well as the Great Plains. Equatorward of the subtropical ridge, tropical cyclones enhance precipitation (mostly from August to October) across southern and eastern sections of the country, as well as Puerto Rico, the United States Virgin Islands, the Northern Mariana Islands, Guam, and American Samoa.
Climate of the United States | Precipitation
Vertical wind shear forces the rainfall pattern around a tropical cyclone to become highly asymmetric, with most of the precipitation falling to the left and downwind of the shear vector, or downshear left. In other words, southwesterly shear forces the bulk of the rainfall north-northeast of the center. If the wind shear is strong enough, the bulk of the rainfall will move away from the center leading to what is known as an exposed circulation center. When this occurs, the potential magnitude of rainfall with the tropical cyclone will be significantly reduced.
Tropical cyclone rainfall climatology | Vertical wind shear impact on rainfall shield
Large thunderstorm areas known as mesoscale convective complexes move through the Plains, Midwest, and Great Lakes during the warm season, contributing up to 10% of the annual precipitation to the region. The El Niño–Southern Oscillation affects the precipitation distribution, by altering rainfall patterns across the West, Midwest, the Southeast, and throughout the tropics. There is also evidence that global warming is leading to increased precipitation to the eastern portions of North America, while droughts are becoming more frequent in the western portions.
United States rainfall climatology | Introduction
Another source shows more precipitation and stabler average temperatures.
Mersa Matruh | Climate
Convective rain, or showery precipitation, occurs from cumulonimbus clouds. It falls as showers with rapidly changing intensity. Convective precipitation falls over a certain area for a relatively short time, as convective clouds such as thunderstorms have limited horizontal extent. Most precipitation in the tropics appears to be convective. Graupel and hail are good indicators of convective precipitation and thunderstorms. In mid-latitudes, convective precipitation is intermittent and often associated with baroclinic boundaries such as cold fronts, squall lines, and warm fronts. High rainfall rates are associated with thunderstorms with larger raindrops. Heavy rainfall leads to fading of microwave transmissions starting above the frequency of 10 gigahertz (GHz), but is more severe above frequencies of 15 GHz.
Air-mass thunderstorm | Convective precipitation
Over elevated surfaces, heating of the ground exceeds the heating of the surrounding air at the same altitude above sea level, creating an associated thermal low over the terrain and enhancing any lows which would have otherwise existed, and changing the wind circulation of the region. In areas where there is rugged topography that significantly interrupts the environmental wind flow, the wind can change direction and accelerate parallel to the wind obstruction. This barrier jet can increase the low level wind by 45%. In mountainous areas, local distortion of the airflow is more severe. Jagged terrain combines to produce unpredictable flow patterns and turbulence, such as rotors.
Prevailing winds | Circulation in elevated regions
Stratiform or dynamic precipitation occurs as a consequence of slow ascent of air in synoptic systems (on the order of cm/s), such as over surface cold fronts, and over and ahead of warm fronts. Similar ascent is seen around tropical cyclones outside of the eyewall, and in comma-head precipitation patterns around mid-latitude cyclones. A wide variety of weather can be found along an occluded front, with thunderstorms possible, but usually their passage is associated with a drying of the air mass. Occluded fronts usually form around mature low-pressure areas. Precipitation may occur on celestial bodies other than Earth. When it gets cold, Mars has precipitation that most likely takes the form of ice needles, rather than rain or snow.
Precipitation | Frontal activity
Orographic precipitation occurs on the windward side of mountains and is caused by the rising air motion of a large-scale flow of moist air across the mountain ridge, resulting in adiabatic cooling and condensation. In mountainous parts of the world subjected to relatively consistent winds (for example, the trade winds), a more moist climate usually prevails on the windward side of a mountain than on the leeward or downwind side. Moisture is removed by orographic lift, leaving drier air (see katabatic wind) on the descending and generally warming, leeward side where a rain shadow is observed. In Hawaii, Mount Waiʻaleʻale, on the island of Kauai, is notable for its extreme rainfall, as it is amongst the places in the world with the highest levels of rainfall, with .
Rain | Orographic effects
Severe thunderstorms, which can spawn tornadoes and hailstorms, require wind shear to organize the storm in such a way as to maintain the thunderstorm for a longer period of time. This occurs as the storm's inflow becomes separated from its rain-cooled outflow. An increasing nocturnal, or overnight, low level jet can increase the severe weather potential by increasing the vertical wind shear through the troposphere. Thunderstorms in an atmosphere with virtually no vertical wind shear weaken as soon as they send out an outflow boundary in all directions, which then quickly cuts off its inflow of relatively warm, moist air and kills the thunderstorm.
Wind shear | Effects on thunderstorms and severe weather
The influence of the adjoining high-pressure areas dominates so the formation of a strong front here is unlikely. The weather is very calm and is mainly determined by the air mass properties. In warm mass, especially in the winter and during summer nights, fog and stratus form. It is usually sunny during daytime in summer but cumulus clouds may persist in colder mass.
Col (meteorology) | Types
Stratiform (a broad shield of precipitation with a relatively similar intensity) and dynamic precipitation (convective precipitation which is showery in nature with large changes in intensity over short distances) occur as a consequence of slow ascent of air in synoptic systems (on the order of cm/s), such as in the vicinity of cold fronts and near and poleward of surface warm fronts. Similar ascent is seen around tropical cyclones outside the eyewall, and in comma-head precipitation patterns around mid-latitude cyclones. A wide variety of weather can be found along an occluded front, with thunderstorms possible, but usually their passage is associated with a drying of the air mass.
Rain | Frontal activity
However, the cold dome behind the trough is at 850 mbar instead of a higher level and must be at least -13 °F (-25 °C). The presence of surface moisture from bodies of water or preexisting liquid precipitation is also a significant contributing factor helping to raise the dew point temperature and saturate the boundary layer. This saturate can significantly increase the amount of convective available potential energy leading to deeper vertical growth and higher precipitable water levels increasing the volume of snow which can be produced by the squall. In cases where there is a large amount of vertical growth and mixing the squall may develop embedded cumulonimbus clouds resulting in lightning and thunder which is dubbed thundersnow.
Snowsquall | Frontal snowsquall
Climatological temperatures substantially affect cloud cover and precipitation. At lower temperatures, air can hold less water vapour, which can lead to decreased precipitation. During the Last Glacial Maximum of 18,000 years ago, thermal-driven evaporation from the oceans onto continental landmasses was low, causing large areas of extreme desert, including polar deserts (cold but with low rates of cloud cover and precipitation). In contrast, the world's climate was cloudier and wetter than today near the start of the warm Atlantic Period of 8000 years ago.
Climate variability and change | The Cenozoic
It may sometimes occur in advance of warm fronts moving northward to the east of mountainous terrain. However, precipitation along warm fronts is relatively steady, as in rain or drizzle. Fog, sometimes extensive and dense, often occurs in pre-warm-frontal areas. Although, not all fronts produce precipitation or even clouds because moisture must be present in the air mass which is being lifted.
Weather front | Precipitation produced
Vertical wind shear forces the rainfall pattern around a tropical cyclone to become highly asymmetric, with most of the precipitation falling to the left and downwind of the shear vector, or downshear left. In other words, southwesterly shear forces the bulk of the rainfall north-northeast of the center. If the wind shear is strong enough, the bulk of the rainfall will move away from the center leading to what is known as an exposed circulation center. When this occurs, the potential magnitude of rainfall with the tropical cyclone will be significantly reduced.
Tropical cyclone rainfall forecasting | Vertical wind shear
In mountainous areas, heavy precipitation is possible where upslope flow is maximized within windward sides of the terrain at elevation which forces moist air to condense and fall out as rainfall along the sides of mountains. On the leeward side of mountains, desert climates can exist due to the dry air caused by downslope flow which causes heating and drying of the air mass. The movement of the monsoon trough, or intertropical convergence zone, brings rainy seasons to savannah climes. The urban heat island effect leads to increased rainfall, both in amounts and intensity, downwind of cities. Global warming is also causing changes in the precipitation pattern globally, including wetter conditions across eastern North America and drier conditions in the tropics.
Rain | Introduction
In contrast to temperature control at high latitudes, the isotopic composition of precipitation in the tropics is mainly influenced by rainfall amount (negative correlation). This "amount effect" is also observed for summer precipitation in the subtropics. Willi Dansgaard, who first proposed the term "amount effect", suggested several possible reasons for the correlation: (1) As cooling and condensation progress, the rainfall isotopic composition reflects an integrated isotopic depletion by the Rayleigh process; (2) A small amount of rainfall is more likely to be influenced by evaporation and exchange with surrounding moisture, which tend to make it more isotopically enriched. At low latitudes, the amount effect for δO is around −1.6‰ per 100 mm precipitation increase at island stations, and −2.0‰ per 100 mm at continental stations.
Hydrogen isotope biogeochemistry | Precipitation amount effect
Orographic or relief rainfall is caused when masses of air are forced up the side of elevated land formations, such as large mountains (often referred to as an upslope effect). The lift of the air up the side of the mountain results in adiabatic cooling, and ultimately condensation and precipitation. In mountainous parts of the world subjected to relatively consistent winds (for example, the trade winds), a more moist climate usually prevails on the windward side of a mountain than on the leeward (downwind) side. Moisture is removed by orographic lift, leaving drier air (see Foehn) on the descending (generally warming), leeward side where a rain shadow is observed.
Precipitation types | Orographic
Low-level outflow boundaries from thunderstorms are cooler and more moist than the air mass the thunderstorm originally formed within due to its wet bulbing by rain, forming a wedge of denser air which spreads out from the base of the parent thunderstorm. If wind speeds are high enough, such as during microburst events, dust and sand can be carried into the troposphere, reducing visibility. This type of weather event is known as a haboob, and is most common in the late spring within Sudan. Upper-level outflow can consist of thick cirrus clouds which would then obscure the sun and reduce solar insolation around the outermost edge of tropical cyclones.
Outflow (meteorology) | Local effects
As the cooler air cannot hold as much moisture as warm air, clouds form and rain occurs.
Cold front | Temperature changes
Insects are swept along by the prevailing winds, while birds follow their own course. As such, fine line patterns within weather radar imagery, associated with converging winds, are dominated by insect returns. In the Great Plains, wind erosion of agricultural land is a significant problem, and is mainly driven by the prevailing wind. Because of this, wind barrier strips have been developed to minimize this type of erosion. The strips can be in the form of soil ridges, crop strips, crops rows, or trees which act as wind breaks. They are oriented perpendicular to the wind in order to be most effective.
Prevailing winds | Effect on nature
The dew point is the temperature to which a parcel of air must be cooled in order to become saturated, and (unless super-saturation occurs) condenses to water. Water vapor normally begins to condense on condensation nuclei such as dust, ice, and salt in order to form clouds. An elevated portion of a frontal zone forces broad areas of lift, which form cloud decks such as altostratus or cirrostratus. Stratus is a stable cloud deck which tends to form when a cool, stable air mass is trapped underneath a warm air mass. It can also form due to the lifting of advection fog during breezy conditions.
Precipitation | Cooling air to its dew point
Convective rain, or showery precipitation, occurs from convective clouds, e.g. cumulonimbus or cumulus congestus. It falls as showers with rapidly changing intensity. Convective precipitation falls over a certain area for a relatively short time, as convective clouds have limited horizontal extent. Most precipitation in the tropics appears to be convective; however, it has been suggested that stratiform precipitation also occurs. Graupel and hail indicate convection. In mid-latitudes, convective precipitation is intermittent and often associated with baroclinic boundaries such as cold fronts, squall lines, and warm fronts.
Precipitation | Convection
Precipitation increases with elevation. Once the moisture is wrung from the air, it descends on the eastern side of the crest, which causes the air to be warmer and drier. On the western slopes, precipitation ranges from annually, while precipitation over the eastern slopes varies from in the east. Temperature extremes reach in summers and during the winters.
Mount Bachelor | Geography
After a period of four weeks, the growth of the seedling once again became uniform throughout the plant, as it was before the windblown sand abrasion occurred.
Wind | Effect on plants
On the leeward side of mountains, desert climates can exist due to the dry air caused by compressional heating. Most precipitation occurs within the tropics and is caused by convection. The movement of the monsoon trough, or intertropical convergence zone, brings rainy seasons to savannah regions. Precipitation is a major component of the water cycle, and is responsible for depositing the fresh water on the planet. Approximately of water falls as precipitation each year; of it over the oceans and over land. Given the Earth's surface area, that means the globally averaged annual precipitation is , but over land it is only .
Precipitation | Introduction
Such drops may have condensational growth rates more than two times faster than drops formed on small aerosol particles, and unlike normal cloud drops, drops formed on the largest of the giant sea salt aerosols may even grow by condensation in otherwise subsaturated cloudy downdrafts.
Sea salt aerosol | Influencing precipitation process
Many of the world's deserts are located in these subtropical latitudes. However the deserts do not extend to the eastern side of the various continents because of ocean currents caused by the Trade Winds.
Hadley cell | Major impacts on precipitation by latitude
The same process promotes the development of a low level jet to the east of the dryline. During the daytime, if heating and/or convergence are sufficient, the cap can be broken, resulting in convective clouds.
Dry line | Associated weather
Convection occurs when the Earth's surface, especially within a conditionally unstable or moist atmosphere, becomes heated more than its surroundings and in turn leading to significant evaporation. Convective rain and light precipitation are the result of convective clouds, for example cumulonimbus or cumulus congestus. In the initial stages of this precipitation, it generally falls as showers with rapidly changing intensity. Convective precipitation falls over a certain area for a relatively short time, as convective clouds have limited vertical and horizontal extent. Most precipitation in the tropics appears to be convective; however, it has been suggested that stratiform and convective precipitation often both occur within the same complex of convection-generated cumulonimbus.
Precipitation types | Convectional
The climate is influenced by the Northeast Monsoon winds and the Pacific trade winds. These Monsoon winds create the low pressure area in the Pacific Ocean, resulting in tropical cyclones that affect the area in November and December. Rainfall occurs throughout the year with low rainfall between January to May and high rainfall between June and December. In the lower basin, mean annual rainfall ranges between . The main factors influencing rainfall are the air streams, the intertropical convergence zone and topography. The mean temperatures are high throughout the year with the daily mean temperatures in the range of and an annual mean temperature of .
Bicol River | Climate
Though precipitation rates during monsoonal circulations are difficult to predict, there is evidence for a large orographic barrier within the interior of Pangaea during the late Paleozoic The possibility of the SW-NE trending Appalachian-Hercynian Mountains makes the region's monsoonal circulations potentially relatable to present day monsoonal circulations surrounding the Tibetan Plateau, which is known to positively influence the magnitude of monsoonal periods within Eurasia. It is therefore somewhat expected that lower topography in other regions of the supercontinent during the Jurassic would negatively influence precipitation variations. The breakup of supercontinents may have affected local precipitation. When any supercontinent breaks up, there will be an increase in precipitation runoff over the surface of the continental land masses, increasing silicate weathering and the consumption of CO.
Supercontinent | Precipitation
When there is stronger wind shear, this means that the storms become more slanted and the latent heat release is dispersed over a much larger area.
Wind shear | Effects on tropical cyclones
Effects of such a strong wind can be seen in the area of Adolfovac as of early 2010s.
Medvednica | Precipitation
In general, winds are strongest immediately under the tropopause (except locally, during tornadoes, tropical cyclones or other anomalous situations). If two air masses of different temperatures or densities meet, the resulting pressure difference caused by the density difference (which ultimately causes wind) is highest within the transition zone. The wind does not flow directly from the hot to the cold area, but is deflected by the Coriolis effect and flows along the boundary of the two air masses. All these facts are consequences of the thermal wind relation. The balance of forces acting on an atmospheric air parcel in the vertical direction is primarily between the gravitational force acting on the mass of the parcel and the buoyancy force, or the difference in pressure between the top and bottom surfaces of the parcel.
Jet stream | Cause
The reduced dust emission warms the North Atlantic and increases westerly flow into the North American Monsoon, strengthening it. The far-field precipitation changes reach as far as Europe and Australia. Discrepancies between modelled and reconstructed northward extension and precipitation in the Asian monsoon regions and the North American Monsoon area may be explained through these remote effects.
African humid period | Effect on other climate modes
Highly elevated surfaces can induce a thermal low, which then augments the environmental wind flow Wind roses are tools used to display the direction of the prevailing wind. Knowledge of the prevailing wind allows the development of prevention strategies for wind erosion of agricultural land, such as across the Great Plains. Sand dunes can orient themselves perpendicular to the prevailing wind direction in coastal and desert locations. Insects drift along with the prevailing wind, but the flight of birds is less dependent on it. Prevailing winds in mountain locations can lead to significant rainfall gradients, ranging from wet across windward-facing slopes to desert-like conditions along their lee slopes.
Prevailing winds | Introduction
A cold front commonly brings a narrow band of precipitation that follows along the leading edge of the cold front. These bands of precipitation are often very strong, and can bring severe thunderstorms, hailstorms, snow squalls, and/or tornadoes. In the spring, these cold fronts can be very strong, and can bring strong winds when the pressure gradient is higher than normal. During the winter months, cold fronts sometimes come through an area with little or no precipitation. Wider rain bands can occur behind cold fronts which tend to have more stratiform, and less convective, precipitation. These rainstorms sometimes bring flooding, and can move very slowly when the storm steering it is strong and embedded within a meridional flow pattern (with more pole to equator motion rather than west to east motion).
Cold front | Precipitation
Graupel and hail indicate convection; when either are present at the surface, it is indicative that some form of precipitation is extant at the freezing level, a varying point in the atmosphere in which the temperature is 0°C. In mid-latitude regions, convective precipitation is often associated with cold fronts where as it is often found behind the front, occasionally initiating a squall line.
Precipitation types | Convectional
For example, in Tucuman Province, these cold fronts are responsible for 70% of the rainfall in that province. In contrast, during the winter months, the Chaco low attracts air masses from the South Pacific high, creating a dry and cold wind. This effect is more prominent in the winter months, when the intensity of the cold is more stronger. The Intertropical convergence zone reaches the region during the summer months, leading to low pressure, and unstable air masses due to the relatively high land temperatures compared to the sea during this season. This leads to enhanced precipitation in the form of convective thunderstorms during the summer months.
Argentine Northwest | Climate
The Berkeley Hills affect rainfall; when the wind is perpendicular to the hills (southwest wind) during a storm, air is forced to rise, cooling and condensing additional moisture, increasing the precipitation on the western slopes and leaving areas east of the hills drier. Especially cold storms occasionally deposit wet snow on the peaks.
Berkeley Hills | Climatic effects
During thunderstorm formation, winds move in a direction opposite to the storm's travel, and they move from all directions into the thunderstorm. When the storm collapses and begins to release precipitation, wind directions reverse, gusting outward from the storm and generally gusting the strongest in the direction of the storm's travel. When this downdraft of cold air, or downburst, reaches the ground, it blows dry, loose silt and clay (collectively, dust) up from the desert, creating a wall of sediment that precedes the storm cloud. This wall of dust can be up to wide and several kilometers in elevation. At their strongest, haboob winds often travel at , and they may approach with little or no warning.
Haboob | Description
Systems known as Kona storms affect the state with heavy rains between October and April. Local climates vary considerably on each island due to their topography, divisible into windward ("Koolau") and leeward ("Kona") regions based upon location relative to the higher mountains. Windward sides face the east to northeast trade winds and receive much more rainfall; leeward sides are drier and sunnier, with less rain and less cloud cover. In South America, the Andes mountain range blocks Pacific moisture that arrives in that continent, resulting in a desertlike climate just downwind across western Argentina. The Sierra Nevada range creates the same effect in North America forming the Great Basin and Mojave Deserts.
Rain | Orographic effects
The systems begin in the afternoon as scattered thunderstorms which organize overnight in the presence of wind shear (wind speed and direction changes with height). The probability for severe weather is highest in the early stages of formation, during the afternoon. The MCC persists at its mature and strongest stage overnight and into the early morning in which the rainfall is characterized as stratiform rainfall (rather than convective rainfall which occurs with thunderstorms). Dissipation of the MCC commonly occurs during the morning hours. After dissipation, a remnant mid-level circulation known as a mesoscale convective vortex can initiate another round of thunderstorms later in the day.
Mesoscale convective complex | Development
Tropical cyclones enhance precipitation across southern sections of the country, as well as Puerto Rico, the United States Virgin Islands, the Northern Mariana Islands, Guam, and American Samoa. Over the top of the ridge, the jet stream brings a summer precipitation maximum to the Great Lakes. Large thunderstorm areas known as mesoscale convective complexes move through the Plains, Midwest, and Great Lakes during the warm season, contributing up to 10% of the annual precipitation to the region. The El Niño-Southern Oscillation affects the precipitation distribution, by altering rainfall patterns across the West, Midwest, the Southeast, and throughout the tropics. There is also evidence that global warming is leading to increased precipitation to the eastern portions of North America, while droughts are becoming more frequent in the tropics and subtropics.
Earth rainfall climatology | United States
Strong winds often develop around dry thunderstorms as the evaporating precipitation causes excessive cooling of the air beneath the storm, which increases its density and thereby its weight relative to the surrounding air. This cool air then descends rapidly and fans out upon impacting the ground, an event often described as a dry microburst. As the gusty winds expand outward from the storm, dry soil and sand are often picked up by the strong winds, creating dust and sand storms known as haboobs.
Dry thunderstorm | Dust storms
However, when the air rises, and while it is still over the land, the air cools. This decreases the air's ability to hold water, and this causes precipitation over the land. This is why summer monsoons cause so much rain over land. In the colder months, the cycle is reversed. Then the land cools faster than the oceans and the air over the land has higher pressure than air over the ocean. This causes the air over the land to flow to the ocean. When humid air rises over the ocean, it cools, and this causes precipitation over the oceans.
Monsoon | Process
Warm fronts occur where warm air pushes out a previously extant cold air mass. The warm air overrides the cooler air and moves upward dud . Warm fronts are followed by extended periods of light rain and drizzle due to the fact that, after the warm air rises above the cooler air (which remains on the ground), it gradually cools due to the air's expansion while being lifted, which forms clouds and leads to precipitation.
Precipitation types | Cyclonic
Temperature tends to decrease and moisture content to increase in the immediate vicinity of a tornado.
Tornado | Electromagnetic, lightning, and other effects
As the equatorward portion of the system approaches the cold air wedge, persistent low cloudiness, such as stratus, and precipitation such as drizzle develop, which can linger for long periods of time; as long as ten days. The precipitation itself can create or enhance a damming signature, if the poleward high is relatively weak. If such events accelerate through mountain passes, dangerously accelerated mountain-gap winds can result, such as the Tehuantepecer and Santa Ana winds. These events are seen commonly in the northern Hemisphere across central and eastern North America, south of the Alps in Italy, and near Taiwan and Korea in Asia.
Cold-air damming | Introduction
Like all other soluble aerosols, increasing normal-sized sea salts suppresses the precipitation process in warm clouds by increasing cloud droplet number concentration and reducing the cloud droplet size. Also, they invigorate precipitation in mix-phase clouds because once the suppressed smaller cloud droplets are lifted above freezing level, more latent heat content would be released due to the freezing of cloud drops. Besides that, adding giant sea salt aerosols to polluted clouds can accelerate the precipitation process because giant CCNs could be nucleated into large particles which collect other smaller cloud drops and grow into rain droplets. Cloud drops formed on giant sea salt aerosols may grow much more rapidly by condensation that cloud drops formed on small soluble aerosol particles, as giant sea salt cloud drops may remain concentrated solution drops for long times after they are carried into cloud.
Sea salt aerosol | Influencing precipitation process
Precipitation The primary meteorological characteristics of a rainfall event that influence stemflow are: Species
Stemflow | Determining factors
A mesohigh forms underneath the downdraft in a squall line and is associated with the cold pool of a thunderstorm. It is largely formed by hydrostatic phenomenon, specifically the evaporation of falling precipitation. As precipitation, primarily rain, falls in the downdraft, it evaporates in the unsaturated air, leading to cooling in the downdraft due to an absorption of latent heat. The cooling of the air leads to an increase in the pressure as the air becomes denser. While not the primary mechanism behind the mesohigh, melting or sensible cooling of hail can also lead to increased pressure in the mesohigh.
Mesohigh | Formation
Mean annual precipitation is . The wettest months are from October to January, with average precipitation per month, while the driest months are from May to July, with average precipitation per month. The sun is up for 24 hours per day between 13 May and 31 July, and remains below the horizon continuously from 21 November to 21 January. On December 29, 2008, winds were recorded at 81 mph.
Honningsvåg | Climate
Climate models show that while some regions should expect an increase in precipitation, such as in the tropics and higher latitudes, other areas are expected to see a decrease, such as in the subtropics. This will ultimately cause a latitudinal variation in water distribution. The areas receiving more precipitation are also expected to receive this increase during their winter and actually become drier during their summer, creating even more of a variation of precipitation distribution. Naturally, the distribution of precipitation across the planet is very uneven, causing constant variations in water availability in respective locations. Changes in precipitation affect the timing and magnitude of floods and droughts, shift runoff processes, and alter groundwater recharge rates.
Environmental degradation | Climate change and precipitation
In addition, these clouds flow with atmospheric circulation and spend little time above a point on the ground. This explains the variations in intensity and the short duration of the showers. The type of precipitation will depend on the temperature structure in the cloud and below it:
Shower (precipitation) | Formation
Mechanisms of producing precipitation include convective, stratiform, and orographic rainfall. Convective processes involve strong vertical motions that can cause the overturning of the atmosphere in that location within an hour and cause heavy precipitation, while stratiform processes involve weaker upward motions and less intense precipitation over a longer duration. Precipitation can be divided into three categories, based on whether it falls as liquid water, liquid water that freezes on contact with the surface, or ice. If these factors do not support precipitation volumes sufficient to reach the surface over a sufficient period of time, the result is a drought. Drought can be triggered by a high level of reflected sunlight and above average prevalence of high pressure systems, winds carrying continental, rather than oceanic air masses, and ridges of high pressure areas aloft can prevent or restrict the developing of thunderstorm activity or rainfall over one certain region.
Droughts in the United States | Causes
The reverse transition can occur behind the departing low pressure system, though it is more common for precipitation to change directly from rain to snow, or for it to stop before a transition back.
Rain and snow mixed | "Wintry showers" or "wintry mixes"
This increases the voltage difference between the base and the top of the cloud, helping to produce lightning.
Dry thunderstorm | Fires
"Frequency" of a precipitation threshold of interest may be determined from the number of measurements exceeding that threshold value within the total time period for which observations are available. Individual data points are converted to "intensity" by dividing each measured depth by the period of time between observations. This intensity will be less than the actual peak intensity if the "duration" of the rainfall event was less than the fixed time interval for which measurements are reported. Convective precipitation events (thunderstorms) tend to produce shorter duration storm events than orographic precipitation. Duration, intensity, and frequency of rainfall events are important to flood prediction.
Flood | Upslope factors
The Southwest appears to have seen an increase in precipitation, as well, also with an average 2° of cooling.
Younger Dryas | West
Prevailing winds flow from the south from February through June and from the east from June through February. Wind velocity averages 8.7 km/h from December through May and 7.4 km/h from May through December.
Jonacatepec | Climate
On the large scale, the highest precipitation amounts outside topography fall in the tropics, closely tied to the Intertropical Convergence Zone, itself the ascending branch of the Hadley cell. Mountainous locales near the equator in Colombia are amongst the wettest places on Earth. North and south of this are regions of descending air that form subtropical ridges where precipitation is low; the land surface underneath these ridges is usually arid, and these regions make up most of the Earth's deserts. An exception to this rule is in Hawaii, where upslope flow due to the trade winds lead to one of the wettest locations on Earth.
Precipitation | Large-scale geographical distribution
Converging sea breeze fronts from the north and south coasts of Cornwall can lead to very rapid convection developing over the moor, which may result in very heavy showers or thunderstorms. Once the showers have formed, they often travel in an easterly or north-easterly direction, driven by the prevailing wind. This causes a thin line of showers to occur along the length of the south-west peninsula, all originating from the Bodmin Moor area, which are often intensified further by the downwind high ground of Dartmoor and the Blackdown Hills. Places directly in the path of these showers suffer a series of heavy downpours, whilst nearby locations remain dry.
Brown Willy effect | Cause