People often get freezing rain and hail confused with sleet. Freezing rain occurs when liquid precipitation freezes upon contact with a cold surface. Hail occurs during a thunderstorm when ice forms into a hailstone, layer by layer like an onion, until it gets too heavy and finally falls to the ground. Snow is made of ice crystals and is very fine in texture. Sleep is actually small pellets of ice that form when rain droplets freeze in the air before reaching the ground and is common during winter storms.
Air pressure is measured using a barometer. One type of a barometer uses a tube full of mercury and another type of barometer uses a box partially full of air to measure air pressure. A hygrometer is used to measure humidity in the air. A thermometer is used to measure temperature and a hydrometer is used to measure the density of liquids such as in an aquarium tank.
The NAM, GFS and NGM art types of computer models. NAM stands for a North American Mesoscale, GFS stands for Global Forecasting System and NGM stands for Nested Grid Model. These computer models are maintained by the National Centers for Environmental Prediction and are used widely by the National Weather Service to forecast our weather. Over time these models 'learn' more weather patterns and their eventual outcome, are run on more powerful computers, are fed increasing amounts of data and their software improved with more sophisticated algorithms. This has allowed the computer models to become more accurate and enable them to forecast further into the future.
Clouds that are convective in nature are part of the Cumulus family of clouds. Clouds that produce precipitation are part of the Nimbus family of clouds. Since most cloud types are the combination of two or more cloud families, the words can be combined to create a new cloud name. So, a convective cloud that produces rain is called a cumulonimbus cloud. Cumulonimbus clouds are responsible for spring showers, thunderstorms and even the dreaded hurricane.
The river of air several thousand feet above the surface is called the jet stream. Another name for this important weather feature is steering currents. The winds in the jet stream are often very fast and can exceed 200 mph. These winds help to steer weather systems such as storms, high pressure systems and fronts. Knowing how fast and in which direction these winds are blowing is vital to forecasting the tracks and the timing of storms and fronts. Since ground instruments are unable to measure wind several thousand feet up in the air, weather balloons need to be released periodically with a radio device attached to measure the direction and speed of the jet stream.
Weather fronts are closely associated with storms and are where most of the inclement weather occurs. The three main fronts of a storm are the cold front, warm front and occluded front. The warm front proceeds the storm and brings in warmer air and a large area of light rain. The cold front is behind the storm and brings convective rain and thunderstorms and colder air. As a storm matures, the cold air starts to catch up with the warm air and forms an occluded front. The area where the three fronts meet is called the triple point and can often spawn a brand new storm system call the secondary low.
When a very large area of the equatorial Pacific Ocean surface temperature is above normal an El Niño forms. This phenomenon can cause widespread changes in weather patterns in many parts of the world. Droughts, flooding and major storms are often the result of El Niño. The associated weather isn't always bad as hurricane formation in the Atlantic is suppressed and some areas can get beneficial rains. The opposite of El Niño is La Niña which occurs when the same area of the Pacific Ocean surface temperature is colder than normal. A Willy-Willy and Typhoon are types of tropical storms.
The location where the equatorial tradewinds merge is called the intertropical convergence zone. This zone fluctuates north and south in conjunction with the sun angle at the equator. In the summer the intertropical convergence zone shifts to the north and spawns tropical waves. These waves contain thunderstorms that feed off the warm ocean water. Quite often these waves will become depressions and tropical storms. If the winds aloft are favorable and the storms stays over warm water then a hurricane could form. This happens most often in late summer.
The anemometer is used to measure wind speed. It's most common form is a series of three cups arranged at 120° angles to measure wind velocity based on how fast the cups are spinning. The data is then sent to a digital gauge. A barometer is used to measure air pressure. A hydrometer measures the humidity in the air and an aerometer measures the weight and density of a gas.
When liquid precipitation freezes upon contact of a surface it is called freezing rain. Sleet is caused when the liquid precipitation freezes before reaching the ground forming pellets of ice. Ice pellets is another name for sleet. Rime ice is caused by extended periods of freezing drizzle and strong winds. This phenomenon is common in high elevation weather stations such as Mount Washington, New Hampshire.