Understanding the Weather
"It is easy to forget that you live in the sky - not beneath it, but within it"
-Opening Line from A Cloud A Day- Gavin Pretor-Pinney
The Geographic location of Ireland, a rocky Island set at the edge of continental Europe next to the expanse of the Atlantic ocean. We sit north of the equator, (about 52 degrees) in line with Poland and Newfoundland, but we don't experience the same harsh winters. We call our climate a temperate maritime or oceanic one. This means mild, changeable and humid. We are at the mercy of; the Jet Stream - a current of air several miles above and; the Gulf Stream - a giant ocean current carrying warm water from the equator.
The map you see on a weather forecast is called a synoptic chart. The chart is a weather map that similar to contours on a mountaineering map but in the synoptic chart, air pressure is represented in the same way but is constantly moving. The lines in this case are called isobars. The closer these lines are together the stronger the wind is moving in that area.
Meteoblue is very detailed website initially developed at the University of Basel, Switzerland. Is very popular with paraglider pilots and mountaineers alike. The meteogram is very handy for cloud cover forecast for hikers
Forecasting the weather is the complex science of using all available technology eg. ocean buoys, satellite, local weather stations to predict the future state of the atmosphere.
Data on wind speed, direction, precipitation, temperature, humidity is gathered and fed into a super computer. Then displayed on charts and computer simulations
Clouds are a major part of our everyday life. When we crawl into bed after a tough day in the hills there is always colossal movement in all the many layers of the atmosphere above. Let’s look at these fluffy formations for a moment and see what they can tell us.
What is a cloud?
Cumulus, stratus, nimbus… clouds have as many names as they do shapes and sizes so we will start by looking at the translation of these names.
- Cumulus - from the Latin heap or pile
- Stratus - to spread out or cover with a layer
- Nimbus - Nimbus means rainy cloud
As humans we love to put things into categories. Clouds can be most simply broken down into 3 categories - low, mid and high or stratus, alto and cirrus.
Very simply clouds are water, and water has some special properties. What’s interesting is it is very stable. Other liquid with similar (molecular) weight boils at around -62 deg Celcius, water we know boils at 100 Celcius. This is important to ecosystems that live in water, even small changes in oceans and lakes can cause organisms to die.
Which is heavier, Air containing more or less water?
When air has as much water as it can hold we call it saturation point. So, either the water content reaches capacity or the plume of air is cooled reducing its capacity to hold vapour. When the water gets too heavy gravity takes over and that’s when we get rain. As we dive in deeper we find they are also dynamic, ever changing and sometimes even high voltage.
So to answer the question, think of steam from a kettle. Water vapour molecules are lighter than the other molecules that make up the atmosphere ie oxygen and nitrogen etc.
And so it rises!
Ways Clouds can form
- The Sun's radiation heats the ground causing hot air to rise. These ‘thermals’ reach a point called dew point. The air cools and we get clouds.
- Air mass forced up a mountain cools and condenses. This can cause rain shadows to occur - this is an arid or dry area found on the leeward side of a mountain. The Gobi Desert, Mongolia on the Lee of the Himalayas or even east Wicklow and south county Dublin are in the rain shadow of the Wicklow Mountains.
- Weather fronts happen when two blocks of air meet or when one catches up to the other. Much like in a mountain scenario, the lighter, less dense warm air is forced upwards. This is called a front.
There is more to this however, you also need particles for the droplets to form. For the droplets to form aerosols (suspended air particles) like dust, pollen or salt in the air are needed, these are called cloud condensation nuclei (CCN).
UK Met office is very educational with an array of maps, charts and forecasts (including a mountain forecast)
What can clouds tell us as hikers?
Before setting out, it is always best to check the weather in advance. There is a great selection of weather apps out there such as MET eireann, Meteoblue, mountain-forecast, Windy etc. you can pick one, and get familiar with it or use a variety of apps for different purposes, eg one for wind, one for expected cloud cover and another for temperature. You can use these along with a Met mountain forecast (more links below). If you really want to impress you could learn to read synoptic charts yourself.
When we are on the hills you are more likely to encounter clouds, they can affect your day in a number of ways; navigating can become more of a challenge, we have less reference points as visibility is reduced and we can become disorientated. In snow, a whiteout can cause extreme disorientation as it becomes difficult to distinguish land from sky.
Wind Effects on Weather
Turn to face the wind you should hear the wind in both ears or use a blade of grass to judge this. The direction you are facing is what gives the wind its name, if the wind is blowing from the east, it’s an easterly wind. This is important because in a way, the wind transports the weather.
What causes wind
Wind is caused by a change in atmospheric pressure. The sun heats the earths uneven surface, much of this heat is concentrated around the equator.
Hot air is lighter than cool air and this causes it to rise creating a low pressure system. When air is cooled it sinks. It's this rising and falling of air that causes air to move from on area to fill another an this creates wind. So in essence wind doesn't blow, it suck's! Filling a vacuum of unoccupied air.
Wind Chill is something we must consider when venturing into the hills. Wind chill is what the weather feels like when it blows on your skin. There is currently no definitive way to measure wind chill but the stronger the wind the faster the cooling effect, a wind breaker reduces this effect trapping a layer of warm air
In Ireland the wind blows most frequently from the south and west, this is known as the prevailing wind. Wind from the Artic north is generally cold and dry. during the winter we sometimes get a north-easterly airflow from Siberia which can carry moisture with it known now as the "Beast From The East". West and south westerly (maritime) wind can often bring moisture in from the the Atlantic. High pressure systems that travel from the Azores often bring welcome dry settled periods.
There are no modern named winds in Ireland. In some ancient texts there was twelve winds and each had their own colour. The Gaoithe sidhe or Fairy wind in Irish Folklore is a sudden gust of wind or a whirlwind was interpreted as being caused by the fairies.
Some Clouds Types to Look out for
Cirrus - The high wispy clouds are an indication of fine weather, they also mean a change is likely to occur within 24 hours.
Altocumulus - High mid level clouds that mean settled weather, can develop into Altostratus as they thicken, these normally form ahead of what’s called a warm front occluded front. If the cloud thickens further it becomes Nimbostratus are a cloud we know all too well in Ireland, these clouds normally mean get the rain gear out, you are in for a wet one.
Stratus Clouds - Low clouds that mean stable conditions, usually associated with light rainfall or drizzle.
Cumulonimbus - Keep an eye on these guys, they are recognisable in the Cumulus Incus stage where they take on their famous “anvil” shape, this shape is created when the cloud grows so tall it hits the atmosphere and is forced to spread out. They are normally associated with extreme weather conditions.
Other unique cloud forms
Kelvin Helmholz clouds - Friction in the atmosphere that is known as shear between two air streams causes waves to form in the clouds.
Lenticulars - In a stable air mass when strong winds blow across an obstruction like a mountain. The stationary cloud indicates strong wind up high. Like a lens or lentil.
Cloud Inversions - one of the great pleasures is hiking out of a valley on what seems like a cloudy, grey day and to emerge over what looks like a blanket sitting calmly among the peaks. This occurs when cooler air is trapped by a cap of warmer air.
Windy app is very colourful and intuitive with a ton of different weather options including
Useful Links & References
- https://cloudatlas.wmo.int/en/appendix-1-etymology-of-latin-names-of-clouds.html#:~:text=with a layer-,Cumulus,accumulation, a heap, a pile