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 position of Ireland; a rocky Island set on 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.
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 in the hills. It is humbling to imagine 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 formations for a moment and see what they can tell us.
What Is a Cloud?
Cumulus, Stratus, Nimbus, Cirrus… clouds have as many names as they do shapes and sizes. A 19th century pharmasist and armchair meteorologist by the name Luke Howard is credited for the names we still use today. Let's start by looking at the translation of some of these names.
- Cumulus - from the Latin heap or pile
- Stratus - to spread out or cover with a layer
- Cirrus - meaning fiber or hair
- Nimbus - Nimbus means rainy cloud
Science bit... Very simply put, clouds are water, and water has some special properties... 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 an important characteristic for life that live in water, even small changes in an ocean or lakes can cause organisms to die.
Which is heavier, Air containing more or less water?
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 they rise!
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 (sheltered from the prevailing wind) side of a mountain. The Gobi Desert, Mongolia is on the Lee of the Himalayas or 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. Aerosols (suspended air particles) like dust, pollen or salt in the air are is needed, these are called cloud condensation nuclei (CCN).
WHAT CAN CLOUDS TELL US AS HIKERS?
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
What causes wind
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! Constantly filling a vacuum of unoccupied air.
Turn to face the wind you should hear the wind in both ears or use a blade. 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.
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, now known as the "Beast From The East". West and south westerly (maritime) wind carries 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, whirlwind or water spout, this 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 (Mackeral sky) - High, mid level clouds that mean settled weather. These can develop into Altostratus as they thicken, these normally form ahead of what’s called a warm occluded front.
If the cloud thickens further it becomes Nimbostratus, these are a cloud we know all too well in Ireland. They normally mean, out with the sallopettes, 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 when 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 wave shapes 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.
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