Ever-changing shape of clouds is fascinating

BY Jim Hurley - NAture Trail

Clouds often share a straight-line cloud base.
Clouds often share a straight-line cloud base.

Sunrise, sunset, the colours of the sky and the ever-changing shapes of clouds are among the very beautiful experiences, albeit fleeting, of the natural world.

How clouds form is particularly fascinating. Pressure systems are large masses of air, so large at times that they can be as big as whole continents. These pressure systems constantly move about in the atmosphere. Warm ones from the south move up over us, cold ones from the north move down over us and both dry ones from the east and wet ones from the west regularly slide across us.

The island of Ireland happens to be positioned on the globe at a point where several pressure systems meet and manoeuvre around each other.

These ever-moving pressure systems waltz about in an apparently random dance. To paraphrase the goings-on at Lanigan's Ball, one steps out and another steps in again challenging weather forecasters to keep track of the endless movements in the great ocean of air above us.

However, the fact that the planet is rotating guarantees that wet pressure systems that originated in the west over the Atlantic Ocean constantly pass over, or very close to, our green and pleasant land often releasing the moisture they picked up over the ocean to bucket down on us as rain.

Pressure systems fall into two great groups: low ones and high ones. Low-pressure systems push air upwards. As the air rises it cools. When the cooling reaches a particular height and temperature, the invisible water vapour in the air condenses to form water droplets and become the visible collection of water droplets that we call a cloud.

Often when clouds form they share a common altitude at which their water vapour began to condense. That uniformity in the temperature of the air results in all the clouds having the same base as in the image above. No such uniformity exists within the clouds so they all have tops towering up to different heights depending on the strength of local convection within each cloud.

While low-pressure systems push air upwards, on the other hand, high-pressure systems, like the Azores High, push air downwards onto the ground. As the air sinks it doesn't cool. Consequently, water vapour doesn't condense into visible water droplets and clouds don't normally form. As the Azores High builds from the south it tends to bring cloud-free, clear blue skies, no rain and bright sunshine.

Wexford People

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