24 Mar 2014

What drives heavy spring showers?

After more than a week of spring warmth, there was a definite change in the feel of the weather at the weekend. Despite some sunshine, there were heavy showers, accompanied by the arrival of colder air.

Snow returned to the hills, mountains and moors, as far south as Dartmoor. Also, there was a widespread frost last night, with temperatures in some places falling to around -5C.

However, the most notable feature of the weather this weekend was definitely the large cumulonimbus clouds roaming the skies, delivering hail, thunder and gusty winds.

Cumulonimbus clouds and heavy showers are common during spring in the UK, but why? 

cumulonimbus_sky_g_wp

Unstable air

In weather, things happen when there are contrasts in temperature, pressure or humidity – often a combination of these three factors.

The main driver of the cumulonimbus clouds and heavy showers at the weekend was a difference in temperature in the atmosphere.

When a bubble of air is warmer than the air surrounding it, it becomes buoyant and wants to rise upwards – something described as unstable air in meteorology.

The greater the difference in temperature between the bubble of warm air and the colder air surrounding it, the higher and more quickly it shoots up into the air.

During the weekend, the UK was bathed in cold polar maritime air. At the same time, the now relatively strong late-March sunshine heated bubbles of air near the surface.

In addition, low pressure was sitting over the UK, which itself allows air to move upwards more readily.

These factors combined sent large bubbles of air high into the sky, which eventually cooled and condensed, forming the cumulonimbus clouds that we saw.

Cumulonimbus cloud in action

On Saturday, @SpireWeather tweeted the beautiful time-lapse video below of a cumulonimbus cloud moving over Salisbury.

If you look closely, you can see the front edge of the cloud growing, as air within it continues to rise upwards and spread outwards at the top.

The cloud appears dark grey from the bottom because it is so thick and most of the sunlight is being reflected back up into the sky.

The showers won’t be as big this coming week, but there’ll still be some rain for all of us in some point. In the meantime, you can send me your cloud pictures on Twitter – @liamdutton

Tweets by @liamdutton