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Does April bring the sweet spring showers?

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John Hammond Met Office Forecaster John Hammond Met Office Forecaster | 08:30 UK time, Monday, 11 April 2011

d ~ 259'852'800 km: day 101

April is a month always linked with the occurrence of showers - but are showers really more likely in April than any other month of the year? Of course not, although there are reasons why April might be more associated with them.

Pentax K20D.28-80mm SFX Macro Lens.Lesser Celandine Flower In The Bog.March 20th 2011.

As far as weather patterns are concerned, the energy received by the Sun plays a pivotal role on weather systems around the world. The differential heating of the Earth's atmosphere ultimately produces jetstreams - narrow but very long bands of strong winds high in the troposphere.

Jetstreams meander north and south through the year and as a rule during the northern hemisphere spring, jetstreams generally drift northwards and away from the British Isles.

As the main drivers of changeable weather, jetstreams often bring spells of continuous heavy rain. However, as the jetstream moves away from the UK, it leaves the country more likely to have weaker bands of rain and more days when showers are likely to develop.

At this time of year the energy from the Sun is intensifying. As a result, ground temperatures begin to respond to the extra solar heating and warm quickly.

Strong surface heating is another factor typical for generating showers. The other required element is colder air aloft. Although warm at the surface in any sunshine, April is a month when the air higher up in the troposphere can be much colder.

On showery days, air temperatures always fall with height in the troposphere. Thanks to the strong heating, air at the surface has much more energy when compared to the colder air above it - and rises. If conditions allow, the air will be able to continue rising and reach the Tropopause at a level of 30,000 feet or more.

As this air rises up through the atmosphere, it cools and condenses into clouds. Once these clouds develop in sub freezing air and are thick enough they produce showers of rain, some of which will be heavy with thunderstorms and hail.

Showery - or convective days are often accompanied by intervals of sunshine as part of the atmosphere displaying 'convective overturning'. This is because not all air can rise at the same time - it's a case of what goes up must come down. So, a shower is where the air is rising, but areas where the air descends will often be clearer or have thinner cloud.

Clouds associated with showery days are Cumulus and sometimes the thunderstorm cloud Cumulonimbus.

During an April shower, temperatures can vary widely. Outside the shower and in the spring sunshine, temperatures can easily reach 20 Celsius. However, a heavy shower with hail can see these temperatures fall rapidly to around 10 Celsius.

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