What gives showers oomph?
Over the past week, the weather across the UK has been characterised by sunshine and showers, with some of them heavy accompanied by hail and thunder. But what actually determines how heavy a shower will be?
It’s all related to the stability of the atmosphere, which effectively means how much air wants to rise in a given situation. If the air is stable then there will be little upward motion, but if the air is unstable, it will want to rise, eventually condensing to form clouds and subsequently showers.
It may sound quite straight forward, but there are a number of factors that contribute to air rising in a sunshine and showers situation – each of which can increase or decrease shower intensity. Effectively, it’s like doing a calculation on the extent to which air will rise over a particular location from a number of contributing factors.
Warming of the ground
You’ve probably noticed that on showery days the mornings often start off dry with sunshine, before the cloud builds through the day with showers often from lunchtime onwards.
The sun heats the ground, which in turn heats the air just above it, which causes it to rise, condense and form shower clouds.
However, the greater the amount of heating in a given situation, i.e. the higher the temperature at the surface, the more readily the air will rise, resulting in more developed clouds and heavier showers.
Cold air aloft
The temperature difference between the ground and the air high up in the sky has a big influence on the intensity of showers. The greater this difference, the more unstable the air tends to be.
This results in air having more buoyancy and therefore rising quicker and higher, resulting in bigger clouds and heavier downpours – sometimes with hail and thunder.
When low pressure is present air will happily rise upwards, condensing and forming clouds. The deeper the area of low pressure, the more readily this tends to happen.
During this week, low pressure has got stuck across the UK, rather than moving through as it usually tends to. This has meant the there has been a more permanent driver for air to rise, allowing lots of showers to form.
Convergence of winds near the ground
Whilst this hasn’t been a huge factor in this week’s weather, it can also give showers potency.
Sometimes due to topographical reasons such as coasts or mountains, winds at the surface can collide, which results in the air having nowhere to go except upwards.
As a result, air over a particular area rises more readily and makes showers heavier. This process often occurs in concentrated lines and showers can get stuck, giving the same location hours of rain. This is what happened with the Boscastle floods in 2004.
Presence of a trough or cold front
A trough or cold front provides a little kick in the upward motion of air in the atmosphere, as well as providing added moisture for shower clouds to tap into.
During this week, troughs have been spinning around the area of low pressure stuck across the UK, helping the showers to form into bands and become heavy with hail and thunder.
Each of these factors have to be considered when working out whether or not showers are likely for a particular location, which is part of the reason why showers can be hard to forecast.
Whilst identifying a region at risk from showers is relatively easy, getting the detail right for an exact location is a much greater challenge.