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Summer showers

Summer rain

In weather forecasting we often refer to 'summer showers' or mention spells of showers and sunshine. These are neat and short phrases which actually capture a complex meteorological situation.

The first thing to briefly note is the difference between showers and rain.

Rain comes from stratus clouds which are usually part of a larger frontal system. Rain is therefore usually relatively widespread and lasts for longer period of times.

Showers on the other hand originate from cumulus clouds as a result of convection and are characterised by short duration and rapid fluctuations of intensity across localised areas.

Why 'summer' showers?

In winter our weather is largely driven by large scale low pressure systems and weather fronts which move across the UK.  In summer however, when the big weather systems head further north and the weather is relatively quiet it's the local effects of the hills and valleys, land and sea and subtle variations in heat and moisture that dominate what weather we will see.

Over the UK, the atmosphere is often finely balanced in summer so that when air ascends, it can keep rising until it cools and forms shower clouds.

And this is where the topography (the shape and features of the earth's surface) comes into play. Showers will often form over hills where moving air is forced to rise over them and also on the coasts where the land heats up and draws in cooler air from the sea to replace it.

If the conditions are right, as this air rises inland showers can form. They will often form into lines (or 'bands') meaning depending on your location and direction of travel you could either see shower after shower or simply wander through them and see no more.

Showers forecast over the UK on 16 June 2016

Predicting summer showers

On a nationwide level, the idea of showers popping up across the country in between spells of sunshine is quite easy to visualise, but when you zoom in to the local level, it is much harder to see exactly what is going on.

Let's work through a quick example. The UK covers 246,610 square kilometres while the average shower only covers around 1 square kilometre, and will likely travel no more than 64 kilometres in the hour or so that it lasts. This means each individual shower will only impact 0.026% of the UK.  The real challenge for the forecaster is to identify accurately which 64 square kilometres each shower will impact.

One analogy we use to picture the complexity of this forecasting challenge is comparing it to boiling a pan of water and having to guess exactly where and when the bubbles will appear.

This kind of detail is quite difficult to forecast, but is exactly what most of us are interested in during the summer.

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