Why are showers one of the hardest things to forecast?

Cumulus clouds tower above Ernesettle Creek, Plymouth Credit: Claire Russell

You hear weather presenters say it all the time

But why is it so hard to predict exactly where they're going to crop up? As with most forecasts it all comes down to the finer detail. Predicting a showery day is relatively easy but pinpointing exactly where each shower is going to form and pour down is a different matter.

Showers over Bellstone Tor Credit: Laura Davis

That's because the sky above our heads during a showery day has the potential to make a shower cloud almost anywhere but it'll only form where there's exactly the right amount of moisture and heat - the ideal conditions.

You can think of it like a pan of boiling water. All of the water in the pan is close to 100 °C and therefore it all has the potential to produce bubbles as it boils. However, bubbles only form in some places and trying to predict where the next ones will pop up is almost impossible, even though you know there will be more.

Rainbow between showers in Abbottsham Credit: Simon James

That's why a lot of weather forecasters will use words like 'widespread' or 'isolated', 'occasional' or 'frequent' to try and give an idea as to how likely and how often showers will develop. Either way it's always safe to expect that at some point you might get wet when you hear the S-word in a forecast.