Is it going to rain? What a popular question.
The key to longer-term forecasts lies in studying changes in clouds and fronts over hours, but we will often find ourselves looking at an individual cloud and wondering if it’s about to soak us.
Rain rarely falls from small cumulus clouds and if they’re wider than they are tall it’s very unlikely indeed. But we can be a bit more forensic about this by studying the bottom of the clouds.
The bottom of all clouds marks the height at which the temperature is cool enough to condense the water vapour in the atmosphere, the dew point. Since this level tends to be consistent it leads to flat-bottomed clouds. It follows that a cloud with a ragged bottom is trying to tell us something.
We’re all used to the idea that clouds give us rain. It is less well known that rain creates clouds. When rain falls, it cools the air just below the cloud. This leads to more condensation and creates jagged areas of new cloud just below the main cloud, which gives the base a ruffled rough appearance. Hence, smooth bases on cumulus clouds mean no rain falling and ragged bottoms indicate rain falling.
These craggy, uneven fragments of cloud are known in formal circles as ‘pannus’ and are described as ‘accessory clouds’. They are rain’s footprints and can be seen under all clouds that rain falls from.
However sombre a cloud appears, if it has a neat horizontal base and good visibility below it is unlikely to rain on you.
Extracted from the book The Secret World of Weather by Tristan Gooley.
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