Weather forecasts often refer to rain or showers and there are crucial differences between the two which explain what weather is expected.
While showers are technically a type of rain, they behave differently to the typical frontal rainfall we receive in the UK and so it it useful to distinguish between the two.
The key points of difference relate to the types of cloud that lead to their formation and the resulting difference in the duration and intensities of the downpours.
A difference in size and duration
What we usually mean when referring to 'rain' is the precipitation that falls from a Weather fronts. A front is the boundary between two bodies of air that have different properties. Frontal cloud may stretch for hundreds of miles usually consisting of various types of stratus cloud (including stratus, altostratus and nimbostratus).
Frontal rain tends to stretch across large areas bringing precipitation to a wider area.
This makes frontal rain hard to avoid because the rain usually stretches along the entire length of the front. These fronts can typically last for a number of days and they are commonly associated with unsettled weather.
Showers by contrast are produced by convective clouds known as cumulus. Rain-bearing cumulus clouds are commonly taller than they are wide and have a fluffy, cauliflower-like appearance to their tops.
The largest cumulus clouds are called cumulonimbus and they reach high enough into the atmosphere to contain ice, rather than water droplets. Whilst cumulonimbus clouds are large relative to an observer on the ground, they are not on the same scale as the body of cloud associated with a front.
A mature cumulonimbus cloud can be tens of thousands of feet high and many miles in diameter, but, unlike frontal cloud they are relatively narrow. A cumulonimbus cloud will last for tens of minutes to hours, and so showers tend to be sudden and brief whereas frontal rain can last for days.
A difference in process and formation
Fronts produce most of their rain through mass ascent. Mass ascent is where one body of air is lifted, bodily, upwards by another body of air. The lifted body of air cools and the water vapour it contains condenses to form clouds and then rain.
Showers produce their precipitation via convection. A cumulus or cumulonimbus cloud will begin life as a 'bubble' of buoyant air that rises upwards in the atmosphere. The bubble of air will cool as it gains height and this will lead to the formation of the cumulus cloud with its distinctive cauliflower-like top. The cauliflower-like head of the cloud represents where buoyant air is continuing to ascend into the air of the surrounding atmosphere. Once the cumulus cloud reaches a certain depth or attains a cold enough cloud top temperature it will produce rain. Rain often falls in a downdraught within the cumulus cloud. The downdraught usually acts to suppress the updraught that originally formed the cumulus cloud which limits the life-span of a cumulus cloud.
In our forecasts
The difference between showers and rain becomes readily apparent when we take a top-down look at the weather map. In the examples below, the difference is clearly evident in a large organised band of rainfall moving across the UK, and isolated showers spouting up and disappearing across Scotland.
We also distinguish between the two in our forecasts with different symbols referencing showers or rain as in the table below:
Last updated: Jul 22, 2016 1:24 PM