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"Dry and sunny" has been replaced with "rain and showers"

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Liam Dutton Liam Dutton | 08:00 UK time, Wednesday, 27 July 2011

(Liam returned to the BBC Weather Centre in September 2007 and now broadcasts across a wide range of BBC channels. He can be seen regularly on BBC News, BBC World News and BBC Breakfast on BBC One at the weekends, in addition to being an established voice on the Today programme on Radio 4.)

Distance travelled ~ 533'427'200 km

After the driest spring in some parts of the UK for more than 100 years, the switch to more unsettled weather of late has necessitated a change in vocabulary for us weather folk.

bbc weather forecast 27/07/11

Dry and sunny has been replaced with rain and showers, especially during last week with low pressure driving our weather. And there's more rain and showers set for today and tomorrow.

But what exactly is the difference between rain and showers? It sounds like a simple question with an obvious answer, but ponder for a little longer and your thoughts may start to waiver.

Whilst the inevitability of getting caught outdoors in rain or showers without an umbrella is the same - a good soaking, the associated clouds and how they form in each case are quite different!

Rain vs. showers - clouds types, their formation and scale
Rain clouds form when a huge wedge of warm, moist air is forced to gently rise above cooler, denser air over hundreds of miles, in a process known as mass ascent.

As air rises, it condenses to form layers of cloud. Initially the cloud is high, thin, cirrus, but eventually thickens and lowers to nimbostratus producing outbreaks of increasingly persistent and heavy rain.

Shower clouds form when parcels of air just a few miles across rise into the atmosphere having been warmed by the Sun heated ground below, in a process known as convection.

The air rises and condenses to form heaped, cauliflower-like clouds. The smaller, fluffy white cumulus clouds produce few, if any showers, but the bigger, taller towering cumulus or cumulonimbus clouds can give intense downpours.

Rain vs. showers - speed of development
The easiest way to visualise the difference in how rain, showers and their associated
Empire State Building
clouds form, along with the speed at which air rises, is to think of making a journey to the top of the Empire State Building.

Imagine walking to the top of the building on an extremely long, gently sloping ramp. It would take a long time (hours) to reach the top and a great distance would have been covered. This is what happens to air when mass ascent occurs - a slow, gradual development of cloud and rain.

Now imagine instead taking a lift to the top of the building. It would take very little time (minutes) to reach the top and very little distance would have been covered. This is what happens to parcels of air with convection - a relatively quick, sudden development of cloud and showers.

Rain vs. showers - duration and intensity
With the formation of rain clouds being gradual and slow, a spell of rain can last for as long as six hours. Rain tends to start off light, but turns heavier and more persistent as cloud thickens and lowers. Eventually, the rain eases and it becomes dry again, but remains quite cloudy.

Conversely, shower clouds tend to form quickly. As parcels of air rise, energy is transferred upwards into the atmosphere. The higher and quicker the parcel of air rises into the sky (i.e. the greater the rate of energy transfer), the more intense the shower is likely to be.

Showers are often moved along by the wind, hence their short duration, in most cases, of not much more than tens of minutes. However, there are cases when showers form where there is little or no wind. When this happens, the same shower can sit over the same location for a long time and give the impression of rain, when technically it's just a shower that's got stuck!

So there you have it - rain and showers in a nutshell. Now, where did I put my umbrella...?



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