Why of weather: What is the Rain Shadow Effect?
Tuesday, November 10, 2015, 11:49 GMT -
This weather pattern is highlighted on Monday afternoons radar image (above) where western areas experienced persistent rain and localised flooding, whilst eastern areas remained largely dry.
So what caused such a noticeable difference across a relatively small geographical area? The Rain Shadow Effect.
If an air mass is forced to rise, for example over a mountain range, it expands because of the lower pressure at higher altitudes and this expansion leads to cooling.
As the air rises and cools it eventually reaches its dew point. This is the temperature at which water vapour in the air condenses into water and enables clouds to form. So the rising, cooling air forms clouds which then bring rain or snow to the higher ground.
Once the air mass crosses over the higher ground it descends and reverses the process, causing the air temperature to rise. By then, the airmass has already emptied itself of any rain/snow and therefore is drier than when it began its ascent over the higher ground.
And that’s the reason why northeast England had a largely dry day on Monday as the mild westerly air mass was forced over the Pennines – effectively the mountain range blocked the weather system from dropping rain on the other side.
In its extreme, the weather shadow effect can create desert environments such as Death Valley, which formed due to the Sierra Nevada and Pacific Coast Range to the west blocking rainfall almost entirely. As a result the average annual rainfall in Death Valley is just 60mm.
Fortunately for northern England, the Rain Shadow Effect isn't as extreme across the Pennines but the effect can be significant on days like Monday.
It looks like we’ll see the Rain Shadow Effect again on Tuesday as the mild southwesterly flow continues to affect the UK.
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