Weather Topic #6

 Mountainous terrain and its impact on Precipitation totals

Impact #1


Yearly, the Western slopes of the Cascade mountain range receives up to 50 inches more rainfall than the Puget sound region. Why is this? This is due to its terrain features, and proximity to large bodies of water. As warm, moist air at the surface moves over the Puget sound region, it is forced to rise upwards as it smacks into the western slopes of the Cascades. This causes the relatively moist air to condense and change from a vapor to a liquid/solid during its path as an upslope wind climbing in order to get over the cascades. It condenses because as it is forced upwards, it enters cooler air, and freezes. This extra liquid falls in very heavy downpours, and has mostly all fallen by the time it reaches the Cascade crest. This phenomenon is known as Orographic lifting. 


Impact #2


The second impact of mountainous terrain is an effect of orographic lifting, and it is the rain shadow effect. The rain shadow effect is the reason why the Columbia basin scarcely receives rainfall. The rain shadow effect is caused by all the moisture in the air being released on the windward side of the cascades, so by the time the air flows down the leeward side, it is too dry for any precipitation to fall. 


Impact #3

A final way that mountains impact precipitation totals is through convection. Mountains breed convection, which is essentially thunderstorm fuel. You can read my entries about CAPE for more information regarding this. Mountains breed convection because their large terrain features force air parcels to displace rapidly. This elevated convection can lead to heavier showers/downpours, and higher rainfall totals. 

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