Mountain Weather: The Rain Shadow Effect

Continued from Mountain Weather: Forecasting for the Windward Side discussion...

Weather at the summit…

The air mass forced up the mountain beyond the 3,000 foot elevation on the windward side remains saturated and keeps a steady cooling rate of about 3.5°F per 1,000 foot climb, creating a deeper cloud and potential precipitation along the way. The rest of the 6,000-foot journey to the top of the mountain (point B, from Part I) then would yield an additional 21 degrees of cooling, so the air mass at the summit is anticipated to be cold (36°F) and saturated (Td is also 36°F with RH of 100%). Not quite cold enough for snow at the summit being warmer than the freezing mark of 32°F. All rain this time, perhaps a wintry mix. Had the mountain been about 10,000 feet tall (another ~3.5 degrees of cooling), the summit would have been near the expected snow level.

Heading down the mountain…

A crucial thing to note is that heating and cooling rates for an air parcel apply to both ascent and decent! With this in mind, the once cold and saturated air mass at the summit has to begin warming going back down in elevation. Soon it will become unsaturated as the temperature rises above the dewpoint and relative humidity drops below 100%. From your neighbor’s perspective (point C, from Part I) clouds at the summit will appear to have it a barrier. The name of this terminating cloud is a “Foehn wall”.

The cause of the sudden loss of clouds on the leeward side of the summit is that the conversion of condensation to precipitation on the windward side of the mountain actually deducts from available water vapor content in an air mass attempting to flow to the other side! Dewpoint measures water vapor content. Less water vapor content means lower dewpoint readings on the leeward side. The combination of lower water vapor content and a return to warming at the unsaturated rate of 5.5°F per 1,000 feet is why the mountain rain shadow effect exists!

Finishing the calculation, your neighbor’s weather at 1,000 feet elevation on the leeward side would be warmer (80°F) and drier (Td stays 36°F, unless a new source of water vapor is added; RH is now ~20%). That’s quite the weather contrast from what you are experiencing on the windward side!

Photo Credit: Jonny William Malloy