Rain Shadows Around the World

2012-11-04 18:53:52.000 – Mike Dorfman,  Summit Intern

Clouds formed by orthographic uplift.

One of the many important characteristics of Mount Washington that bring extreme weather to the summit is its prominence in the Northeast. Mount Washington, combined with the surrounding summits of the Presidential Range, offer a substantial barrier to air barreling toward it. As air is forced to rise upwards, it cools and water molecules that were energetic enough to be shooting around on their own suddenly don’t have enough energy to do this, and they succumb to the intermolecular forces between other water molecules. In other words, a cloud forms. This often puts clouds on the summit when there can be crystal clear skies in the valley. I recall a trip I made to Tuckerman’s Ravine when it snowed 2.5 feet in 3 days in April, but the valley didn’t get any precipitation!

Mt Washington isn’t the only place to have extreme orthographic uplift! In the southern hemisphere, New Zealand’s South Island does just that. At 40 degrees south, wind can whip around the world, running into very little land to slow it down. New Zealand’s south island has a spine of mountains running down the center of it and similar to Mt Washington, this pushes air up into the atmosphere. Accomplishing two significant things, this both increases wind speed and forces the maritime air to condense and form clouds and precipitation. Due to the prevailing west winds, the west coast of New Zealand is soaked with rain. As the air descends down the eastern side of the mountain range, very little moisture falls. Appropriately, the west coast contains lush rain forests while the area directly east of the mountains consists of dry grasslands. This dry area is called a ‘rain shadow’, and areas like these are found everywhere around the world where there is a significant barrier blocking moist air.

Another area with a significant rain shadow is the Pacific Northwest. Heavy rain and snow often fall as extremely moist maritime air rises over the many mountain ranges in Western Washington state. Eastern Washington state, in turn, is left relatively arid and dry. One simple metaphor (although this metaphor is not accurate when looking further into the thermodynamics of our atmosphere) is thinking of the air as a sponge. Forcing air to rise ‘wrings’ the sponge out. As the air descends, it is relatively drier and less likely to condense and form precipitation.

The presidential range is a bit more complicated than the two examples above. Mount Washington’s winds are often from the West, but storms can hit the mountain from almost any direction. Prevailing Westerly wind is often relatively dry, forming a minimal rain shadow on the east side of the mountains. The summit can get intense storms (the remnants of hurricane Sandy being an example) that bring extremely moist air from the east. This then would form a rain shadow on the western side of the Presidentials. Due to the varying direction of our storm systems, average precipitation total does not show a large rain shadow near the Presidential range.

Observer footnote: If you think you have what it takes to predict when wintry weather will hit the area, enter our snow contest. Purchase a ticket for $2 and register your prediction of the day and time that Jackson, New Hampshire will receive its first 6′ of snow in a single storm. If your prediction is correct, you’ll win half of the ticket sale proceeds! The other half will be split equally between the Observatory and Jackson Area Chamber of Commerce.


Mike Dorfman,  Summit Intern

Find Older Posts