AY Honors/Weather/Answer Key

This wave cloud pattern formed off of the Île Amsterdam in the far southern Indian Ocean, due to orthographic lift of an airmass with alternating bands of high and low humidity over the island.

Orographic lift occurs when an air mass is forced from a low elevation to a higher elevation as it moves over rising terrain. As the air mass gains altitude it expands and cools adiabatically. This cooler air cannot hold the moisture as well as warm air and this effectively raises the relative humidity to 100%, creating clouds and frequently precipitation.

Orographic lift is demonstrated in many places in the world, but few examples are as clear as those in the Northwestern United States. In the states of Washington and Oregon, places on the sea-facing side of coastal mountains see over 100 inches (over 2.5 m) of precipitation per year. These locales are on the side of the mountains which are in the path of storm systems, and therefore receive the moisture which is effectively squeezed from the clouds. However, on the other side of the mountains, sometimes as little as 15 miles (25 km) away, annual precipitation can be as low as 8 inches (200 mm) per year.

Also it happens on the Hawaiian island of Kauai.

Orthographic lift can create lenticular clouds and wave clouds on the leeward side of mountains.

See also

• Orography
• Rain shadow

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