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Ask the Meteorologist: Mountain Impacts

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How are the mountain ranges a factor in the type of weather you may have in a region?

Adam Bickel

Central Point

 

The mountains have a tremendous impact on the type of weather a distinct area sees. A perfect example is the Pacific Northwest where mountain ranges can bring inches of rainfall to the Coastline and barely a drop further inland. This is known as a rain shadow, and is commonly seen in our forecast area. It’s the main reason why the Rogue Valley sees so little rainfall year round.

As weather disturbances come off the ocean, they encounter the Coastal Range Mountains. The higher terrain forces this air to rise, cool and condense. Clouds form as a result, and precipitation falls as the system moves over the mountain. On the leeward side, the easterly facing slopes, these clouds then evaporate as the air sinks and warms through compression. This is why lots of vegetation and rainfall can be seen along the Oregon Coast, and very dry conditions are common in the Valley.

Another common weather phenomena associated with mountains is downsloping winds. This is the reason behind the Chetco Effect at Brookings. As winds move down the slopes of mountainous regions, they adiabatically warm through compression. That air warms at a rate of 5.5 degrees Fahrenheit every 1000 foot in elevation drop. These downsloping winds can bring breezy conditions to areas, but also warm towns up in a matter of minutes.

When easterly or northeasterly winds develop (these are offshore winds), the air warms as it descends the windward side of the Coastal Range Mountains. Brookings can hit the 80’s and 90’s within an hour from downsloping winds. They also contribute to the evaporation of clouds and lack of precipitation in the valleys.