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Sunday, June 16, 2019  Spokane, Washington  Est. May 19, 1883
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News >  Idaho

Rainfall influenced by mountains

By Michelle Boss Correspondent

Weather and climate are affected by many factors, including latitude, proximity to large bodies of water, and even topography. Mountains, or the orography of a region, have a big impact as well, resulting in large variations of weather in cities that are relatively close to one another. Seattle is soggy, central Washington is arid, and Spokane is rather dry when compared to Coeur d’Alene.

To understand how mountains affect precipitation amounts, you have to understand the process by which clouds and precipitation form. In such a process, moist air must be lifted until it cools and condenses into the water droplets that form clouds. Imagine air moving from west to east toward a mountain range such as the Cascades or the Bitterroots (or any other north/south-oriented mountain range). The air is forced to rise up with the sloping terrain, and if it has sufficient moisture, the rising air forms a cloud and eventually precipitates. This happens on the windward (or west side) of a mountain range. On the east side (or lee side) of the mountains, this same air having precipitated much of its moisture, continues eastward and down the mountain slope. This sinking air warms, what moisture is left evaporates, and dry conditions remain. This is what is called the “rain shadow” effect. To the west of the Cascades, Seattle averages nearly 40 inches of rain per year. Contrast that with the 7.9 inches of rain annually in Moses Lake on the east side. The change in elevation need not be so drastic to have an impact on precipitation. Air continuing eastward across central Washington will once again start to climb in elevation toward Spokane, which averages about 17 inches of rain per year. Moving closer to the Bitterroots (Coeur d’Alene being on the windward side) the air continues to rise with the terrain, boosting Coeur d’Alene’s annual precipitation to around 26 inches.

Mountains also can influence the formation of warm-season thunderstorms, which are sustained by rising moist air. A great example of this occurs when moist southerly winds approach the Oregon/Washington Blue Mountains. Thunderstorms often form and appear on radar as if they are making a beeline toward the Spokane/Coeur d’Alene area. You anticipate rain only to see these storms “disappear” on radar as they move farther away from the mountains and the source of lift. I should point out, however, that as some of these storms dissipate, their outflow can cause damaging wind/dust storms in our area.

Just as air rising up a mountain slope cools and moisture condenses, the opposite happens as air travels downward. Across the Rockies “chinook winds” or “snow eaters” are warm and dry winds that come down off the east side of the mountains. The air is dry because most of the moisture precipitated out on the windward side. The air is warm due to compressional heating as it descends the mountain (lower elevation equals higher air pressure). Winds can become very strong (reaching speeds of between 40 and 60 miles per hour), and the temperature change can be drastic as well, warming 40 degrees or more in just minutes. These drastic temperature changes can be felt east of the Rockies from Calgary to the Dakotas, and to Denver on south. In California, these same warm, dry winds are called Santa Ana winds and can greatly worsen fire weather conditions.

January weather

We are now in the middle of the coldest part of the year with average highs just above freezing. January is also the snowiest month on average, with about 21 inches of snow and a total of 3.61 inches of precipitation.

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