Climate of the Salish Sea Bioregion

The Salish Sea Bioregion is located within the large-scale wind belt known as the Westerlies, within which winds are expected to blow most frequently from the southwest to the northeast on average over the year. At a local scale, topographic barriers and seasonal shifts in atmospheric pressure patterns lead to spatial and seasonal variability in the prevailing wind direction experienced at any given location.

This map shows the prevailing, or most frequent, direction of wind for specific climate stations in January (purple arrows) and July (brown arrows) based on 1991-2020 averages. You can zoom and pan to explore the map, and use the buttons below to view months individually.

In July, winds blow most often from the south or southwest across much of the region. In January, winds most frequently blow from the southwest in the Puget Sound region and from the west along the Strait of Juan De Fuca, bringing cool, humid air inland from the Pacific Ocean. In the area near the Fraser River flood plain, January winds are most frequently from the east or northeast, bringing cold, dry air from the interior of British Columbia. These winds converge and head northwest in the central and northern Salish Sea.

Winters are generally mild in the Salish Sea Bioregion in part because the mountain ranges to the east block cold air from the interior. However, easterly winds can bring cold interior air to the coast via some large, low-lying valleys. Wind direction in January is a major control of daily weather conditions, with heavy precipitation most frequently associated with westerly or southwesterly winds and extreme cold spells most frequently associated with easterly or northeasterly winds.

This map shows 1991-2020 average total annual precipitation as continuous contours along with monthly records for individual climate stations marked with a + sign. Zoom and pan to explore precipitation patterns across this map. Click on the map to see the total annual precipitation ranges represented by each colored contour. Click on the + signs to access monthly precipitation records for individual climate stations.

Total annual precipitation varies considerably across the Salish Sea Bioregion in response to differences in topographic setting and prevailing wind direction. As wind encounters a topographic barrier and is forced to rise, it cools as it moves up in the atmosphere. If the air cools enough, condensation occurs, creating clouds and precipitation. Precipitation therefore generally increases with elevation and is higher on the windward side of mountains. As wind moves down the leeward side of a topographic barriers, it is heated through compression. This often creates a rain shadow effect, where precipitation is dramatically reduced on the leeward side of mountains. Particularly striking examples of rain shadows are found on the northeastern corner of the Olympic Peninsula and in the San Juan - Gulf Islands Archipelago.

Seasonal patterns of precipitation are pronounced in this region due to large-scale fluctuations in atmospheric circulation and pressure patterns. On average across the region, 80% of total annual precipitation arrives between October and April. Summers are typically characterized by high atmospheric pressure, descending air, and thus very low levels of precipitation. During late autumn and into early spring, the midlatitude jet stream steers storms onshore, bringing humid, cool air and heavy precipitation.