English: On January 27, 2022, the Moderate Resolution Imaging Spectroradiometer (MODIS) on board NASA’s Aqua satellite acquired a true-color image of snow, sediment, and fog in British Columbia, Canada. The Northern Pacific Ocean sits in the southeast (lower left) in this image while the snow-topped Coast Mountains visible inland. In the south, a large bank of fog sits over the state of Washington, U.S.A., and large fingers of that fog extend northward into the mountain valleys of British Columbia. Another substantial fog bank sits over the Strait of Georgia and Salish Sea, between Vancouver Island and the mainland. Mud-colored sediment fills the Fraser River and pours into the Strait of Georgia.

Fog is simply a cloud lying close to the ground. Like all clouds, it forms when the air reaches its dew point, which is the temperature to which an air mass must be cooled for the water vapor in it to condense into liquid droplets.

Valley fog is common in British Columbia and the Pacific Northwest. On clear winter nights, the ground and overlying air cool off rapidly, especially at high elevations. Cold air is denser than warm air, and it sinks down into the valleys. The moist air in the valleys gets chilled to its dew point, and fog forms. If undisturbed by winds, such fog may persist for days.