When Sunny Means Foggy

This time of year on Powell Lake in Coastal BC a forecast for sunny days isn't always good news. During winter months it's common for Powell Lake to become enshrouded with a mantle of low lying fog. You might call it ground fog, but it starts on the surface of the lake. Ground fog and valley fog are forms of radiation fog. Conditions leading to radiation fog include:

• clear skies
• light winds
• shallow layer of cool moist air near the ground
• long nights

Powell Lake fills a steep glacial carved valley and is susceptible to valley fog. The NASA website states:

Valley fog forms on clear winter days when heavy cold air settles into the mountain valleys while warmer air moves over the surrounding mountains. Fog forms when the frigid ground cools the air immediately above it. The cooling air thickens into fog as water condenses. Air farther from the Earth’s surface is warmed by the sun, trapping the layer of cold air beneath it. The condition is called a temperature inversion, and it can last several days.

When the fog begins to clear it typically moves from north to south down the lake. From the deck of our float cabin we can often see it flowing back through First Narrows.

Thanks for visiting! -- Margy