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Why is it always so cloudy in B.C.'s Okanagan Valley during winter?

Thursday, January 28th 2021, 5:30 am - Here's the science behind the Okanagan's persistent valley fog.

Grey skies seem to be a daily occurrence as of late if you call the Okanagan Valley home. I would agree, I live here too.

As a newbie who moved here from Ontario back in the summer, I wanted to explore just how unusual or common it is to get this many dreary days in the valley. Here's what I learned.

VALLEY FOG: A REGULAR WEATHER FEATURE HERE

When I moved to Vernon, B.C., in August, there was not a cloud in the sky for weeks! Then the fall/winter season began and I didn't see the sun for long stretches.

A trapped valley fog is a common sight for many locals here in the Okanagan Valley, where there is a band of cloud that sits about halfway up from the lake and the top of the mountains. It occurs when cold air piles in throughout the late fall and winter season and it becomes trapped.

Cold air is more dense than warm air so it sinks to the valley floor, and stays there, for almost the whole season, never to mix out with the heat of the sun. The reason for this is that the sun angle keeps most of the daytime heating behind the mountains, never to truly mix out the layer of fog or cloud.

okanagan lake explainer

SUN ANGLE SEEMS TO ALWAYS SIT BEHIND THE MOUNTAINS

As we progress into our dark and long winter season, the sun seems to be lower in the sky than what I remember in Ontario. That's because it is!

Vernon, B.C., sits at 50.2670°N latitude, whereas my former home of Brantford, Ont., is at 43.1394°N latitude – Enough of a difference that we get about 50 minutes less daylight than our friends in Ontario in the winter. That, coupled with the fact that the sun is so low that it cannot heat the surface of the valley bottom, keeps a layer of cold air trapped over the lake. With the warm air above it, the cloud cover does not budge.

WHY THE VAST DIFFERENCE IN THE SUMMER SEASON?

Summer is not only glorious in the Okanagan while vacationing at a cottage, it's also glorious aloft. Things operate as they should this time of year. The sun is powerful high in the sky and heats the surface, including the valley floor. This allows cooler air that can be settled over the lakes to "mix out" and the sun wins this battle.

Additionally, when a ridge of high pressure dominates B.C. in the summer, the Okanagan can feature some of the best weather Canada has to offer. Sunny and dry days for weeks, with temperatures regularly into the 30s, with low humidity.

The valley plays a role once again in the heat part of this weather story. Air comes down the mountainside and warms and compresses as it progresses, giving an extra boost to summer heat.

With endless views of the waterfront and vineyards lining the valley, it's a much better place to call home in the summer season.

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