When most people think of snowmelt, the first thing that might come to mind as the culprit is rain. Believe it or not, other factors contribute much more efficiently to the snow melting process.

The temperature of fresh snowpack is usually below freezing. When rain first falls on the snowpack, it freezes and creates a thin layer of ice on top of the snow. While that ice can compact the snow a bit, it doesn’t contribute to the melting process. A snowpack colder than the freezing mark has very little or no ability to absorb rainwater.

Snowpack that has been on the ground for quite some time, especially during a transition to a warmer air mass, will warm above the freezing mark. When a rainstorm arrives, the water will go right through that snowpack because it’s ripe, having warmed above the freezing mark. A ripe snowpack will easily melt at the expense of rain.

So, if rain is only effective at melting snowpack that is ripe or warmer than freezing, what other factors contribute the most to snow melt? The answer is simply fog and wind. In advance of a warm front, a warm, moist air mass pushes over a colder surface and usually develops fog. This condensation releases heat, which is much more efficient at melting snow than rain.

Warm fronts are usually associated with a wintry mix that changes to rain. So, the fog can help warm the snowpack above freezing and begin to thin the snowpack. Then, once the transition to rain occurs in the weather system, the water will flow through the snow and further accelerate the melting process. Fog can also occur on a clear night with light wind during a mild winter pattern.

Wind is also a big contributor to the melting process. Take an extreme example: a chinook wind. This is a downsloping, warm wind coming off the Rockies into the northern Plains. The air mass is already much warmer than freezing when it descends into the Dakotas. Now, add in the friction created between the wind and surface of the snowpack and within hours, several inches of snow can disappear. The snow doesn’t even have time to melt, but rather sublimes or vaporizes. This particular type of wind is often referred to as a “snow eater” because of how quickly it can eradicate an existing snowpack.

In a typical winter, a warmer wind can occur several days to a week after a fresh snowfall during a pattern transition. The combination of sunshine and a milder wind will accelerate the loss of the snowpack. So, the next time a fresh snowfall occurs in your neighborhood and the weather report several days later calls for warmer temperatures and a breezy wind (usually a southerly component), know that is the perfect recipe for snow around your property to rapidly melt or at least thin considerably.

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Story Image: A person jogs along the Rideau Canal in Ottawa, on Saturday, Dec. 31, 2022. According to Environment Canada a rainfall and fog advisory is in effect for Ottawa and the surrounding regions calling for 20 to 30 mm of rain and reduced visibility. (Spencer Colby /The Canadian Press via AP)

Fog: Number One Contributor to Melting Snow