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Snow falls when the temperature drops below the freezing point of water, but when it's really cold you may hear people say, "It's too cold to snow!" Can this be true? The answer is a qualified "yes" because snowfall becomes unlikely once the temperature of the air at ground level drops below -10 degrees Fahrenheit (-20 degrees Celsius). However, it's not technically the temperature that keeps snow from falling, but a complex relationship between temperature, humidity, and cloud formation. If you're a stickler for details, you'd say "no" because it's not just the temperature that determines whether it will snow. Here's how it works…
Why It Doesn't Snow When It's Really Cold
Snow forms from water, so you need water vapor in the air to form snow. The amount of water vapor in air depends on its temperature. Hot air can hold a lot of water, which is why it can get extremely humid during summer months. Cold air, on the other hand, holds much less water vapor.
However, in the mid-latitudes, it's still possible to see significant snowfall because advection can bring in water vapor from other areas and because the temperature at higher altitudes can be warmer than at the surface. Warmer air forms clouds in a process called expansion cooling. The warm air rises and expands because there is lower pressure at higher altitudes. As it expands, it grows cooler (check the ideal gas law if you need a refresher on why), making the air less able to hold water vapor. Water vapor condenses out of the cold air to form a cloud. Whether the cloud can produce snow depends partially on how cold the air was when it formed. Clouds that form at cold temperatures contain fewer ice crystals because the air had less water to give. Ice crystals are needed to serve as nucleation sites to build the larger crystals we call snowflakes. If there are too few ice crystals, they can't stick together to form snow. However, they can still produce ice needles or ice fog.
At truly low temperatures, like -40 degrees Fahrenheit and Celsius (the point at which the temperature scales are the same), there is so little moisture in the air it becomes extremely unlikely any snow will form. The air is so cold it's not likely it will rise. If it did, it wouldn't contain enough water to form clouds. You could say it's too cold to snow. Meteorologists would say the atmosphere is too stable for snow to occur.