Weather Imagery

This could be a very technical discussion, but I will try and keep it simple. The answer to the question of can it be too cold to snow is no, it can’t be too cold to snow, but is gets increasingly difficult the colder it gets.

First, an important concept. Warm air can contain more water vapor than colder air. That is to say, there is more water vapor present for producing precipitation in warm air than in cold air. When any air mass reaches the point where it can’t contain any more vapor, it becomes saturated (also called the dew point). When the dew point is reached condensation occurs, clouds form, and the more droplets that form, the greater the chance of them colliding and forming bigger drops. When they get big enough, they fall to the ground as precipitation and if the air is cold enough, they fall as snow. But when the air is really cold, it can’t absorb as much water vapor and therefore has less precipitable content for making snow.

You can see from the chart below, that as the air gets colder there is less water vapor present. The less water vapor in the air, the less precipitation that can be produced. When it is 32°F (0°C) there is a lot more water vapor present than at 0°F (-18°C), so a lot of snow can be produced and the snowflakes are usually bigger. When the air gets really cold, there really isn’t enough water vapor in the air to get a whole lot of precipitable accumulations in a short amount of time and the snowflakes decrease in size. Therefore, as it gets colder it takes much longer for there snow to accumulate on the ground. And when it’s really cold, say below -40°F (-40°C) very little precipitation can be produced. Ice crystals can form, but they are extremely small.

There are a couple other reason why it’s hard for snow to form in really cold temperatures. One of them being, there is less evaporation taking place. For this reason, cold air is usually much drier than its dew point, unless you happen to live next to a large body of water that hasn’t frozen over. The drier the air, the colder it has to get to reach its dew point and ultimately before condensation will occur.

The other reason snow has a hard time forming in really cold temperatures is that the atmosphere is usually a lot more stable. It’s hard to get really cold air to rise and expand so that the little amount of water vapor will condense and form snow. There are exceptions (there always are!). One of them being a mountain which can force the air upwards causing expansive cooling at which point the air will achieve its dew point. Another being a warm air mass moving over top of a very cold airmass.

So putting this all together, snow can form at any temperature provided the right conditions exist, but it has an increasingly difficult time forming in really cold air because there is less water vapor present. Right around freezing is where you will see the most snow. This is because the air can contain the most moisture yet still be cold enough to freeze. Once it drops below -20°F, your chances of snow are virtually nil (but still possible). Ironically, when temperatures are less than -40°F, snow can form without ice nuclei. However, these ice crystals are extremely small and accumulations are virtually unnoticeable.

Where I’m at in Colorado, I’ve seen it “snow” at about -5°F. I put snow in quotation marks because the snow wasn’t composed of the typical looking snow flakes. Instead, itty-bitty ice crystals that looked like shards of glass were drifting with the wind. I could only see them when looking at the porch light.