Have you ever heard anyone say that it is too cold to snow? Well, that is right and wrong. Actually the statement is “true” but it is a consequence and not a cause. I am a chemist. You have to humor me on occasion.
Snow, as we probably all know comes from water vapor (not steam that we see, but water in the gas form–invisible) in the air. Most often we refer to the “humidity,” when actually we mean the relative humidity. Now for the chemistry.
The amount of water vapor that the air can “hold” is dependent on the temperature. Warm air can hold more water. So when you dry your hair, you usually use a “dryer” that warms the air, making it thirstier, if you please. More water can evaporate in warm air than in cold. The “relative” part comes from comparing the actual amount of water vapor in the air compared to the theoretical amount that it can hold according to its temperature.
When it reaches 100% relative humidity it is ready to rain. Any change, like more water evaporating, or more likely, a drop in temperature will cause the water to condense and fall to the earth. This is precipitation and snow if the temperature is right.
When the air temperature is below freezing, the condensation is frozen. Rain that is cooled and condenses is hail or sleet. Water vapor that changes directly into a solid is snow. As more water molecules accumulate on the flakes, they eventually become heavy enough to fall to earth. Big globby flakes are formed when there is a lot of water in the air, i.e. warmer air. If it condenses very rapidly when cooled to a lower temperature, it will result in big flakes.
If the actual amount of water vapor and the relative limit are lower, or it is cooled more slowly, the water solidifies more slowly and forms smaller flakes. This is why some snow is big heavy flakes and other is light and fluffy.
Now to the temperature. As the temperature decreases, the amount of water vapor the air can hold also decreases. At temperatures below 0 degrees F, or so, there is very little water vapor, so even cooling it farther causes very little snow to form. Or, it is too cold to snow.
One of the common scenarios for snow is “warm” moist air is in place and a cold mass drives under it, raising it up and cooling it at the same time. (Cold air is heavier, denser than warm air.) As the air continues to cool, the amount of water left in the air is less and the snow diminishes. Eventually it will stop as the air becomes so dry that further temperature depression does not produce precipitation.
Occasionally a cold air mass is in place and a warm mass rides up over it. The cooling air then can produce snow. Most of the “really cold air” snow is caused by this rather than the previous method described. A “blizzard” consisting of extremely large amounts of snow accompanied by very cold temperatures and high winds usually results from this situation. If the “warm mass” has a good supply behind it, the snow can last for a longer time as the water is “pumped” up over the cold air mass to replenish the water that has condensed.
Too cold to snow? Not really, but once the water vapor is removed from the air, even much colder temperatures cannot “wring,” if you please, more water from the dry air. Hence the observation that it rarely snows in really cold weather.
You may now return to your normal web surfing. Keep warm.
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