Weather Whys; Snow

This is weather wise. In the United States, the heaviest snowfalls tend to occur when the air temperature is between 15 and 30 degrees Fahrenheit. But if the temperature is below that range, it doesn't mean it's too cold to snow. In fact, snow can occur at almost any temperature below freezing. But the colder the air, the less moisture it can hold. That means a given volume of 25 degree air can hold about twice as much water vapor as the same volume of 10 degree air. So a winter storm with warmer air usually has a greater amount of moisture which could become snow. And as a result, heavier snows are more likely to occur. But did you realize it can snow and temperatures are slightly above freezing too? That can happen when the air is dry, or in other words, when it contains a lot less water vapor than it's capable of holding. After all, a substance requires more energy to exist in a gaseous state than as liquid. So when water evaporates into vapor, the additional energy needed for the transformation comes from the surrounding air where the energy exists in the form of heat. The removal of heat is considered to be cooling and so we refer to evaporation as a cooling process.

And that's why a snow storm can begin even when temperatures are above freezing. Snowflakes fall from a cloud into warmer air and melt. As the melted snowflakes descend into the dry air, some droplets evaporate, which serves to cool the air and to make it less dry. The air temperature may then decrease to the point where other snowflakes fall and do not melt. Also, the air is now moist enough so the snowflakes which do fall won't tend to evaporate. The air relatively rare phenomenon since it requires extremely dry air to produce the effect. But if conditions are just right, a winter rain may actually convert itself into snow. Weatherwise is made possible by a grant from the National Science Foundation and is a service of the University of Oklahoma. For Weatherwise, I'm Drew Barla.