Winter storms can leave behind a thick layer of snow.
Credit: UCAR

Are you, or someone you know, wishing for a snow day? Are you hoping to wake up tomorrow morning and run outside to play in the snow rather than sit in a classroom or office? Well, while you are wishing for a storm, perhaps you should consider the circumstances that would be needed for you to get your wish.

First, it is essential that the air be very cold. It has to be below freezing for snow to fall. What’s more, the temperature must be cold both up in the clouds where snowflakes form, and down at ground level. If the air near ground level is too warm, the snow will melt on its way down, changing to rain or freezing rain.

Second, there has to be enough moisture in the air for all those snowflakes to form. Moisture in the air is called water vapor. Air blowing across a body of water, such as a large lake or the ocean, is an excellent source of water vapor. As wind blows over the water, some water leaves the surface and hops into the air through the process of evaporation. “Lake effect snowstorms” and “Nor’easters” pick up so much moisture by traveling over bodies of water. However, very cold air is not able to hold much water vapor and can’t make much snow. So it needs to be cold, but not so cold that moisture can’t be held in the air.

For a snowstorm to form, warm air must rise over cold air. It might seem illogical that warm air would have anything to do with making a snowstorm, but it does. When warm air and cold air are brought together, a front is formed and precipitation occurs. Winds pull cold air toward the equator from the poles and bring warm air toward the poles from the equator. Warm air can also rise to form clouds and blizzard snows as it flows up a mountainside.

Snowstorms are one type of winter storm. Icy winter storms bring freezing rain or sleet as well as snow. Blizzards are snowstorms with high winds.