What a Blizzard Needs
Three things are needed to make a large snowstorm or blizzard.
1. Cold air (below freezing) is needed to make snow.
For snow to fall to the ground, 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.
2. Moisture is needed to form clouds and precipitation.
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 moves air over the water, some water evaporates from the surface, putting vapor into the air. This is how “lake effect snowstorms” and “Nor’easters” pick up so much moisture. However, cold air is not able to hold much water vapor. In fact, very cold air does not make very much snow.
3. Moist air needs to rise, over very cold air, making clouds and snow.
There are two ways that moist air might rise over cold air. Winds pull cold air toward the equator from the poles and bring warmer, moist air toward the poles from the equator. When warm air and cold air are brought together, a front is formed and snow can form. Warm air can also rise to form clouds and blizzard snows as it flows up a mountainside.