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How Do Blizzards Form?

Three things are needed to make a 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. Warm, rising air is needed to form clouds and cause precipitation.
For a blizzard to form, warm air must rise over cold air. There are two ways that this may happen. Winds pull cold air toward the equator from the poles and bring warm air toward the poles from the equator. When warm air and cold air are brought together, a front is formed and precipitation occurs. Warm air can also rise to form clouds and blizzard snows as it flows up a mountainside.

What Are Snowflakes?

Snowflakes are made of ice crystals. Each snowflake is made of as many as 200 ice crystals.

Some snow crystals are symmetrical, like the type that you cut from paper. They form a hexagonal shape because that is how water molecules organize themselves as they freeze. Others are small and irregularly shaped. If they spin like tops as they fall to the ground, they may be perfectly symmetrical when they hit the Earth. But if they fall sideways, they will end up lopsided.

Even though most have a hexagonal structure, there are so many ways that water molecules can arrange themselves as the water freezes, that some people say that there are no two snowflakes alike. Probably no two snowflakes have exactly the same arrangement of molecules. But they can look alike. Many snow crystals are like the two in the photograph below. They are simple in shape and look alike.

How Snowflakes Form

Snowflakes form in clouds where the temperature is below freezing (0ºC, or 32ºF). The ice crystals form around tiny bits of dirt that have been carried up into the atmosphere by the wind. As the snow crystals grow, they become heavier and fall toward Earth. Different types of snowflakes form in different conditions. Temperature determines if the crystals become a flat plate, a long column, or a prism shape.

On average, 10 inches of snow melt down to about an inch of water; however, not all snow is the same. Some places receive very heavy snow. For instance, only five and a half inches of January snow on Mount Washington, New Hampshire, melt down to an inch of water. In contrast, over 15 inches of January snow at Crested Butte, Colorado, melt down to an inch of water.

Other Icy Precipitation

Snow is not the only type of ice that falls from the sky! Other types are described below.

Graupel: Snowflakes that have become encrusted with ice. This happens when snowflakes pass through a chilly cloud on their way down and water droplets freeze on them.

Ice pellets: Frozen raindrops, also known as sleet. They are usually quite small in size and unlike snowflakes, do not have a crystal shape.

Hail: A frozen raindrop or graupel that is kept from falling to the ground by the upward flowing air of a thunderstorm. The more droplets that freeze onto the hailstone, the longer the hailstone spends in the sky. When it finally grows too heavy to be held up by the flowing air, it falls to the ground like those in the picture to the right.