Scientists Say: Snow line

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Snow line (noun, “SNOH line”)

The term “snow line” is used in both earth science and astronomy. It has similar, but slightly different, meanings in each field.

On Earth, a snow line is the boundary between snow-covered and bare ground.

Imagine you are hiking up a craggy, white-capped mountain. When you reach the edge of the snowpack, you’ve reached the snow line. This is an elevation-based snow line. Likewise, if you were to march toward one of Earth’s poles, you would eventually pass a point beyond which the ground remains snow-covered. This is a snow line based on latitude.

It is important to note that a snow line does not mark a place where snowmelt never occurs. It just marks a point at which the snow falls at least as fast as it melts and evaporates.

Some snow lines are permanent. Others are seasonal. They move as snow melts faster during warmer months and builds up faster during colder months.

Many factors determine the elevation or latitude where a snow line occurs. For example, it’s easier for snow to build up on mountains closer to Earth’s poles. So here, snow lines often occur at lower elevations — sometimes as low as sea level. But near the equator, it’s warmer and harder for snow to build up. There, a mountain’s snow line may occur over 6,000 meters (19,700 feet) above sea level.

Snow lines can even vary on two different sides of the same mountain. Intense sunlight on one side can raise a snow line. Harsh wind also makes it harder for a mountain to hold onto snow. So a wind-swept mountainside usually has a higher snow line.

In astronomy, a snow line refers to the distance from a star where it’s cold enough for a certain type of molecule to freeze. For example, water freezes the closest to a star. Carbon dioxide freezes farther out than that. Methane, carbon monoxide and other chemicals become solid farther and farther out.

The snow lines of different molecules play a key role in what types of planets form across a solar system. They also impact the chemical makeups of objects such as comets.