Weather Alerts for New York

This map shows recent moisture content over North America. Bright and colored areas show high moisture (ie, clouds); brown indicates very little moisture present; black indicates no moisture.

Weather Topic: What are Shelf Clouds?

Home - Education - Cloud Types - Shelf Clouds

Shelf Clouds Next Topic: Sleet

A shelf cloud is similar to a wall cloud, but forms at the front of a storm cloud, instead of at the rear, where wall clouds form.

A shelf cloud is caused by a series of events set into motion by the advancing storm; first, cool air settles along the ground where precipitation has just fallen. As the cool air is brought in, the warmer air is displaced, and rises above it, because it is less dense. When the warmer air reaches the bottom of the storm cloud, it begins to cool again, and the resulting condensation is a visible shelf cloud.

Next Topic: Sleet

Weather Topic: What is Snow?

Home - Education - Precipitation - Snow

Snow Next Topic: Stratocumulus Clouds

Snow is precipitation taking the form of ice crystals. Each ice crystal, or snowflake, has unique characteristics, but all of them grow in a hexagonal structure. Snowfall can last for sustained periods of time and result in significant buildup of snow on the ground.

On the earth's surface, snow starts out light and powdery, but as it begins to melt it tends to become more granular, producing small bits of ice which have the consistency of sand. After several cycles of melting and freezing, snow can become very dense and ice-like, commonly known as snow pack.

Next Topic: Stratocumulus Clouds