Welcome to the National Snow and Ice Data Center

When the temperature of the ground drops below 0° Celsius (32° Fahrenheit), it becomes frozen ground. Permafrost is a permanently frozen layer below Earth’s surface. It consists of soil, gravel, and sand, usually bound together by ice. 

Glaciers are huge masses of ice, snow, rock, sediment, and often liquid water that originate on land and move down slope under the influence of their own weight and gravity. The two main types are continental glaciers (or ice sheets) and alpine glaciers. 

Ice sheets are masses of glacial land ice extending more than 50,000 square kilometers (20,000 square miles). There are only two ice sheets worldwide today, the Antarctic and Greenland ice sheets. They contain more than 99 percent of the land ice on Earth.

An ice shelf is a large floating platform of ice that forms where a glacier or ice sheet flows down to a coastline and onto the ocean surface. Ice shelves play an important role in slowing the flow of ice from glaciers or ice sheets into the ocean. When ice shelves collapse, research has shown that glaciers accelerate, flowing into the ocean and adding to sea level rise. 

Sea ice is frozen seawater that floats on the surface of polar oceans and seas. It grows during each hemisphere’s winter; and it retreats in the summer, but does not completely disappear. The highly reflective white surface of sea ice reflects solar energy, cooling the planet. When it melts, the darker ocean absorbs more heat, reinforcing the cycle of melting sea ice.

Snow is composed of frozen water crystals. It is not frozen rain, but a crystallization of ice that typically forms in clouds. Highly reflective, white snow bounces solar energy back into space instead of warming the planet. Mountain snowpacks also provide a slow release of fresh water into rivers and waterways.

Soil moisture is the water content of soil and is key to Earth's terrestrial water, carbon, and water cycles. It also influences how and when the soil may freeze or thaw, a key component of understanding frozen ground and its extent.