Permafrost is a permanently frozen layer below the Earths surface. It consists of soil, gravel, and sand, usually bound together by ice. Permafrost usually remains at or below 0C (32F) for at least two years.

Permafrost can be found on land and below the ocean floor. It is found in areas where temperatures rarely rise above freezing. This means permafrost is often found in Arctic regions such as Greenland, the U.S. state of Alaska, Russia, China, and Eastern Europe.

Permafrost thickness can range from 1 meter (3 feet) to more than 1,000 meters (3,281 feet). Permafrost covers approximately 22.8 million square kilometers (8.8 million square miles) in the Earth's Northern Hemisphere. Frozen ground is not always the same as permafrost. A layer of soil that freezes for more than 15 days per year is called "seasonally frozen ground." A layer of soil that freezes between one and 15 days a year is called "intermittently frozen ground." Permafrost is frozen for two years or more.

Permafrost does not always form in one solid sheet. There are two major ways to describe its distribution: continuous and discontinuous.

Continuous permafrost is just what it sounds like: a continuous sheet of frozen material. Continuous permafrost extends under all surfaces except large bodies of water in the area. The part of Russia known as Siberia has continuous permafrost.

Discontinuous permafrost is broken up into separate areas. Some permafrost, in the shadow of a mountain or thick vegetation, stays all year. In other areas of discontinuous permafrost, the summer sun melts the permafrost for several weeks or months. The land near the southern shore of Hudson Bay, Canada, has discontinuous permafrost.

Scientists who study permafrost are able to understand changes in the Earth's climate by observing changes in permafrost. Studies show the Earth's permafrost warmed by 6C during the 20th century. Scientists predict widespread melting of permafrost by 2100.

Melting permafrost would raise water levels in the Earth's oceans and increase erosion. Erosion happens when permafrost melts because soil and sediment are easily washed away without the ice binding them together.

permafrost
Lakes and marshes in the Arctic are often shallow due to permafrost.

Permafrost Architecture
Though people do live in permafrost regions, such as Siberia, Canada, and Alaska, building on top of permafrost is difficult. Buildings raise the temperature of the ground beneath themwhich can melt permafrost and cause the building to sink in the mud. Engineers have found ways to build on top of permafrost without raising the ground temperature. These methods include building on top of wood piles or on top of thick gravel pads.

approximately
Adjective

generally or near an exact figure.

Noun

region at Earth's extreme north, encompassed by the Arctic Circle.

Noun

all weather conditions for a given location over a period of time.

continuous permafrost
Noun

layer of soil that is permanently frozen.

discontinuous permafrost
Noun

frozen layer of soil that thaws during the summer.

Noun

act in which earth is worn away, often by water, wind, or ice.

freezing
Adjective

at or below 0 degrees Celsius (32 degrees Fahrenheit).

gravel
Noun

small stones or pebbles.

Noun

water in its solid form.

intermittently frozen ground
Noun

earth that freezes between one and fifteen days a year.

mountain
Noun

landmass that forms as tectonic plates interact with each other.

Northern Hemisphere
Noun

half of the Earth between the North Pole and the Equator.

Noun

permanently frozen layer of the Earth's surface.

sand
Noun

small, loose grains of disintegrated rocks.

seasonally frozen ground
Noun

earth that freezes more than fifteen days a year, but less than two continuous years.

Noun

solid material transported and deposited by water, ice, and wind.

shadow
Noun

dark area where an object prevents light from reaching a surface.

Siberia
Noun

region of land stretching across Russia from the Ural Mountains to the Pacific Ocean.

soil
Noun

top layer of the Earth's surface where plants can grow.

Noun

degree of hotness or coldness measured by a thermometer with a numerical scale.

vegetation
Noun

all the plant life of a specific place.