Frozen ground occurs when ground water freezes due to the temperature of the ground being below 0°C. The ground freezes when the water freezes between rocks, soil, and pebbles. In this context, this frozen water is called pore ice. The ground freezes when the water in the ground becomes ice and the ground thaws when the pore ice melts. When the ground thaws, it's not melting; thawed soil is still solid.
In the Northern Hemisphere, more than half of the entire land surface freezes and thaws every year, which is called seasonally frozen ground. A quarter of the land in the Northern Hemisphere has an underground layer that stays frozen all year long. Ground that stays frozen for at least two years in a row is called permafrost. The freeze and thaw cycle of the ground has the power to split fence posts, crack rock and gravel causing roads to become bumpy. In certain areas, entire towns are built on frozen ground, which stays frozen the entire year; this means there are dependences on the ground staying frozen.
When ground water freezes becoming ice, it expands pushing soil in the ground causing it to swell. Often times, in frigid regions, a layer of pure ice, which is formed by attracting water, forms under the soil. This process causes more water to be attracted and thus more ice is formed. The segregated ice layer can be several meters thick. It forms when pore ice attracts water, which freezes and attracts even more water as the cycle continues. This effect is called cryosuction. Cryosuction makes the frozen layer grow, and the growing layer expands the soil even more. Cryosuction can make permanently frozen ground expand by 50%.
The ground temperature is often different from the temperature of the air above it. Layers deep within the ground may be colder or warmer than layers near the surface of the ground. The top layer of the ground reacts to conditions on the surface, but the lower layers respond more slowly to changes at the surface. In summer, the ground surface absorbs heat and becomes warmer than the ambient air. However, the temperature a meter below the surface is usually much cooler than the air. In contrast, during the winter season, the ground surface cools, but the layer underground usually stays warmer than the surface. The upper layer of ground traps heat, preventing it from moving between the cold air and the deeper layers of the ground ultimately displaying that the earth insulates itself. The type of soil also impacts how the ground stores heat. Loose soils such as sand have more pore space for water retention. Looser soils have larger particles, so ice forms more easily. Dense soils have smaller particles and do not have as much space for water. This means that clay does not freeze as easily as sand.
Changes in solar radiation also affects frozen ground. Since the ground receives no heat from the Sun at night, it releases stored heat during the day. The ground may freeze overnight and then thaw the next day with the Sun's heat. In some places, the ground only absorbs enough heat in the summer to thaw the top layer of the ground. This top layer of ground is called the active layer. The layer underneath stays frozen. Snow and ice are light colored and reflect more heat away. Ocean water and bare ground absorb more heat and reflect less. The amount of light that's reflected is called the albedo. This transfer of heat between the ground and the air is called the surface energy flux. Heat is also coming from the Earth's core which is very hot, and its heat moves towards the surface. This movement of heat to the surface is called the geothermal heat flux. The geothermal heat flux can stop the ground from freezing. However, even in very cold areas, the ground can only freeze so far before the geothermal heat flux stops it. A thick layer of snow acts like a blanket so that heat does not leave the ground. Only a thin layer of ground will freeze under a thick layer of snow. Light-colored soils freeze sooner and stay frozen longer than dark soils because these surfaces reflect sunlight, keeping the ground cooler. Peat is soil that forms when dead plants do not decompose all the way. Peat is found in marshy areas that form when the active layer thaws. The ground beneath the peat is usually colder than ground not covered by a peat layer. In the winter, peat freezes and allows heat to leave the ground. Because the heat escapes, more frozen ground and permafrost form.
Places with higher altitudes are colder because Earth's atmosphere is thinner there therefore not trapping as much heat as in lower elevations or thicker areas of the atmosphere. Earth's atmosphere is like a blanket as it holds heat near Earth's surface. For every 100 meters increase in altitude, the average air temperature drop is 0.6°C. This is called the lapse rate.
When frozen ground thaws, the environment changes dramatically. For example, thawing permafrost on mountain slopes can lead to landslides. When permafrost thaws on level ground, the ground buckles, trees growing nearby can be at risk as their roots become so weak that trees tilt and fall over. Trees growing nearby can be at risk. Sometimes, their roots become so weak that trees tilt and fall over. These trees are sometimes called a drunken forest.