On December 31, 1967, the Green Bay Packers and the Dallas Cowboys competed for the NFL Championship in Green Bay, Wisconsin. They played in temperatures as low as -25° Celsius (-13º Fahrenheit). The players noticed that the field became so hard their cleats could not dig into the normally soft soil. They slipped and struggled to stay upright. Why? The ground had frozen solid. This historic game is known as the Ice Bowl. Frozen ground played havoc with the game.
Frozen ground occurs when the ground contains water, and the temperature of the ground goes below 0° Celsius (32° Fahrenheit). More than half of all the land in the Northern Hemisphere freezes and thaws every year, and is called seasonally frozen ground. One-fourth of the land in the Northern Hemisphere has an underground layer that stays frozen all year long. If the ground remains frozen for at least two years in a row it is called permafrost.
What makes the ground freeze?
When ground is frozen solid, the water between the rocks, soil, and pebbles, and even inside the rocks, has frozen and becomes pore ice. So officially, the ground freezes when the water in the ground becomes ice.
How does the density of water affect frozen ground?
When water turns into ice, it can expand with great force and cause the ground to swell. In areas with a cold winter season frozen ground can damage roads. For example, water turning to ice under roads sometime creates frost heave. The expanding ice pushes up the road and creates a hump, which later, after a thaw, will create potholes and sunken sections in a roadway.
The ground below is not all the same temperature
When the temperature of the ground drops below 0° Celsius (32° Fahrenheit), it freezes; however, the ground temperature can be 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 ground may respond to conditions on the surface, but the layers below may not change as quickly. On a warm summer day, the surface of the ground absorbs heat and becomes hotter than the air. But the temperature a few feet underground may be much lower than the air. It is the opposite in the winter; the surface of the ground cools, but the layer deep underground may stay warmer than the surface. The upper layer of ground stops heat from moving between the cold air and the deeper layers of the ground, insulating itself.
The ground is not the only thing that insulates itself from the air. For example, imagine a lake on a hot summer day. The first few feet of the lake will be warm. But closer to the bottom of the lake, the water will be much cooler. The Sun’s heat has less effect on the water deeper below the surface. This layering of temperatures is called a temperature gradient.
The type of soil in an area also affects how the ground will store heat. Loose soils like sand have more space for water and ice forms more easily. Dense soils with small particles do not have as much space for water. Clay, for example, does not freeze as easily as sand.
How deep does the ground freeze?
The depth of frozen ground depends on the length of time the air is cold. The longer the cold period, the deeper the ground will freeze. But the depth of frozen ground is limited, because Earth is warm deep inside.
Most of Earth’s heat comes from the Sun (Figure 1). The ground stores a lot of the Sun’s heat and reflects the rest into the air. Snow and ice are light colored and reflect more heat away. Ocean water and bare ground reflect less heat, instead absorbing it. This transfer of heat between the ground and the air is called the surface energy flux.
Figure 1. This diagram shows how the Earth’s atmosphere and the ground reflects and absorbs the Sun’s energy.
—Credit: NASA Atmospheric Science Data Center
Heat is also coming from the inside of the Earth. The Earth’s core is very hot, and its heat moves towards the surface–geothermal heat flux (Figure 2).
Figure 2. Deep inside, the Earth is hot. The mantle and liquid outer core are molten rock. The inner core is solid, but it too is hot. This heat moves through Earth’s layers to the surface.
—Credit: Lawrence Livermore National Laboratory
Heat from volcanoes, rivers, lakes, and other sources can also spread through the ground. This heat keeps some areas unfrozen, even though surface temperatures are low.
In general, deeper permafrost is very old. One researcher found that the deepest part of the permafrost underneath Prudhoe Bay, Alaska, is more than 500,000 years old.
How does the local landscape affect frozen ground?
Temperature swings, seasonal changes, and location are not the only things that affect frozen ground. Snow, soil, plants, and other aspects of the local landscape also affect frozen ground.
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.
Some soils freeze more easily than others. Light-colored soils freeze sooner and stay frozen longer than dark soils. Light-colored soils and rocks 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 under 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.
In the summer, plants keep the soil underneath them cooler because they block some sunlight from reaching the ground. Evergreen trees especially keep the ground cooler. Evergreen trees do not lose their leaves in the winter. This means that the trees block sunlight from warming the ground. Plus, their branches block snow from reaching the ground underneath. The bare ground loses heat more easily. Permafrost often forms under evergreen trees.
Hillsides and mountain slopes can affect frozen ground and permafrost.
If a slope gets more sunlight because of the way it faces, the ground will be warmer and will be less likely to freeze. In the Northern Hemisphere, slopes that face south, towards the Sun, get more sunlight than shady slopes that face north. The opposite is true in the Southern Hemisphere.
Steep slopes are likely to contain frozen ground. The steepness of the slope affects how much sunlight it gets. Steep slopes do not get as much direct sunlight, so they are colder. Steep slopes do not hold snow cover very well, so the bare ground loses more heat. Wind direction also affects whether frozen ground forms. If a slope faces into the wind, the ground will lose more heat. Plus, the wind will blow snow away making the ground even colder.
Lakes and rivers
Lakes and rivers are sources of heat in cold places. The water is warmer than the surrounding air and can keep the ground beneath it warmer in the winter. Lakes and rivers might not have frozen ground under them. Or, they might have a thicker active layer compared to nearby land.
The Powerblanket® Solution for Frozen Ground
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- Saves time, money, and labor
“All About Frozen Ground.” National Snow and Ice Data Center. Accessed 20 December 2016. https://nsidc.org/cryosphere/frozenground.