Frozen Ground: What Makes It Freeze?

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 -13º Fahrenheit (-25° Celsius). The players noticed that the field became so hard that their cleats could not dig into the normally soft soil. They slipped and struggled to stay upright. Why? The ground had frozen solid.

At what temperature does the ground freeze and what makes it freeze? Frozen ground occurs when the water in the pores of soil or rock turns into ice (pore ice) as the temperature drops below 0° Celsius (32° Fahrenheit).

Understanding frozen ground is important because it can impact outside work and job sites. Frozen soils are waterproof and help to hold moisture, allowing them to act like a concrete layer.

When does the ground freeze? More than half of the land in the Northern Hemisphere freezes and thaws every year. This annual freezing and thawing is called seasonally frozen ground.

One-fourth of the land has an underground layer of soil or rock that stays frozen all year long. If the ground remains frozen for at least two years in a row, it is called permafrost. Permafrost is typically found in Arctic regions.

The main difference between seasonal frozen ground and permafrost is that permafrost stays frozen year-round and seasonal frozen ground thaws every summer.

Soil undergoes freeze-thaw cycles during the winter which can impact soil structure and porosity. When soil freezes, the water expands and can create cracks.

Dry soils freeze deep more quickly and easily than moist soils. When this happens, nearby plants and vegetation will have less water availability. Additionally, cold soil makes it difficult for plant roots to absorb nutrients and water.

Cold temperatures can also affect nutrients within the soil. Frequently changing winter temperatures can cause the nutrients to go between the soil and water sources. This can lead to nutrient runoff which can pollute nearby bodies of water.

Frozen ground affects various human activities and environmental aspects. 

When permafrost thaws, the ground can become unstable and soil can shift and collapse. This can cause issues such as buildings tilting, roads buckling, and pipelines rupturing. Engineering and careful planning are key to ensuring stability when looking to build on permafrost.

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 sometimes creates frost heave, where expanding ice pushes up the road and makes a hump, which later, after a thaw, will create potholes and sunken sections in a roadway.

The depth of frozen ground depends on the length of time that the air is cold for. The longer the cold period, the deeper the ground will freeze. However, the depth of frozen ground is limited because of Earth’s warm core.

When the temperature of the ground drops below 0° Celsius (32° Fahrenheit), it freezes. However, the ground temperature can be different from the air temperature. 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 while the temperature a few feet underground may be much lower. 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 the 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, think of 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 to form 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. Light-colored soils freeze sooner and stay frozen longer than dark soils because they reflect sunlight.

While frozen ground can be impacted by various factors such as temperature swings, seasonal changes and location, it can also be affected by snow, soil, plants and other aspects of the local landscape.

A thick layer of snow acts like a blanket, keeping heat from leaving the ground. Typically, only a thin layer of ground will become frozen under the thick layer of snow.

In the summer, plants keep the soil underneath them cooler because they block some of the sunlight from reaching the ground. Since evergreen trees do not lose their leaves in the winter, they block sunlight from warming the ground in all seasons. The branches of evergreen trees also block snow from reaching the ground which makes the bare ground lose heat easily. Permafrost often forms under evergreen trees.

Peat is a type of soil that forms when dead plants do not decompose all the way and it is often 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.

Hillsides and mountain slopes can affect ground and permafrost. If a slope gets more sunlight because of the way it faces, the ground will be warmer and less likely to freeze. In the Northern Hemisphere, slopes that face south towards the sun get more sunlight than slopes that face north.

Steep slopes are likely to contain frozen ground and the steepness of the slope affects how the amount of sunlight it gets. Steep slopes do not get much direct sunlight and do not hold snow cover very well, so the bare ground loses more heat. Wind direction also helps determine whether frozen ground forms. If a slope faces into the wind, the ground will lose more heat and the wind will blow snow away making the ground even more cold.

Lakes and rivers can be sources of heat in cold places since the water is warmer than the surrounding air and therefore can keep the ground beneath warmer in the winter. Lakes and rivers might not have frozen ground under them, or, they might have a ticker active layer compared to nearby land.

When it gets cold outside and the ground freezes, jobs can become more difficult and time-consuming. Prep for winter construction often takes longer than the job itself. Some industrial companies try heating the ground with massive coils or large heating systems, which become expensive and cumbersome. This approach requires costs including rental fees and transporting equipment. Powerblanket® has a solution for the difficulties and delays associated with winter construction as well as cold and freezing temperatures. Powerblanket®’s patented heating technology allows you to easily and efficiently prep the ground with little to no effort or supervision. 

Powerblanket® ground thawing and industrial heating blankets have a higher power density and hotter temperature than Powerblanket®’s concrete curing blankets. The higher power and temperature are ideal for ground thawing applications and curing epoxy and resins. Using a Powerblanket® ground heater will help you save time, money and stress.

“Your blankets are absolutely excellent. Thanks to the Powerblankets, we could quickly thaw the ground and complete our job. In fact, we estimate a savings of 10 hours per site equaling a savings of $5,000 already. Calculating this to our thousands of sites, the savings is huge! We are excited about the time and money Powerblanket has saved us and look forward to future savings.” — Kim Herman OSP/COEI Operations Manager Precision Utilities Group

At The Cary Company, we understand how important it is that you find the right Powerblanket® for your industrial and winter construction needs. Have any questions or are unsure which Powerblanket® is the best for your business? Contact our product specialists at 630.629.6600 or email sales@thecarycompany.com.