Can snow melt when the temperature is below freezing?

Answered by Alana Jensen, INL ESER Program

This is not simple question to answer because they are several factors that help determine how fast snow and ice will melt. This conditions are constantly changing and form kind of a tug-of-war in the melting process.

The most obvious factor is temperature. If the air temperature is above o, snow and ice will start to melt. If the air temperature is above 32o, snow and ice will remain frozen.

The amount of sunshine also plays a big factor. On a sunny day, the energy from the sun warms the surfaces near, on, or under the snow and ice. If the surface temperatures warm above 32o, the snow and ice touching the surface will warm and begin to melt. This is why we can get melting snow on days when the air temperature is still below freezing.

Rainfall can be helpful for melting snow. Rain is liquid and, therefore, above freezing. As the above 32o raindrops falls on the below 32o snow and ice, heat will transfer and in most cases result in more melting.

The last factor is sublimation. The transition of water from the ice phase (or snow) to the gas phase (or water vapor) is called sublimation and it’s a common way for snow to melt in Idaho winters. You may have seen sublimation at work in your freezer, where the ice cubes tend to shrink with time.

We do not see the sublimation process because the snow goes directly into water vapor without first melting into liquid water. However, we do notice that the snow amount is decreasing, so snow sometimes seems to disappear on cold winter days.

The rate of sublimation is a function of the weather conditions. It takes a lot of energy to turn ice into a gas called water vapor—about 7 times the amount of energy needed to boil that water. The energy needed to sublimate the snow comes primarily from the sun, so sunny weather is the best weather for sublimating snow. Windy days are also good, as the wind helps to remove the water molecules once they leave the snow and enter into the atmosphere. A low humidity also helps to increase the rate of snow loss.

Sublimation
There are three generally known states of matter: solid, liquid, and gas. Each of these states also is known as a phase. Elements and compounds can move from one phase to another phase. Sublimation is a phase transition that occurs when a solid changes into a gas, or as chemists say, when a solid sublimes.

Safety Note: We’re going to use dry ice in the following experiment. Dry ice is the solid form of carbon dioxide and is extremely cold (about -110oF). Always handle dry ice with care and wear protective cloth or leather gloves whenever touching it. If touched for any amount of time, the dry ice will freeze the skin and cause an injury similar to a burn.

Materials: newspaper, ice cubes, dry ice, 2 glass measuring cups, warm water, food coloring, liquid soap, thick gloves

  1. Spread the newspaper on the counter. Put on a pair of gloves. Break off a piece of dry ice about the size of the ice cube. (You may need to use a hammer.)
  2. Compare the dry ice chip and the ice cube. Dry ice sublimates and ice melts. Can you see gas sublimating from the dry ice? Why do you think the gas is drifting down towards the counter instead of up? (Carbon dioxide is heavier than air.)
  3. Fill a glass measuring cup with 1/2 cup warm water.
  4. With thick gloves on, place a piece of dry ice in the measuring cup. What is happening to the size of the dry ice sliver? Did you see any bubbling in the water? Observe the experiment for several minutes. Why does the rate of bubbling change from fast to slow?
  5. Fill the other measuring cup with 1/2 cup warm water. Add a few pumps of liquid soap and a few drops of food coloring.
  6. Add the dry ice to the measuring cup. Observe the bubbles. If they spill out of the cup, why do they fall downward instead of floating away?
  7. Pop some of the bubbles from the top of the bubble mass. (Make sure you don’t put your fingers inside the measuring cup with the dry ice in it.) What do you observe?

Unlike ice made from frozen water, dry ice does not melt; it sublimates. Dry ice bypasses changing into a liquid state before converting to gas. That is why it is called ”dry“ ice.

Placing dry ice in a container of warm water will cause the formation of lots of bubbles. The bubbles are created when heat from the water enters the dry ice, causing it to sublime — change to its gaseous form. The water may appear to be boiling, but these bubbles are actually carbon dioxide gas being released into the air. Adding dry ice to soapy water will cause the carbon dioxide to become “trapped” in the soap that is dissolved in the water.

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