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The wondrous process of frost formation

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By Paul G. Wiegman
Sunday, Nov. 23, 2008
 

The formation of frost is a beautiful phenomena resulting in crystalline-like paintings on windows.

To understand how frost forms, imagine a sponge in a bowl of water. Held under for a long enough time, the sponge traps water in its web-like structure until it is saturated.

When you take the sponge out of the water and squeeze, the volume of the sponge decreases, and water is released.

Now, visualize a block of air the size of your sponge. Air is made up of atoms of gases, but there is space between the atoms, just like the voids in a sponge.

Molecules of water vapor can fit in the spaces between the atoms of gases that make up air. When those spaces are filled, like the sponge, the air is saturated.

if you cool air, the volume decreases, forcing the water out.

Meteorologists have specific terms related to this phenomena. Saturated air is air that holds all the water vapor it can. We don't hear the term "saturated air" often, but we do hear about humidity.

Humidity is the amount of water vapor in the air. Generally, when you hear humidity during a weather forecast, the more precise meteorological term is relative humidity. It is relative because it refers to the ratio of the amount of water in the air to the amount the air can hold.

Think again about a sponge.

If the sponge can hold a cup of water, but when you wipe up a spill you only fill it with a half a cup of water, the sponge is only 50 percent full. Similarly, if air is holding only one-half the amount of water vapor it can potentially hold, the relative humidity is 50 percent.

The upper limit is reached when the air has all the water it can possibly hold; then the relative humidity is 100 percent.

We usually associate 100 percent humidity with muggy summer days. However, even the coldest winter day can have 100 percent humidity. The difference is that the amount of water vapor the air can hold on a summer day is far more than the amount of water vapor the air can hold on a winter day.

What does relative humidity have to do with frost?

Temperature of the air is critical. It is the way to squeeze the air.

If the relative humidity on a fall day is 90 percent, as night falls, the air cools. Cooling the air makes the volume smaller, and the amount of water a set volume of air can hold decreases. After the air reaches 100 percent relative humidity, it is saturated. At that point, any water vapor the air cannot contain must go elsewhere.

It goes onto anything cold enough to hold crystallized water -- ice. Cooling squeezes out the excess air as water, and frost forms.

It's a magical process. Seemingly from nothing, a beautiful crystalline pattern appears on leaves, twigs, roofs, vehicles and anything else outside.

What happens in the summer?

When the temperature of the air is above freezing, the result of cooling saturated air overnight is dew.

That brings up another meteorological term -- dewpoint. This is the temperature to which air must be cooled for it to become saturated.

The higher the relative humidity, the closer the dewpoint temperature is to the actual temperature. At 100 percent relative humidity, the dewpoint temperature is equal to the actual temperature, and any cooling of the air will create dew or frost.

Take a pitcher of ice water onto the porch on a hot summer day with the relative humidity at or near 100 percent. The surface of the pitcher is cold, below the dewpoint of the warm outside air. When the hot air contacts the surface of the glass pitcher, it is cooled. When the air is cooled below the dewpoint, the excess water has to go somewhere. That somewhere is the surface of the pitcher.

Another example of water being wrung out of air by lowering the temperature below the dewpoint occurs during winter driving.

On a cold day, the windshield of your car is cold. Your breath after going into and out of your lungs picks up a lot of water vapor and has a high relative humidity when you exhale. When your breath hits the cold windshield, it cools below its dewpoint, and the water is squeezed out. The water forms on the inside of the windshield, and if the glass is cold enough, frost forms.

Frost that forms on windows in our homes is created in the same way that frost forms on a windshield. Warm, wet indoor air hits windows cold from the outside, and window or fern frost forms.

 

 
 


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