METEOROLOGIST JEFF HABY
When temperatures drop below freezing and the temperature reaches the dew or frost point, the ice on the ground
is termed frost or frozen dew. "Frost" can form in two ways: Either by deposition or freezing. Depositional frost
is also known as white frost or hoar frost. It occurs when the dewpoint (now called the frost point) is below
freezing. When this frost forms the water vapor goes directly to the solid state. Depositional frost covers the
vegetation, cars, etc. with ice crystal patterns (treelike branching pattern). If the depositional frost is thick
enough, it resembles a light snowfall.
Frost that forms due to the freezing of liquid water is best referred to
as frozen dew. Initially, both the dewpoint and temperature are above freezing when dew forms. Longwave radiational
cooling gradually lowers the temperature to at or below freezing during the night. Cold air advection can also do
the trick (e.g. Cold front moving through in the middle of the night after dew has formed). Once the temperature
falls to freezing, the condensed dew droplets freeze. Frozen dew looks different from white frost. Frozen dew does
not have the crystal patterns of white frost. White frost tends to looks whiter while frozen dew tends to look
slicker and more difficult to see.
Frost and frozen dew can delay people in the morning if it covers their car. Some
frosts or frozen dews
are much easier to scrape off the car than others. When the temperature is near freezing (29 to 32 F), the ice is
fairly easy to scrap off the car windows. It is also quicker to warm up the car windows to above freezing with
the defroster when temperatures are near freezing. The bonding of ice crystals is weaker in warm ice than in cold
ice. Once temperatures drop into the mid-20's and below, the ice becomes more difficult to remove. It requires
more "elbow grease" to remove the ice. It also takes longer to warm up the car windows to above freezing. At these
temperatures ice is well bonded. Next time you witness ice in the morning, think about the processes that produced
the frost or frozen dew.
Q: Can frost occur at temperatures above 32°F?
A1: No, frost is defined as a layer of ice that forms on surfaces that are at or below 32°F. Sometimes frost can occur on your lawn overnight, even though your thermometer may never have dropped to the freezing mark. This is because cold air on clear, calm nights sinks to ground level. Temperatures at the ground can be lower than the temperature only a few feet higher where your thermometer may be located.
Since official weather measurements are taken in an instrument shelter four to five feet above the ground, frost can form even when the official temperature is above freezing.
A2: The ground, or any surface, must be at or below 32 for frost to form.
However, if your thermometer was just a few feet above the ground, it may not have given an accurate reading for frost. A thermometer shows the temperature where the thermometer is located.
Because cool air sinks and the ground can quickly cool, the ground temperature on clear, still nights is invariably lower than the temperature only a few feet higher. This is especially common in the fall and winter when nights are long, which allows extra time for cooling. Thus, frost can form even when a thermometer gives a reading in the upper 30s.
Since official weather measurements are taken in an instrument shelter four to five feet above the ground, frost can form even when the official temperature is above freezing. (Related: measuring weather).
Additionally, frost will only form if the ground temperature matches the dew point. (Related: understanding humidity).
A3: Yes and no: It depends on how you define "ambient temperature", and, of course whether the temperature is below the frost point.
You see, when temperatures are officially recorded for hourly weather observations and climate reports, they are measured at a height of between 1.25 and 2 metres (4.1 and 6.6 ft) above the ground in special shelters, called Stevenson screens. (The shelter is named after the father of writer and poet Robert Louis Stevenson.) Meteorologists call this temperature the "surface temperature," and it is what is reported on the radio and TV (and internet and newspapers, reports, etc.).
The distinction is important for the following reason.
During clear and calm nights, the temperature at the ground or some surface near the ground can become much cooler than the "surface temperature". The radiation of heat away from the ground is the cause of this drop. The coldest air, therefore, forms near the ground, and being heavier than the air above it remains there.
If we were to make measurements of temperature from the surface to the height of the official "surface temperature" measurement every few centimetres or inches, we would find the air temperature increases as we move upward from the ground. Meteorologists call this a surface temperature inversion.
Since cold air is heavy air, in the absence of wind, the coldest air will remain nearest the ground, thus allowing surface temperatures to continue to fall. Thus, under such conditions -- clear and calm nights -- the ground temperature may fall below the freezing point while the temperature measured officially at was still above freezing. This is particularly common in the autumn and winter when nights are long allowing more time for cooling to occur.
Now frost is a covering of ice crystals on the surface produced by the depositing of water vapor to a surface cooler than 0° C (32° F). The deposition occurs when the temperature of the surface falls below the frost point. Similarly, dew forms when the air or surface temperature falls below the dew point temperature. (Note that the water vapor goes directly from gas to ice. Therefore, frost is not frozen dew.)
Thus, if the temperature on the ground or an object such as a bush or a car windshield near the ground falls below the frost point, frost crystals may form. But the measured "surface temperature" may still be above freezing.
This is the most common way in which frost may form when the official surface temperature is still above the freezing point.
A4: You also see frost on the rooftops of houses on nights when the temperature never goes below freezing.
Every warm object loses energy by radiating electromagnetic energy (e.g., infrared photons). If it receives an equal amount of energy from other objects, it is in radiative equilibrium; if it receives less from other objects, it loses energy and cools down.
Consider the view from the roof of your car or a house rooftop. If you were lying on this surface, you'd see the sky. The dark sky has an effective temperature of three degrees above absolute zero -- very cold! Your car is much, much warmer; so the roof of your car loses more energy than it gets, and it cools off.
On a cloudy night, the clouds are much warmer than the universe beyond it. If the temperature stays above freezing, the effect you describe generally occurs only on clear nights. Also, it doesn't happen to objects under trees, but only to objects under the open sky.
There are two other methods of heat transfer -- conduction and convection. It is radiative transfer, however, that is causes the effect you described.
-- added later:
I have to comment on the mention of wind in other answers. Wind actually reduces the formation of frost from radiative cooling. Consider the roof of the car as it loses energy to the clear sky. As the temperature of the roof goes down, the air adjacent to the roof also cools off. (The molecules in the air are bouncing off the roof, and always tending toward thermal equilibrium.) If the wind starts blowing, it will bring air from other locations and will displace the cool air just above the car.
One other thing I haven't mentioned is that the humidity has to be high; you need a high dew point so that as the car roof cools, moisture will condense on it (and eventually freeze as the roof continues to cool).
Something I implied above but didn't say explicitly is that when frost forms on the roof, the roof is below freezing. In other words, the sequence might be as follows:
1) Initially, the air and the car are at temperature 37 F, and the dew point is 35.
2) The car roof cools radiatively. As it cools below 35 degrees, water condenses on it.
3) The car roof continues to cool. As it cools below 32, the water on the roof freezes. This is what you see when you look at your car early in the morning.
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