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The sun was warm but the wind was chill. You know how it is with an April Day When the sun is out and the wind is still, You're one month on in the middle of May. But if you so much as dare to speak, A cloud comes over the sunlit arch, A wind comes off the frozen peak, And you're two months back in the middle of March. Two Tramps in Mud Time (1936)
Warmth, warmth, more warmth! for we are dying of cold and not darkness. It is not the night that kills, but the frost. The Tragic Sense of Life (1913)
Frost is ice that sublimates directly on the surfaces on which it is found. Sublimation occurs when water vapor, goes directly from the vaporous state to the solid state. To fully understand frost deposition you must think in terms of energy. Water vapor is in a higher energy state than liquid water and liquid is in turn in a higher energy state than solid water (ice). For water to be maintained in a vaporous state, there must be a certain amount of energy available. As energy is removed from the air, i.e. the air is cooled, all the gasses loose heat. Nitrogen, Oxygen, Carbon Dioxide, Water Vapor and all other components loose energy. Eventually when enough energy is removed the vapor must change state and it becomes either liquid or solid. Each liquid water molecule has less energy than each vapor molecule and each solid water molecule has less energy than either a molecule of liquid or gas.
The temperature at which the change of state occurs as the air is cooled is called the dew point temperature when the change of state occurs at a temperature above freezing or the frost point temperature when the change of state occurs at a temperature below freezing.
What determines the temperature of condensation or sublimation is the amount of moisture in the air, the greater the amount of water vapor, the higher the condensation temperature. During summer it is not uncommon to have a dew point of 70°F in the eastern United States. But in the cooler seasons the dew point can be below freezing, in which case it is called the frost point temperature.
Whether dew or frost will form is determined by two factors:
For more on dew point click here
There are other types of ice that can be found on surfaces.
If after dew forms the temperature falls below freezing a coating of glaze ice may result, sometimes called black ice or a black frost it is just frozen dew. If the amount of dew is scanty individual frozen drops will result.
What is important is not what the temperature was at the thermometer but what the temperature was where the frost formed, and if frost formed the local temperature was below freezing.
A thermometer indicates the temperature where the thermometer is, usually a few feet above the ground. Because cold air sinks relative to warmer air and because the ground can cool very quickly the temperature at ground level can be cooler than a few feet higher where the thermometer is, eventhough the thermometer indicates a temperature that is above freezing, it can easily be below freezing a few feet lower.
For more on LOW TEMPERATURES click here
Certain materials like glass and car metal radiate heat quickly and therefore cool quickly. Also, areas like rooftops or high plateaus, because of their exposure lose heat through re-radiation very quickly and tend to receive frost before sheltered areas.
On marginal nights frost may only be seen on rooftops and cars and spotty grassy areas, because of location or materials these could cool below freezing while nearby areas remained just above freezing and received no frost.
In the same way temperature varies in very areas in close proximity to one another, the amount of water vapor varies too. An exposed parking lot may have less vapor in the air than a nearby garden because the a plant canopy traps water vapor and plants transpire (emit) water vapor.
In the garden there may be enough vapor so the condensation temperature is a dew point temperature, while in the parking lot the condensation temperature is a frost point temperature. If the air over each cools to the condensation temperature dew would be found in the garden and frost on cars in the parking lot.
If the temperature continued to fall after dew and frost formation, the parking lot may end up with a heavy frost and the garden with black ice, that is frozen dew.
© 2008 Steven L. Horstmeyer, all rights reserved