Weather IQ: Explaining dew points and frost points

The dew point is the temperature the air needs to be cooled to (at constant pressure) in order to achieve a relative humidity (RH) of 100%. At this point the air cannot hold more water in the gas form. If the air were to be cooled even more, water vapor would have to come out of the atmosphere in the liquid form, usually as fog or precipitation.

The higher the dew point rises, the greater the amount of moisture in the air. This directly affects how "comfortable" it will feel outside. Many times, relative humidity can be misleading. For example, a temperature of 30 and a dew point of 30 will give you a relative humidity of 100%, but a temperature of 80 and a dew point of 60 produces a relative humidity of 50%. It would feel much more "humid" on the 80 degree day with 50% relative humidity than on the 30 degree day with a 100% relative humidity. This is because of the higher dew point.

So if you want a real judge of just how "dry" or "humid" it will feel outside, look at the dew point instead of the RH. The higher the dew point, the muggier it will feel.

General comfort levels USING DEW POINT that can be expected during the summer months:

less than or equal to 55: dry and comfortable
between 55 and 65: becoming "sticky" with muggy evenings
greater than or equal to 65: lots of moisture in the air, becoming oppressive

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.

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