Dew Point and Relative Humidity
The dew point is a measure of air temperature when the atmosphere becomes completely saturated. Relative humidity is important to the dew point, as it is considered the amount of water vapor in the air, relative to its point of saturation. It indicates the amount of moisture in the air. Relative humidity can be calculated by knowing dew point values. When the air temperature meets the dew point, humidity is 100 percent. More water vapor exists in warm air than in cold air.
Dew Point Depression
The difference between the dew point and the temperature of the air is called the dew point depression. When there is a large difference between the air temperature and the dew point temperature, this means the air is drier, with a lower change of saturation, or condensation. A small distance between the air temperature and the dew point temperature means the moisture content in the air is high, with a greater chance of saturation or condensation.
Dew
Dew forms when the air temperature reaches the dew point. Water vapor condenses on surfaces when the moisture in the air becomes saturated.
Frozen Dew
Frozen dew, appearing as layers of ice, occurs when dew condenses and then the air temperature goes below freezing. Frozen dew is also known as black ice.
Frost Formation
Frost is formed by the process of deposition or sublimation, when water vapor changes directly to solid or ice. Condensation happens when water vapor changes to its liquid state. Frost forms when the dew point is at or below freezing, and the air temperature comes to or below the dew point temperature. In this case, the dew point is called the frost point. Frost usually forms on higher surfaces first, as the air just above the ground maybe a few degrees warmer. Ideal conditions for frost are:
- Clear skies
- Light or no wind
- Temperature near or below freezing
For example, if the dew point is 20 degrees F (-6 C), then the air temperature would have to drop to this point or near it in order for frost to form. Frost formation depends greatly on the relative humidity of the air. In some areas like those which are covered, or those with more vegetation the formation of frost may vary.
NWS Frost Warnings
The National Weather Service issues Frost Warnings when significant or widespread frost is expected. A Frost Advisory can be issued if frost is expected in the spring, during the growing season. The Frost Warning and Frost Advisory are important to agriculture especially after crops have been planted.
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