Dew Point & Pressure
Dewpoint is the temperature at which air forms dew at a particular pressure. The dew point is also connected to the relative humidity--a high relative humidity indicates that the dew point is close to the current air temperature. For example, a humidity of 100 percent indicates that the due point is equal to the current air temperature because the air is maximally saturated with water. Dew point is important to airline pilots because it indicates the level of ice, which will build up on the aircraft, as well as the potential for fog and the height of the cloud base.
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Pressure
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Varying the pressure of air under laboratory conditions to simulate atmospheric pressure alters the dew point. Air at 21 degrees Celsius with air pressure at 50 pounds per square inch (psi) forms dew at -40 degrees Celsius. If the pressure is reduced to 20 psi (maintaining the 21-degree temperature), the dew point is reduced to -47 degrees Celsius. If the air pressure is increased to 80 psi at 21 degrees Celsius, the dew point increases to -35 degrees Celsius.
Humidity
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Relative humidity also plays an important role in calculating dew point because warm air can hold more water than cool air. The greater the difference between temperature and dew point the greater difference between the amount of water the air can hold and the amount it could potentially hold. Larry Krengel, an aviation instructor in Elmhurst, Illinois, explains it as "The [difference between temperature and dew point is] the same as relative humidity, but relative humidity is given as a percent. At 75 percent relative humidity, the air has three-fourths of the water vapor it can hold."
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Comfort Range
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Extremely high dew points can cause discomfort and even pose health dangers to asthmatics. A normal "comfortable" dew point temperature is considered to be between 10 and 18 degrees Celsius. Below 10 degrees Celsius, skin can become uncomfortably dried out and lips chapped. Above 18 degrees Celsius to 24 degrees Celsius would be considered "muggy" and slightly uncomfortable. Above 24 degrees Celsius is considered sultry and oppressive--asthmatics may have difficulty breathing with a dew point this high.
Related: Frost point
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Related to the dew point is the frost point--the temperature at which ice forms on the cool body. The frost point of liquid water is higher than the dew point because for ice to form on skin requires a certain amount of impurity on the surface of the body for the ice to attach itself to. This said, dew is still capable of occurring at subzero temperatures.
Record High Dew Points
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While it is hard to pin down an exact reading confirmed as the highest ever recorded dew point, the dew point of 35 degrees Celsius on July 8, 2003, in Dhahran, Saudi Arabia, must be a contender. This dew point would have been extremely uncomfortable for anyone experiencing it and would have confined many to their homes. The comfort scale rates anything more than 24 degrees Celsius as oppressive and possibly dangerous.
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References
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- Photo Credit Dew image by photorich from Fotolia.com
