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Name: Matt
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Question:
How does altitude affect the frost point and dew point? Would a town at 1300' ASL be more likely to get a frost than a neighboring town (7 miles away)at 1100' ASL?


Replies:
Matt,

Frost and dew formation depends on the temperature and the absolute humidity of the air, and the temperature of whatever dew or frost may form on.

Sometimes grass, branches, vehicles, etc. will loose energy and decrease in temperature more than the air itself, allowing dew or frost to form on these exposed surfaces even though the air temperature itself does not decrease to the frost point or dew point.

Since temperature generally decreases with height (unless there is a nighttime temperature inversion on a clear night), it is more likely that the air temperature will be lower at 1300 feet than at 1100 feet. Dew or frost would then be more likely to form at 1300 feet than at 1100 feet, assuming that the absolute humidity at the two towns is the same.

However, the absolute humidity can also change with altitude, as well as from one place to another depending on soil moisture, whether a water body is nearby, etc. So saturation of the air (dew point, frost point) may occur at the 1100 foot high town and not at the 1300 foot town in some cases. There are just too many variables to make a hard and fast rule on this.

David R. Cook
Climate Research Section
Environmental Science Division
Argonne National Laboratory


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