Archive for August 25th, 2008|Daily archive page

WHY DOES IT FEEL SO HUMID?

Relative humidity figures are deceptive unless one understands that warm air has the energy to possess more water vapor than does cold air.  For example, 77˚F. air at 50% relative humidity will contain more water vapor than 50˚F air at 100% relative humidity.  If you did not know that warm air can contain more water vapor than cold air, you would probably “logically” deduce that the 50° air at 100% relative humidity had the greater amount of moisture because, after all, 100% is greater than 50% – but you would be wrong.

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It is a fact that the warmer the air, the greater can be the percentage of water vapor within the whole mixture of gases, (even though the water vapor always represents but a small fraction of the gaseous content of the air).

If the relative humidity of a kilogram of 77˚F. (25˚C) air is 100%, that air will contain 20.00 grams of water vapor.

If the relative humidity of a kilogram of 86˚F. (30˚C) air is 100% that air will contain 26.50 grams of water vapor.

Therefore, if the relative humidity of 77˚F. air is 50%, the air contains 10.00 grams of water vapor in each kilogram (half of 20.00 grams).

Comparatively, if the relative humidity of 86˚F. air is 50%, that air contains 13.25 grams of water vapor in each kilogram (half of 26.50 grams).

So in this example, the warmer of the two is more humid because it contains more water vapor (EVEN THOUGH THE RELATIVE HUMIDITY IS THE SAME IN BOTH EXAMPLES).

Even if the relative humidity of the 86˚F. air was lower, say 45%, it would still contain more water vapor than the 77˚F. air at 50% relative humidity! 45% of 26.50 = 11.925 grams of water vapor per kilogram of air (which is greater than 10 grams).

The 50°F. air discussed in the first paragraph is at 100% relative humidity when it is holding only 7 grams of water vapor per kilogram.  This clearly shows the limitations of cooler air when it comes to the amount of water vapor within it, even if saturated!

So, in conclusion, as is so noticeable in humid climatic zones like where we live in Florida, the hotter the air the more moisture it usually contains. This is true so long as there are available ways for water vapor to enter the air (e.g. evaporation and transpiration). One measure important to climatologists is the “evapotranspiration” rate.

SPECIAL NOTE: This type of explanation that you just read is considered a simplification by some because water is constantly changing phases in the air. When evaporation exceeds condensation within the air, the relative humidity will be below 100% – but when condensation exceeds evaporation, the relative humidity may reach 100%.

A humid afternoon in the Smoky Mountains

A humid afternoon on a LeConte trail in the Smoky Mountains - by Cloudman23©