How to Calculate Water Vapor
Water vapor is the gaseous phase of water and is one of the states in the water cycle. Water is continually being added to the atmosphere through evaporation and removed through condensation. Scientists such as climatologists and hydrologists frequently wish to determine the maximum partial pressure of water in the atmosphere, and this value is generally dependent upon the temperature of the air although there are significant exceptions. There are a variety of methods for calculating the maximum vapor pressure and the Goff-Gratch equation is one of the most common.
Instructions
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The Steps
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1
Calculate the maximum water vapor pressure in the atmosphere with the Goff-Gratch equation. This equation may be given as Log10 pw = A + B + C + D + Log10(1013.246) where A, B, C and D are functions of the temperature of the moist air and pw is the maximum water vapor pressure in millibars (roughly 1/1000 of an atmosphere.
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2
Derive the value for the function A in step 1. This is given as A = -7.90298 (373.16/T - 1) where T is the temperature in Kelvin.
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3
Evaluate the value for the function B in step 1. This is given as B = 5.02808 Log10(373.16/T) where T is the temperature in Kelvin.
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4
Evaluate the value for the function C in step 1. This is given as B = - 1.3816 x 10^-7 (1011.344 (1-T/373.16) -1) where T is the temperature in Kelvin.
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5
Evaluate the value for the function D in step 1. This is given as B = 8.1328 x 10^-3 (10-3.49149 (373.16/T-1) -1) where T is the temperature in Kelvin.
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6
Use the Goff-Gratch equation for temperatures in the range of '50 to 102 degree Celsius. This equation also does not hold for certain adverse conditions, such as air that is above a body of water when it begins to boil. In this case, the boiling water will immediately dispel a large amount of water into the atmosphere regardless of the relative humidity.
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