### Water Vapor - Mixing Ratio

How much water vapor is there? And how should it be presented?

Basic questions - difficult answers.

Weather reports use relative humidity. At the surface of the Earth, this measure says something about the probability of rain and how comfortable you will feel.

However, for many applications what is really needed is either the mass of water in the atmosphere or the number of molecules. Since I am interested in spectroscopic analysis (the Greenhouse Effect), the number of molecules is what is important to me. Since my programs compute this value from the atmospheric pressure and water vapor expressed as the mixing ratio relative to the total pressure (expressed as ppm), that is what Water_Vapor.exe computes. This calculator is used to convert that value to mixing ratio for dry air which is what many papers use. For some applications, the difference is very large - for my purpose, it is not significant. This is because I want to understand the process, and I am not concerned with a 10% or 20% uncertainty. In this case, the difference is on the order of only 1%.

### Water Vapor Calculator

Saturation Partial Pressure mbar   This depends on only the temperature
Actual Partial Pressure mbar This is computed from the dew point
Total Pressure mbar This is measured
Mixing Ratio - Total Pressure ppm Actual Partial Pressure / (Total Pressure) * 10E6
Mixing Ratio - Dry Air ppm Actual Partial Pressure / (Total Pressure - Actual Partial Pressure) * 10E6
Relative Humidity %RH Actual Partial Pressure / Saturation Partial Pressure * 100

To Use - just type a value into any cell and the others will be automatically computed.

• Changing any of the top 3 fields will change the bottom 3.
• Changing the bottom 3 fields will compute the Actual Partial Pressure.

Author: Robert Clemenzi