Water Vapor Computations


	Over Water	Over Ice
Algorithm
Saturated H₂O Partial Pressure (mbar)
Saturated H₂O Mixing Ratio (ppm)
%RH
Water Mixing Ratio (ppm)
	Dew Point Suppression K/°C °F
	Dew Point Temperature K °C °F

Temperature K °C °F

Pressure (mBar)

Hold This Constant %RH Over Water
%RH Over Ice
Mixing Ratio (ppm)
Dew Point Suppression
Dew Point

Notes

This javascript app is functionally identical to Water_Vapor.exe version 1.4 2014/2016 (Windows only), except that the cloud simulation is not provided.
Use Ctrl, Shift, and Alt keys with the mouse wheel, arrow buttons (Chrome), and up/down cursor keys to change number values.
The saturation partial pressures are red when the temperature is outside the calibration range for the selected algorithm.
All Over Ice values are red for temperatures above freezing.

Comments

The water in the atmosphere is typically expressed in different ways, depending on the source.

Relative humidity (%RH)	Used with some weather reports - TV, radio, and newspapers
Absolute humidity (ppm)	aka Water Mixing Ratio - used with Climate models.
Absolute humidity (g/m³)	This is the actual amount of water in the local atmosphere (not the total column).
Dew Point	Weather reports, directly measured with cold mirror hygrometers
Dew Point Suppression	Radiosonde soundings - subtract positive values from the temperature to determine the dew point
Frost point	Directly measured with cold mirror hygrometers
Wet bulb/Dry bulb	Directly measured with sling psychrometers - this is the old method to determine relative humidity. This technique requires the use of charts (or tables) to convert the 2 temperatures into RH.
Total precipitable water (TPW)	kg/m² - Science papers - refers to the total atmospheric column

One of the purposes of this application is to easily convert between some of these. Unfortunately, Precipitable water refers to the total atmospheric column and, therefore, can not be converted into a local value.

The values calculated above are presented to much higher precision than can be justified by the known precision of the instruments used to make the measurements. While laboratory instruments are fairly accurate, sensors used to collect real world (field) RH values are less than 1% accurate. Therefore, for practical purposes, values within +/-2% RH should be considered identical.

Explaining Dewpoint and Relative Humidity to the Public presents an interesting discussion. Basically,

Dewpoints < 65F feels good
50% < RH < 70% feels good

References

Saturation Water Vapor Formulas: This is my page explaining where the various saturation water vapor formulas come from
Hygrometer - wikipedia: This describes various hygrometers - including the sling psychrometer - and compares their relative accuracies
Psychrometrics - wikipedia: This describes the basics, defines a number of terms, and provides a psychrometric chart
Equilibrium Relative Humidity for various Saturated Salt Solutions: Hygrometers are typically calibrated using saturated salt solutions. In this chart, all the RH values decrease with increasing temperature .. except for NaCl which has a minimum at about 55°C.

Author: Robert Clemenzi