|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|
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,