Midcoast Green

Midcoast Green Collaborative > Home > Articles > Measuring Humidity

Moisture 201: Measuring Humidity

Topher Belknap

Basic fact #1: The amount of water vapor that air can hold is limited, that limit is dependent on temperature, and it is not a linear relationship.

Psychrometric Chart (click for PDF)
pdf credit: Coolerado
Saturation Point:
When the air (at its current temperature) is holding all the water vapor it can.
Psychrometric Chart:
A graph with dry bulb temperature on the X-axis, Humidity Ratio on the Y-axis, Saturation point curve on the left side, and various other metrics plotted on it (including wet bulb temperature, enthalpy, relative humidity, specific volume, dew point temperature, etc.). Atmospheric pressure is also a factor, so they are calibrated for a given elevation.

Metrics (unit type given in parenthesis)

Dew Point (temperature):
The temperature at which air (at it current level of water vapor) will be at its saturation point. Dew point is a measure of absolute humidity (though the scale is not linearly related to either mixing ratio or partial pressure).
Relative Humidity (percentage):
For a given temperature, the percentage of the amount of water vapor actually present, compared to what it would take to be at the saturation point.
Absolute Humidity [Mixing Ratio] (mass / volume):
The mass of the water vapor in a volume of air.
Dry Bulb Temperature (temperature):
Standard temperature, named to differentiate it from Wet Bulb Temperature.
Wet Bulb Temperature (temperature):
Temperature read from a thermometer with a wet sock on it, being swung through the air. Sling psychrometers are generally how it comes. This can be used to determine humidity levels (in conjunction with a psychrometric chart). Wet Bulb temperature is closely correlated with Enthalpy but don't worry about that.
Humidity Ratio (mass of water vapor / mass of dry air):
Another measure of absolute humidity.

So, looking at a psychrometric chart (click on picture above), for your elevation, you can find your current conditions by locating the current temperature on the x-axis, and the humidity level, either by wet bulb or relative humidity (depending on your measuring device) to establish a point on the Y-axis. Wet bulb lines slope negatively (down to the right) on the chart. Relative humidity (since it is a percentage of saturation temperature) follows the saturation curve.

If the point ever hits saturation curve, there is danger of condensation, and further movement in that direction will need to follow the curve. For example if one is cooling (only) and reaches the saturation point, further cooling is only possible by condensing some moisture out of the air.