| Quantity | typical units | What it tells you alone | What it can tell you combined with another quantity |
| Actual vapor pressure (AVP) | millibars (mb) | How much water vapor is in the air | With saturation vapor pressure can be used to calculate relative humidity |
| Saturation vapor pressure | millibars (mb) | How much water vapor the air could hold (how much water vapor would be in the air if the air was saturated) | With actual vapor pressure can be used to calculate relative humidity |
| Dew point temperature (Td) | degrees (°F or °C) | Temperature to which if that air was cooled, it would be saturated (so is another way to describe how much water vapor is in the air) | With air temperature can give a qualitative estimate of relative humidity (whether RH is high as when T & Td values close or low as when T & Td values far apart) |
| Air temperature (T) | degrees (°F or °C) | Can be used to calculate saturation vapor pressure (so is another way to tell how much water vapor that air could contain) | With dew point temperature can give a qualitative estimate of relative humidity (whether RH is high as when T & Td values close or low as when T & Td values far apart) |
| Relative humidity (RH) | percent (%) | How close the air is to being saturated |