Dew point vs relative humidity explained

Weather apps and thermostats usually show relative humidity. Engineers and meteorologists often prefer dew point. Both describe the moisture in the air, but they answer different questions. Knowing the difference explains why a house can feel damp at "only 50%", and why windows fog up in winter.

Relative humidity is relative to temperature

Warm air can hold more water vapour than cold air before it becomes saturated. Relative humidity (RH) compares the water vapour actually in the air with the most the air could hold at its current temperature.

So RH changes whenever the temperature changes, even if no moisture is added or removed. Cool a room and its RH rises. Warm it and the RH falls. The amount of water in the air has not changed at all.

Dew point is the moisture itself

The dew point is the temperature to which air must be cooled, with no change in its moisture, before water starts to condense. It depends only on how much water vapour is in the air. Heating or cooling the room does not change it, until the air touches something colder than the dew point.

That makes dew point the better number for two things: comparing moisture levels between rooms or days, and predicting condensation.

Same moisture, different relative humidity

Take air with a dew point of 55 °F. Its relative humidity at different room temperatures is:

• at 65 °F: about 70%
• at 70 °F: about 59%
• at 75 °F: about 50%
• at 80 °F: about 42%

The water content is identical in all four cases. Only the temperature has changed.

It works the other way too. At 50% RH, the dew point is:

• about 50.5 °F in a 70 °F room
• about 55.1 °F in a 75 °F room
• about 59.7 °F in an 80 °F room

A reading of "50%" therefore means quite different amounts of moisture depending on the temperature. On a summer day, the dew point tells you more about how muggy it will feel than the RH does.

Predicting condensation

Water condenses on any surface colder than the dew point of the air touching it. That one rule explains most indoor moisture problems:

• Winter windows. In a 70 °F room at 40% RH, the dew point is about 44.6 °F. A window pane colder than that will fog or drip. Raise the indoor humidity to 50% and the dew point climbs to about 50.5 °F, so condensation appears on warmer glass and on more of each window.
• Sweating ducts and pipes. Cold supply ducts, refrigerant lines and cold-water pipes in a humid attic, crawlspace or basement collect water when their surface is below the surrounding air's dew point. Insulation with a vapour-tight jacket keeps the surface warm and dry.
• The air conditioner coil. This is condensation on purpose. The indoor coil runs well below the dew point of the return air, so moisture condenses on it and drains away. That is how an air conditioner dehumidifies.

How the numbers are calculated

The dew point comes from the air temperature and RH through the saturation vapour pressure of water: how much vapour pressure saturated air has at a given temperature. A common approximation is the Magnus formula, which works well for normal indoor and outdoor conditions. Our dew point and humidity calculator uses the more exact ASHRAE equations as implemented in PsychroLib. It works from dry-bulb temperature plus RH, wet-bulb or dew point, and also gives the humidity ratio and enthalpy.

To work out where condensation will form, measure the surface temperature, with an infrared thermometer for example, and compare it with the room's dew point. If the surface is colder, expect water.

Key points

• Relative humidity is moisture compared with what the air could hold at its current temperature, so it changes with temperature.
• Dew point is set by the actual amount of moisture, so it stays the same when the room warms or cools.
• The same 50% RH means more moisture in a warm room than a cool one.
• Water condenses on any surface colder than the dew point of the air around it.
• An air conditioner dehumidifies by keeping its coil below the dew point.

Sources

• PsychroLib — psychrometric functions after ASHRAE Handbook—Fundamentals (2017) ch. 1, open source (MIT licence).
• Water vapor pressure formulations, UCAR / NCAR Earth Observing Laboratory.
• Dew point, Wikipedia, for the Magnus approximation and its constants.