How Does Humidity Affect Temperature? (Heat Index and Comfort, 2026)

Humidity doesn't change what a thermometer reads, but it changes how warm you feel. Your body cools itself by evaporating sweat, and the more moisture the air already holds, the slower that evaporation goes.

Humid air feels hotter because sweat evaporates more slowly. The National Weather Service's heat index puts 90°F at 70% humidity at a felt 106°F, and 100°F at 50% at 118°F. Indoors, the EPA recommends keeping humidity between 30% and 50%, and below 60%.

Why humid air feels hotter

Evaporating sweat carries heat away from the skin. When the air is already close to saturated, sweat evaporates slowly and the body sheds less heat, so the same temperature feels hotter. In dry air, sweat evaporates quickly, which is why dry heat feels more bearable.

The heat index

The NWS heat index combines temperature and relative humidity into an apparent temperature, the temperature it feels like in the shade. Computed with the NWS's heat index equation:

The heat index is defined for hot weather, about 80°F and above, and for shade; direct sun can add more. High values mean real risk of heat illness, which is why the NWS issues heat advisories from it.

Dew point

Relative humidity depends on temperature: the same moisture reads as a higher percentage when the air cools. Dew point measures the moisture itself, the temperature at which the air would become saturated. The National Weather Service gives a general summer comfort scale by dew point: 55°F or lower feels dry and comfortable, 55 to 65°F becomes sticky with muggy evenings, and 65°F or higher feels oppressive.

Relative humidity and absolute humidity measure different things. Absolute humidity is the actual mass of water vapor the air holds, while relative humidity expresses that amount as a percentage of the most the air could hold at its current temperature. Warm air can hold far more vapor than cold air, so a fixed amount of water reads as a low percentage when the air is warm and a high percentage once it cools.

This is why a cool basement at the same moisture level as the floor above can read a higher relative humidity: the cooler air sits closer to saturation. When air cools all the way to its dew point, relative humidity reaches 100% and moisture begins to condense, the same process that pulls water onto a cold AC coil or the outside of a glass of ice water.

The same physics explains both seasons. In summer, warm air holds a lot of vapor, so even a moderate relative humidity carries enough moisture to slow sweat evaporation. In winter, cold outdoor air holds very little vapor, so once it is heated indoors its relative humidity falls and the dry air speeds evaporation from skin, which reads as a chill.

Humidity indoors in summer

An air conditioner removes moisture as it cools, because water condenses on its cold indoor coil. If it's oversized and short-cycles, or if the weather is mild but humid, it may cool without drying the air enough, leaving the house cool but clammy. A dehumidifier or a properly sized system fixes that; see running a dehumidifier and AC together.

Raising the thermostat in a drier room

Because humidity changes how warm a room feels, drying the air holds the same comfort at a higher setpoint. At 80°F, the heat index table above puts 60% relative humidity at a felt 82°F and 40% at a felt 80°F.

Dropping the humidity from 60% to around 45%, between the 40% and 50% columns, therefore trims the apparent temperature by roughly 1 to 2°F, back toward the 80°F the thermometer actually reads. A drier room can feel the same at a thermostat setting about 1 to 2°F higher, and a higher setpoint means the air conditioner runs less.

Humidity indoors in winter

Cold outdoor air holds little moisture, so heated indoor air gets very dry, which can feel cooler and irritate skin and sinuses. ENERGY STAR suggests 30% to 40% indoor humidity in cold climates during the heating season, enough for comfort without condensation on windows.

The range to aim for

The EPA recommends 30% to 50% indoors, and below 60% to limit mold and dust mites. See the ideal indoor humidity level.

Frequently asked questions

Does humidity make it feel hotter?

Yes. At 90°F, the heat index rises from about 91°F at 40% humidity to 106°F at 70%, according to the NWS heat index equation.

Why does dry heat feel cooler?

Sweat evaporates quickly in dry air, carrying heat away from your skin efficiently.

What humidity is comfortable indoors?

Between 30% and 50%, according to the EPA; in cold climates during winter, 30% to 40% avoids window condensation.

Does lowering humidity make a room feel cooler?

Yes, at the same temperature. Drier air lets sweat evaporate more easily, which is why dehumidifying can let you set the thermostat a little higher in summer.

At what humidity does it feel muggy?

Mugginess depends on how much moisture the air actually holds, not the relative humidity reading alone. Outdoors, the National Weather Service calls a dew point of 65°F or higher oppressive, with 55 to 65°F feeling sticky. Indoors, relative humidity above 60% feels clammy and invites mold, which is why the EPA keeps the comfortable range below that.

Can high humidity raise the actual room temperature?

Not the temperature a thermometer reads; humidity changes how warm the air feels, not its dry-bulb reading. The moisture does carry stored heat that is released when it condenses, which is why an air conditioner has to pull out both heat and water. A dehumidifier returns that heat to the room plus its own motor heat, so a 590-watt unit adds roughly 2,000 BTU per hour (590 × 3.412 ≈ 2,013).

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