What Is Humidity? Plain-English Guide to Relative, Absolute, and Dew Point

Humidity is water vapor in the air. You cannot see it; humidity is the gaseous form of water, distinct from clouds (which are tiny liquid droplets) and from precipitation (rain or snow that has condensed and fallen). The number on the weather report is almost always relative humidity, a percentage. Below is what that percentage actually means, why two summers at the same air temperature can feel completely different, and the four humidity terms worth knowing.

Three Kinds of Humidity, Plus Dew Point

Relative humidity (RH). The percentage of how much water vapor the air is holding compared to the maximum it could hold at the current temperature. The 65 percent on your weather app. Most people-facing humidity numbers are RH.

Absolute humidity. The actual mass of water vapor per cubic meter of air, measured in grams per cubic meter. A direct measurement, not a ratio.

Specific humidity. The mass of water vapor per kilogram of total air. Used by meteorologists for atmospheric calculations.

Dew point. The temperature at which the air would become 100 percent saturated. Unlike relative humidity, dew point does not change as air temperature changes, which makes it a more reliable single-number measure of how humid the air actually feels. Dew point under 50 degrees Fahrenheit is dry and pleasant; over 70 is dangerous.

Why Warm Air Holds More Water Vapor

The relationship between temperature and water-vapor capacity is described by the Clausius-Clapeyron relation. In plain terms: every 1 degree Celsius of warming lets the air hold about 7 percent more water vapor. Capacity roughly doubles every 20 degrees Fahrenheit. That is why a 90-degree day in Florida feels different from a 90-degree day in Phoenix; warm air has the capacity to hold a lot more moisture, and the Gulf of Mexico provides it.

Heat Index: When Humidity Makes Heat Worse

The heat index is what the temperature feels like to your body when relative humidity is factored in. NOAA publishes a heat index chart that pairs RH with air temperature to give a “feels like” temperature. At 90 degrees Fahrenheit and 70 percent RH, the heat index is roughly 106 degrees. The body cools by sweating, and sweat cools you by evaporating off your skin. High humidity slows evaporation. At dew points above 75, sweating barely cools at all.

Wet-bulb temperature is the more rigorous measure of this risk. A wet-bulb temperature of 35 degrees Celsius (95 degrees Fahrenheit) is generally considered the upper survival limit; the body cannot dump heat fast enough at that level even in shade with full hydration.

How Humidity Feels at Different Levels

Below 30 percent. Dry. Cracked skin, static shocks, nosebleeds, sore throats. Common in the desert Southwest and during winter indoor heating.

30 to 50 percent. Comfortable. The target band for indoor air per EPA and ASHRAE.

50 to 60 percent. Comfortable to slightly humid.

60 to 80 percent. Humid. Sweat does not evaporate efficiently. Hair frizzes.

Above 80 percent. Oppressive. Heat illness risk rises sharply when paired with temperatures above 85 degrees.

Where Humidity Comes From

Almost all atmospheric water vapor originates from oceans (about 90 percent). The rest comes from inland water bodies, transpiration from plants, evaporation from soil, and respiration from animals and humans. The Gulf of Mexico is the major source of US East Coast and Plains humidity. The Great Lakes contribute to Midwest summer humidity. Plant transpiration is significant at the local scale, particularly in dense forest canopies.

Geographic and Seasonal Patterns

Gulf Coast. Some of the highest annual humidity in the US. Summer dew points routinely above 70 degrees.

Southeast. High summer humidity, moderate winter humidity.

Southwest. Very low humidity, often below 20 percent. Even very hot days can feel tolerable because sweat evaporates instantly.

Pacific Northwest. Moderate humidity year round.

Midwest and Plains. Wide swings. High summer humidity from the Gulf, low winter humidity once the cold air settles in.

Winter humidity indoors. Cold outside air holds little water vapor. When that air is heated to room temperature, RH drops dramatically. A 30 percent RH outside at 20 degrees Fahrenheit can read 10 percent RH at 70 degrees indoors. That is why winter heating dries out skin and irritates airways.

How Meteorologists Measure Humidity

The instrument is a hygrometer. Common types: capacitive sensors (most digital home and weather-station hygrometers), resistive sensors, chilled-mirror dew-point hygrometers (the laboratory standard), and the older psychrometer (a wet-bulb / dry-bulb pair, the original method). Modern smart thermostats use capacitive sensors and report indoor RH on the display.

Indoor Humidity Targets

EPA and ASHRAE both recommend an indoor RH of 30 to 50 percent. Below 30, skin and respiratory tracts dry out and flu viruses survive longer in the air. Above 60, mold can germinate within 24 to 48 hours and dust mites multiply 1.5 to 3 times faster. For a longer breakdown of which RH targets fit which use case, see our relative humidity chart.

Frequently Asked Questions

What is humidity in simple terms?

Humidity is water vapor in the air, in invisible gas form. The percentage on your weather app is relative humidity, which compares the actual water vapor present to the maximum the air could hold at that temperature.

What is the difference between humidity and dew point?

Relative humidity is a percentage that depends on the current air temperature. Dew point is a temperature, not a percentage, and stays the same even as air temperature changes. Dew point is the more reliable measure of how humid the air actually feels.

Why does humid heat feel hotter than dry heat?

Your body cools itself by sweating, and sweat cools you by evaporating. High humidity slows evaporation, so sweat sits on the skin and the body cannot dump heat fast enough. A 90-degree day at 70 percent humidity feels like 106 degrees on the heat index.

What humidity is most comfortable?

30 to 50 percent relative humidity, both indoors and outdoors at moderate temperatures. EPA and ASHRAE both recommend this band as the indoor target. Below 30, the air feels dry; above 60, it starts to feel sticky.

Where does humidity come from?

About 90 percent comes from ocean evaporation. The rest comes from inland water, plants (transpiration), soil moisture, and respiration. The Gulf of Mexico is the dominant source of US East Coast and Plains humidity.

Why is winter air so dry indoors?

Cold outside air holds very little water vapor. When that air is heated to room temperature inside, relative humidity drops dramatically. Outside air at 30 percent RH at 20 degrees can read 10 percent RH once heated to 70 degrees indoors. A humidifier offsets this.