Dew Point vs. Humidity Explained

Ask most people how humid it is and they'll quote the relative humidity, the percentage you see on most weather apps, including the temperature and humidity badge on the RadarBug map. But meteorologists usually talk about the dew point instead, because it says far more about how the air actually feels and what the weather is likely to do. Once you understand the difference, a few common weather puzzles make sense: why 90 percent humidity can feel comfortable, and why 40 percent can feel oppressive.

What relative humidity measures

Air can hold only so much water vapor, and the amount depends strongly on temperature: warm air can hold much more than cold air, roughly doubling for every 20 °F of warming. Relative humidity (RH) is how much water vapor the air contains compared with the most it could hold at its current temperature, expressed as a percentage.

The catch is in the word relative. Because the maximum changes with temperature, RH changes even when the actual amount of moisture stays the same:

• On a typical summer day, the amount of water vapor in the air barely changes from dawn to afternoon. But as the temperature climbs from 72 °F to 92 °F, the RH might fall from 95 percent to 50 percent. The air didn't get drier; it got warmer.
• A 30 °F winter morning at 90 percent RH contains far less moisture than a 90 °F summer afternoon at 40 percent.

That's why RH is almost always highest around sunrise, when temperatures are lowest, and lowest in mid-afternoon. RH is still useful — it's what controls how fast things dry and evaporate, and it matters for fire danger and for comfort indoors in winter — but it's a poor guide to how muggy it feels outside.

What the dew point measures

The dew point is the temperature the air would have to cool to, with no change in moisture, for it to become saturated (100 percent RH). Cool it any further and water starts condensing out as dew, fog, or cloud. Unlike RH, the dew point depends only on how much water vapor is actually in the air, so it's a direct measure of moisture. A higher dew point means more moisture, regardless of the temperature.

The dew point can never be higher than the air temperature. When the two are equal, the RH is 100 percent. The gap between them, called the dew point depression, tells you how close the air is to saturation.

A comfort scale

Because sweat evaporates more slowly in moist air, the dew point is the best simple measure of how sticky summer air feels. Individual sensitivity varies, but most people find:

So a 60 °F morning at 90 percent RH (dew point about 57) feels pleasant, while a 95 °F afternoon at 45 percent RH (dew point about 70) feels oppressive. To estimate the dew point from the temperature and RH on the RadarBug badge, a calculator is easiest, but as a rough guide, at around 50 percent RH the dew point is about 20 °F below the temperature, and at 70 percent RH it's about 10 °F below.

What dew point tells forecasters

Fog. On a clear, calm night the temperature falls toward the dew point. If it gets there, fog forms. A small dew point depression at sunset on a calm, clear night is a good sign of fog by morning, especially in valleys and near water.

Overnight lows. The evening dew point is a good first guess for the overnight low on a clear, calm night, because once the air reaches the dew point, condensation releases heat and slows further cooling. Dry air with a low dew point can cool off dramatically at night, which is why deserts can be hot by day and cold at night.

Frost. When the dew point is below freezing and the temperature drops to it, moisture deposits as frost. With a dry air mass and a low dew point, temperatures can fall well below freezing without frost forming, which can still damage plants.

Thunderstorms. Moisture is one of the main ingredients for thunderstorms (see Types of Thunderstorms Explained). Forecasters watch for dew points of 60 °F or higher spreading north from the Gulf of Mexico in spring as a sign the atmosphere can support severe storms. In the Plains, the boundary where moist Gulf air with high dew points meets dry desert air is the dryline, a favorite place for storms to form (see the Tornado Alley Weather Guide).

Fronts. A cold front passing usually brings a sharp drop in dew point, often more noticeable than the temperature drop, as drier air arrives. See Weather Fronts Explained.

Humidity indoors

Indoors, RH is the more useful number. Most people are comfortable at 30 to 50 percent RH indoors. Higher than about 60 percent encourages dust mites and mold growth (a concern for people with allergies; see Pollen Types, Seasons & Counts Explained for outdoor mold spores). In winter, heating cold outdoor air with a low dew point can drive indoor RH down to 20 percent or less, causing dry skin, static shocks, and irritated sinuses. A humidifier helps, but keep it clean and avoid pushing RH so high that condensation forms on windows.

Reading the RadarBug badge

The temperature and humidity badge in pollen mode shows the current temperature and RH at your saved home location, or your current location if no home is set, refreshed every hour. Use it with the patterns above in mind:

• High RH in the early morning is normal and doesn't by itself mean a muggy day ahead; look at what the RH is as the temperature rises.
• Warm temperatures with RH above about 50 percent mean high dew points and sticky, uncomfortable air.
• Low RH on a windy day raises fire danger and helps pollen travel further.