What is dew point, and what does it mean at home?

What dew point means

Dew point is the temperature to which air must be cooled, with no change in its moisture or pressure, before it becomes saturated and water vapor starts turning into liquid water.

Saturated means the relative humidity has reached 100%. Relative humidity (RH) is how much water vapor the air holds compared with the most it could hold at its current temperature. Cool the air and that maximum drops, so the same moisture fills a bigger share of it. Cool it all the way to the dew point and the air is full: any further cooling squeezes water out as dew on the grass, fog in the valley or droplets on your window.

You have seen this on a hot day with a cold drink. The bottle is colder than the dew point of the air around it, so water vapor condenses on the outside of the bottle. Nothing is leaking; the water comes from the air.

The key thing to remember: the higher the dew point, the more moisture is in the air.

Dew point vs. relative humidity: which tells you how muggy it feels?

Dew point is the better measure of how muggy it feels, because it tracks the actual amount of moisture in the air, while relative humidity changes every time the temperature does.

The National Weather Service gives a clear example. At 30 °F with a dew point of 30 °F, the relative humidity is 100%. At 80 °F with a dew point of 60 °F, it is only 50%. Yet the 80-degree day feels much more humid, because its dew point is 30 degrees higher and there is far more water vapor in the air.

Relative humidity also swings over a single day even when the moisture doesn’t change. Take a summer day with a steady 60 °F dew point:

The humidity number drops by more than half between morning and afternoon, but the air holds the same moisture all day. The dew point says so; the RH doesn’t. That’s why forecasters look at the dew point when they talk about how sticky it will feel.

Dew point comfort levels

The National Weather Service rates summer dew points of 55 °F or lower as dry and comfortable, 55 to 65 °F as sticky, and 65 °F or higher as oppressive.

These bands come from the NWS office in La Crosse, Wisconsin, and describe how outdoor air feels in summer. Farther south it gets more extreme: in Florida, the NWS Tampa Bay office notes that summer dew points can climb above 80 °F (27 °C), which can push the heat index to dangerously high levels. In winter, dew points in the same area drop below 30 °F (-1 °C).

Worked example: 72 °F at 60% humidity

At 72 °F (22 °C) and 60% relative humidity, the dew point is about 57 °F (14 °C).

Here’s the math our calculator uses. It’s the Magnus approximation, a standard formula from meteorology that works in °C:

T is the air temperature in °C and RH is the relative humidity in percent.

1. Convert the temperature: 72 °F = (72 - 32) × 5 ÷ 9 = 22.22 °C.
2. Work out γ: ln(0.60) = -0.511, and 17.625 × 22.22 ÷ (243.04 + 22.22) = 1.477. Add them: γ = 0.966.
3. Dew point: 243.04 × 0.966 ÷ (17.625 - 0.966) = 14.09 °C.
4. Convert back: 14.09 × 9 ÷ 5 + 32 = 57.36 °F.

So what does 57 °F mean in that room? Any surface colder than 57 °F will collect moisture: a cold-water pipe, an uninsulated air-conditioning duct, a cold window. On the NWS scale, that dew point is in the “sticky” range, and 60% humidity is right at the EPA’s upper limit for indoor air (more on that below).

Why windows fog up in winter

Windows fog up in winter because the inside of the glass is colder than the dew point of your indoor air, so water vapor from the room condenses on it.

The inside surface of a window sits somewhere between room temperature and the outdoor temperature. The colder it gets outside, the colder the glass gets, and once it drops below the indoor dew point, it gets wet. That’s why condensation shows up on the coldest mornings, and on the coldest parts of the window first.

Here’s how warm or cold the glass can be before it fogs, at a room temperature of 70 °F:

You have two ways to stop it, and they work best together:

• Lower the indoor dew point. Take moisture out at the source: run the bathroom fan while you shower, use the range hood or open a window when you cook, and make sure the clothes dryer vents outside.
• Warm the glass or the air next to it. Raise the room temperature a little, open curtains and blinds so room air reaches the glass, and use a fan if needed. If you’re buying new windows, check the Condensation Resistance (CR) rating on the NFRC label: it runs from 1 to 100, and the higher the number, the less likely the inside of the window is to fog.

Wipe condensation off when you see it. If water runs down the glass and pools on the sill morning after morning, it can be a sign that your indoor humidity is too high: the EPA’s advice is to dry wet surfaces quickly and reduce the moisture source.

Dew point, mold and when to ventilate or dehumidify

The EPA recommends keeping indoor relative humidity below 60%, ideally between 30% and 50%, which at 70 °F means keeping the indoor dew point below about 55 °F, ideally between about 37 and 51 °F.

The reason is condensation. Mold needs moisture, and in a house the moisture often comes from water condensing on cold surfaces. The EPA notes that mold is often found on walls in cold corners behind furniture, where condensation forms, and that a moldy smell can be a sign of hidden mold even when you can’t see it. It also says that if wet or damp materials are dried within 24 to 48 hours, in most cases mold won’t grow.

Dew point makes the next decision easy. To dry out a room, you want to replace its air with air that holds less moisture, and dew point measures exactly that. Compare the outdoor dew point (from your forecast or weather station) with the indoor one:

How home weather stations and hygrometers report dew point

A home weather station, or a hygrometer that shows dew point, calculates the dew point rather than measuring it: it reads the air temperature and relative humidity and works out the dew point from those two numbers.

Davis Instruments, which makes the Vantage Pro2 and Vantage VUE stations, says this plainly in its technical notes: dew point is a derived value with no sensor or circuitry of its own, worked out from the outdoor temperature and humidity. Its software uses a WMO-recommended formula that is very close to ours (for 72 °F and 60% it gives 57.35 °F against our 57.36 °F), and its Vantage Pro2 and Vantage VUE consoles use a lookup table that closely approximates the formula.

Two practical points follow:

• The dew point is only as good as the two readings behind it. At 72 °F, a hygrometer that reads 63% instead of the true 60% shows a dew point of 58.7 °F instead of 57.4 °F, about 1.4 °F too high. Keep the sensor out of direct sun and away from heaters, lamps and kitchens.
• If your hygrometer shows only temperature and humidity, you can get the dew point from those two numbers with the calculator below.

Dew point is also handy for reading tonight’s weather. Davis notes that if the temperature and the dew point are close together in the late afternoon as the air starts to cool, fog is likely overnight. And with no front on the way and an afternoon humidity of 50% or more, the afternoon dew point gives a rough idea of the overnight low.

Calculate your dew point

Enter your air temperature and relative humidity in the calculator below, and it shows the dew point in both °F and °C.

It starts with the example from this article, 72 °F at 60% humidity. Swap in the readings from your thermostat, hygrometer or weather station.