What is dew point?
Dew point is the temperature at which the air you're standing in would have to be cooled for water to start condensing out of it. Cool a glass of iced tea on a humid summer afternoon and watch droplets form on the outside — the glass surface dropped below the dew point of the surrounding air, and the moisture had nowhere to go but onto the glass.
The number matters because it describes how much actual moisture is in the air, not the relative amount. Relative humidity is a percentage of capacity, and capacity changes with temperature. Dew point doesn't. A dew point of 70°F (21°C) in Atlanta feels exactly as oppressive as a dew point of 70°F in Bangkok — regardless of what the thermometer reads. Meteorologists prefer it for that reason. Weather apps that only show RH are hiding the more useful number.
If you've ever wondered why one 85°F day feels fine and another feels like wading through soup, the answer is almost always dew point. The thermometers were the same. The water in the air wasn't.
How to use the Dew Point Calculator
Two inputs, one result. The calculator does the math instantly — there's no submit button to press.
- Enter the air temperature. Pick °F or °C from the dropdown.
- Enter the relative humidity as a percent (1 to 100).
- Read the dew point in both °F and °C.
- Below the number, you'll see the comfort tier: Dry, Comfortable, Noticeable, Uncomfortable, Oppressive, or Extreme.
Your numbers stay in the browser. Nothing's sent anywhere. The calculator works offline once the page has loaded.
The formula behind the number
The Dew Point Calculator uses the August-Roche-Magnus approximation — the same formula the National Weather Service and most meteorology textbooks rely on. It's been refined over more than a century, and the 1980 constants are accurate to about ±0.4°C across the normal weather range.
γ = ln(RH ÷ 100) + (17.625 × T) ÷ (243.04 + T)
T_dp = (243.04 × γ) ÷ (17.625 − γ)
Both temperatures in °C. Convert if your input is in °F.
The constants 17.625 and 243.04 aren't arbitrary — they're fit to laboratory data on water vapor saturation pressure. The original Magnus equation from 1844 used 17.27 and 237.7; the modern constants are tighter and more accurate near tropical conditions where the older version drifted.
Worked example: imagine it's 25°C (77°F) outside with 60% relative humidity — a typical summer afternoon in much of the eastern US.
- γ = ln(60 ÷ 100) + (17.625 × 25) ÷ (243.04 + 25)
- γ = ln(0.60) + 440.625 ÷ 268.04
- γ = −0.5108 + 1.6439 = 1.1331
- T_dp = (243.04 × 1.1331) ÷ (17.625 − 1.1331)
- T_dp = 275.39 ÷ 16.4919 = 16.7°C (62.1°F)
A dew point of 16.7°C lands in the Comfortable-to-Noticeable range. Pleasant outside, but you'd feel it if you went for a run.
The comfort tiers and what they mean
The thresholds below come from the US convention first published by Glickman in 1957 and adopted by meteorologists ever since. They're based on absolute moisture content, so they don't shift with the thermometer reading — a 70°F dew point feels like the tropics whether the air temperature is 75°F or 95°F.
| Dew Point (°F) | Dew Point (°C) | Tier | How it feels |
|---|---|---|---|
| Below 50 | Below 10 | Dry | Comfortable, crisp. Sweat evaporates instantly. Indoor humidifier territory in winter. |
| 50 – 55 | 10 – 13 | Comfortable | Most people don't notice the humidity at all. Ideal outdoor weather. |
| 55 – 60 | 13 – 16 | Noticeable | You can feel the moisture, but it's not a problem. A light shirt sticks if you exercise. |
| 60 – 65 | 16 – 18 | Uncomfortable | Noticeably sticky. Sweat doesn't fully evaporate. Outdoor exertion becomes harder. |
| 65 – 70 | 18 – 21 | Oppressive | The air feels heavy. Sleep without AC is rough. Sweat pools rather than cools. |
| Above 70 | Above 21 | Extreme / Tropical | Soup. Bangkok in May. Houston in August. Active cooling becomes a health issue, not a preference. |
For reference: New Orleans averages a 73°F dew point through July. Phoenix in summer often sits around 45°F — desert dry. The highest dew point ever recorded was about 95°F (35°C) in Dhahran, Saudi Arabia, in 2003. Above that, human survival outdoors gets difficult because sweat can't evaporate fast enough to shed metabolic heat.
Why your air feels heavy
High dew point means the air is already near saturation. Your sweat — the only cooling mechanism you have once the temperature rises above skin temperature — needs somewhere to evaporate into. When the air is full of moisture, evaporation slows to a crawl. You keep producing sweat (losing water and electrolytes) without losing much heat. You feel sticky because you're literally coated in liquid that has nowhere to go.
Sleep is harder at high dew points for the same reason. Your body sheds heat overnight by sweating and radiating; when neither works well, core temperature stays high and sleep architecture breaks down. The first thing an air conditioner does on a sticky night isn't lowering temperature — it's removing water from the air. That's why a dehumidifier can make a 78°F room feel better than an AC that cools it to 75°F without dehumidifying.
Breathing feels heavier too, partly from the humidity itself and partly because high-dew-point days often coincide with low atmospheric pressure (the same conditions that produce afternoon thunderstorms).
Dew point vs. relative humidity
Both describe moisture. They describe it differently.
Relative humidity = (current water vapor) ÷ (maximum the air can hold at this temperature) × 100. It's a ratio, so it shifts as temperature shifts.
Dew point = the temperature at which the current vapor would be 100% saturated. It's an absolute measure, independent of current temperature.
Same air sample, different framings: 50% RH at 90°F means a dew point around 70°F (oppressive). 50% RH at 50°F means a dew point around 32°F (very dry). The relative humidity number is identical in both cases, but the comfort is wildly different. That's why "the humidity was 80%" tells you almost nothing on its own.
When you hear a weather report mention RH without dew point, it's usually because the broadcast is short. The forecasters know which number actually matters.
Edge cases and limitations
A few things worth knowing about the formula and its quirks:
- Dew point can't exceed air temperature. If it did, water would already be condensing — that's what fog and rain are. If your inputs imply a dew point higher than the air temp, your humidity reading is wrong or the temperature reading is wrong.
- When dew point equals air temperature, RH is 100%. That's the saturation point. Below it: dew, fog, condensation on cold surfaces. The cool side of a window in winter often hits dew point first, which is why condensation forms there.
- The formula loses accuracy below −40°C or above +50°C. Both extremes are outside the range of normal weather. For polar or industrial-furnace conditions, other approximations work better.
- Indoor dew point is what matters for mold. If your bathroom dew point exceeds the temperature of the wall behind your shower, you'll grow mold there. Bathroom fans drop indoor dew point fast; running one for 20 minutes after a shower is usually enough.
- Sensor accuracy varies. Cheap consumer hygrometers can drift by 5-10% RH after a year. That translates to a several-degree error in calculated dew point. If your reading seems off, the sensor is the most likely culprit.
Related calculations
Comfort and weather involve more than one number. Pair the dew point with:
- Heat Index Calculator — combines temperature and humidity into a single "feels like" number for hot weather. Uses the NWS Rothfusz formula.
- Wind Chill Calculator — the cold-weather counterpart. NWS 2001 formula, with frostbite-time warnings.
- Temperature Converter — quick °F ↔ °C ↔ K conversions if your thermostat and your weather app disagree.
Frequently asked questions
What's a comfortable dew point?
Most people find dew points below 55°F (13°C) pleasant and below 60°F (16°C) acceptable. Above 65°F (18°C), discomfort sets in for most people; above 70°F (21°C), almost everyone feels it. These thresholds are remarkably consistent across populations because they're driven by sweat-evaporation physics, not preference.
Can dew point be negative?
Yes — very dry winter air or desert conditions routinely produce dew points below freezing. Technically, when the dew point is below freezing, the more precise term is "frost point" (the temperature at which water deposits as frost rather than dew). The formula and the comfort tiers work the same way.
Why is the dew point usually lower at night?
It usually isn't, actually. Dew point stays relatively constant through the day because the absolute moisture in the air doesn't change quickly. What changes is the air temperature — which drops at night, often falling to meet the dew point. That's when dew forms, and that's why "dew point" is named what it is. The number describes the temperature at which dew would form. When morning dew is heavy, you're seeing the air temperature falling to the dew point and water condensing out.
How does dew point relate to fog?
Fog is what happens when the air temperature drops to the dew point in a layer of air near the ground. The relative humidity hits 100% and water condenses into the tiny droplets that make up fog. Morning valley fog forms when cold air pools in low spots and chills below dew point overnight; it burns off when the sun warms the air back above dew point.
What's the dew point in a typical air-conditioned room?
Most home AC systems target a dew point of around 50-55°F (10-13°C). The thermostat reads the dry-bulb temperature, but the dehumidification side of the cooling coil sets the dew point. If your home feels clammy at 72°F, your AC is undersized or running short cycles — it's cooling without dehumidifying enough.
Does altitude affect dew point?
Not directly. Dew point is a function of water vapor content, not pressure or altitude. However, higher-altitude air is generally drier because air loses moisture as it lifts and cools (rain forms). So Denver tends to have lower dew points than Atlanta even at the same air temperature — not because of altitude per se, but because of the airmass typical for that location.
Why doesn't my weather app show dew point?
Many do — usually buried in the "details" view rather than the main display. Some apps (especially older or simpler ones) only show relative humidity because it's a more familiar concept to most users. The data is usually there if you look. If you find yourself checking dew point regularly, look for a settings option to surface it on the main screen.