Science & Units
Dew Point and Humidity Table
Relative humidity is the number weather apps show and the one that misleads most often. It answers a narrow question — how full is the air, as a percentage of what it could hold at this temperature — and because the capacity changes with temperature, the same 60% means very different amounts of water at 10 °C and at 30 °C. Dew point answers the question people actually care about: how much water is in the air, and how will it feel?
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Dew point is the temperature to which air must be cooled, at constant pressure and water content, for it to become saturated. That definition makes it an absolute measure: a dew point of 20 °C means the same amount of water whether the air around it is at 25 °C or 35 °C. It also makes it a physical threshold rather than a comfort index — cool the air to its dew point and you get condensation, which is fog in the open, mist on a cold window, and water dripping from an air-conditioning coil.
The values in the tables are computed from the Magnus formula with the Alduchov and Eskridge coefficients, which hold to within about 0.05 °C between −40 °C and 50 °C. That is accurate enough that any difference from a weather service's figure comes from their use of a different approximation or from rounding, not from a different definition. The absolute humidity column uses Buck's saturation vapour pressure equation and is there because it makes the dew point's meaning concrete: grams of water per cubic metre of air.
Dew point (°C) for each air temperature and relative humidity
| Air temp (°C) | 30% | 40% | 50% | 60% | 70% | 80% | 90% | 100% |
|---|---|---|---|---|---|---|---|---|
| 0 | -15.5 | -12.0 | -9.2 | -6.8 | -4.8 | -3.0 | -1.4 | 0.0 |
| 5 | -11.2 | -7.5 | -4.6 | -2.1 | 0.0 | 1.8 | 3.5 | 5.0 |
| 10 | -6.8 | -3.0 | 0.0 | 2.6 | 4.8 | 6.7 | 8.4 | 10.0 |
| 15 | -2.4 | 1.5 | 4.7 | 7.3 | 9.6 | 11.6 | 13.4 | 15.0 |
| 20 | 1.9 | 6.0 | 9.3 | 12.0 | 14.4 | 16.4 | 18.3 | 20.0 |
| 25 | 6.2 | 10.5 | 13.9 | 16.7 | 19.1 | 21.3 | 23.2 | 25.0 |
| 30 | 10.5 | 14.9 | 18.4 | 21.4 | 23.9 | 26.2 | 28.2 | 30.0 |
| 35 | 14.8 | 19.4 | 23.0 | 26.1 | 28.7 | 31.0 | 33.1 | 35.0 |
Read across a row to see how much humidity matters: at 25 °C, going from 40% to 80% relative humidity lifts the dew point from 10.5 °C to 21.1 °C — the difference between comfortable and muggy.
What a given dew point feels like
| Dew point (°C) | Dew point (°F) | Absolute humidity (g/m³) | How it feels |
|---|---|---|---|
| 5 | 41 | 6.8 | Dry and crisp; static shocks, chapped lips |
| 10 | 50 | 9.4 | Comfortable for most people |
| 13 | 55 | 11.3 | Comfortable indoors; slightly humid outside |
| 16 | 61 | 13.6 | Noticeably humid; sweat evaporates more slowly |
| 18 | 64 | 15.3 | Sticky; most people notice it |
| 21 | 70 | 18.3 | Oppressive; heat index climbs quickly |
| 24 | 75 | 21.7 | Muggy to the point of discomfort |
| 27 | 81 | 25.7 | Dangerous with heat; air feels like a wet blanket |
Dew point is an absolute measure, so it travels: 20 °C dew point feels the same in Manila and in Miami. Relative humidity does not — 60% at 30 °C is far more water than 60% at 10 °C.
Air temperature that turns a dew point into fog or cloud
| Dew point (°C) | Fog forms at or below | Comfort band |
|---|---|---|
| 5 | 5 °C air temp | Dry |
| 10 | 10 °C air temp | Dry |
| 15 | 15 °C air temp | Comfortable |
| 18 | 18 °C air temp | Humid |
| 21 | 21 °C air temp | Oppressive |
| 24 | 24 °C air temp | Oppressive |
| 27 | 27 °C air temp | Oppressive |
Cooling air to its dew point is exactly what fog is: the air reaches saturation, and the water that was invisible as vapour condenses into droplets. This is why fog forms overnight, when the ground cools the air above it.
How to Use These Tables
- To judge comfort, ignore relative humidity and read the dew point: below 13 °C feels dry, 13–18 °C is comfortable, 18–21 °C is sticky, and above 21 °C most people describe the air as oppressive.
- To predict fog or condensation on a surface, compare the dew point with the expected overnight low. If the low is at or below the dew point, expect fog, dew on grass, or water on the inside of single-glazed windows.
- To size a dehumidifier, use the absolute humidity column: a 30 m³ room at 24 °C dew point holds about 21.8 g/m³, so bringing it to 12 g/m³ means removing roughly 290 g of water — before accounting for the continuous inflow from outside.
- To check a weather report against your own sensor, compute the dew point from your temperature and relative humidity and compare. A gap of more than a degree usually means one of the two sensors is uncalibrated, most often the humidity one.
Common Questions
Why is dew point better than relative humidity for comfort?
Because it does not change when the temperature changes. Relative humidity falls as air warms even though the water content is identical, so 70% in the morning can become 40% by afternoon with no water leaving the room. Dew point stays put, which is why it tracks how sticky the air actually feels.
Can the dew point be higher than the air temperature?
Not in air that is in equilibrium — by definition the dew point cannot exceed the air temperature, because the air would already be supersaturated and the excess water would condense out. In practice a reading above the air temperature means one of the two measurements is wrong, or the air is supersaturated with respect to a curved surface, as in a cloud chamber.
What dew point means it will rain?
There is no fixed threshold, but a dew point above about 15 °C means the air holds enough water for meaningful rain if it is lifted, and values above 20 °C are typical of thunderstorms in summer. Rain also needs a lifting mechanism — a front, a mountain, or convection — so a high dew point alone does not guarantee precipitation.
Why does my air conditioner drip water?
Because the coil is colder than the dew point of the air passing over it. Water condenses on the coil exactly as it would on a cold glass, and the drain carries it away. This is also why air conditioning dehumidifies: the water leaving through the drain is water removed from the room.
How accurate is the Magnus formula?
With the Alduchov and Eskridge coefficients used here, the maximum error is about 0.05 °C between −40 °C and 50 °C over water. The older and more commonly quoted coefficients (17.27 and 237.7) are roughly ten times less accurate at low temperatures. Both are approximations to the same physical curve, not competing definitions.
More Science & Units
Sources
- Improved Magnus form approximation of saturation vapor pressure (Alduchov & Eskridge, 1996) — Journal of Applied Meteorology
- New equations for computing vapor pressure and enhancement factor (Buck, 1981) — Journal of Applied Meteorology
- Humidity — dew point and relative humidity explained — US National Weather Service, Louisville
- Guide to Instruments and Methods of Observation (WMO-No. 8), chapter 4 — World Meteorological Organization
Filed with Science calculations.