Relative humidity is the number most weather reports lead with, but it's a moving target — the same moisture content reads as a low percentage on a hot day and a high percentage on a cool one. Dew point measures the actual amount of moisture in the air directly, which is why meteorologists and HVAC engineers reach for it whenever they need to describe how muggy conditions really are.
What the Dew Point Actually Measures
The dew point is the temperature at which air becomes saturated — unable to hold any more water vapor — given its current moisture content. Cool air down to its dew point and water starts condensing out as dew, fog, or cloud droplets. Because this only depends on how much moisture is actually present (not on the current air temperature), the dew point stays roughly constant through the day even as the temperature and relative humidity swing. That stability is exactly what makes it a better comfort indicator: a 60% relative humidity reading means something very different at 15°C than it does at 35°C, but a 20°C dew point feels the same regardless of the air temperature around it.
The Magnus Formula and Its Limits
This calculator uses the Magnus-Tetens approximation, an empirical formula fitted to the physics of water vapor pressure over liquid water. It's simple, fast, and accurate to within about 0.4°C across the range of temperatures and humidities most people encounter day to day (roughly −45°C to 60°C). More rigorous formulas exist (based on the Clausius-Clapeyron relation with additional correction terms), but they add complexity without meaningfully changing the result for everyday weather, cooling, and comfort planning. The one edge case worth knowing: right at or below freezing, the formula technically describes the frost point rather than the dew point, since ice forms instead of liquid dew — the numeric difference is small enough that most practical uses don't need to distinguish them.
Reading the Comfort Scale
The comfort tiers used here — Dry below 10°C, Comfortable from 10–16°C, Sticky from 16–20°C, Muggy from 20–24°C, and Oppressive above 24°C — are a simplified version of the ranges used by the U.S. National Weather Service. They're deliberately based on dew point rather than relative humidity, so the same tier means the same felt mugginess whether it's a 20°C spring day or a 35°C summer afternoon. The Reverse tab flips this around: instead of asking 'how muggy is it right now,' it answers 'how dry would the air need to be to feel comfortable at this temperature' — useful for evaluating a forecast or deciding whether a dehumidifier setting will actually get you into the comfortable range.