ReckonBoxWhat the dew point measures
The dew point is the temperature the air would have to cool to for its water vapour to start condensing into dew, fog, or droplets, which makes it a direct measure of how much moisture is actually in the air. At 85F with 60% relative humidity, the dew point is about 70F.
Think of the air as a sponge. The dew point tells you how wet the sponge is in absolute terms, while relative humidity only tells you how full it is compared to what it could hold at that moment. The closer the dew point sits to the air temperature, the more saturated the air, and the heavier and stickier it feels.
Why forecasters trust dew point over humidity
Relative humidity is a percentage of a moving target, since warm air holds far more moisture than cold air, so the same reading means different things at different temperatures. The dew point is an absolute figure that compares honestly from one day to the next.
A winter morning at 40F and 90% humidity has a dew point near 37F, and it feels crisp and fine. A summer afternoon with a 70F dew point feels oppressive whether the humidity gauge says 60% or 80%. That is the whole case for dew point: 90% humidity sounds alarming until you notice the air is cold and bone-dry in absolute terms. Meteorologists quote dew point precisely because it does not play that trick.
The math behind it
This tool uses the Magnus formula, the standard approximation, with constants a of 17.625 and b of 243.04C. It converts your temperature to Celsius, builds an intermediate term from the temperature and the relative humidity, solves for the dew point, and converts the answer back to Fahrenheit. Across the normal weather range of -40 to 50C the formula carries an uncertainty of about a third of a degree Celsius, which is far tighter than the difference you could feel.
Reading the comfort scale
Dew point maps to comfort more reliably than any humidity percentage, and the bands are easy to carry in your head.
| Dew point | How it feels |
|---|---|
| Below 50F | Dry, even a touch dry for some |
| 50 to 60F | Pleasant, the range most people prefer |
| 60 to 65F | Getting sticky |
| 65 to 70F | Unpleasant and muggy |
| Above 70F | Oppressive, and heavy enough to be dangerous above 75F |
The gap between the air temperature and the dew point has its own name, the dew point depression, and it reads as dryness. A wide spread means dry air with room to cool before anything condenses. A spread near zero means the air is saturated, which is exactly when fog forms, as the temperature falls to meet the dew point overnight. At 85F with a 70F dew point that spread is 15 degrees; on a foggy dawn it is close to nothing.
Frequently asked questions
What is the dew point? The dew point is the temperature the air would have to cool to for its water vapour to start condensing into dew, fog, or droplets. It is a direct measure of how much moisture is actually in the air. At 85F with 60% relative humidity the dew point is about 70F. The closer the dew point is to the air temperature, the more saturated the air and the muggier it feels.
Why is dew point better than relative humidity for comfort? Relative humidity is a percentage of a moving target, since warm air can hold far more moisture than cold air, so the same 90% means very different things at 40F and at 85F. The dew point is an absolute figure, so it compares honestly across days. A 90% humidity morning at 40F has a dew point near 37F and feels fine, while a 70F dew point feels oppressive whatever the humidity reads.
How is the dew point calculated? This tool uses the Magnus formula, the standard approximation, with constants a of 17.625 and b of 243.04C. It converts your temperature to Celsius, computes an intermediate term from the temperature and relative humidity, solves for the dew point, and converts back. The formula carries an uncertainty of about a third of a degree Celsius across the normal range of -40 to 50C, which is plenty accurate for weather.
What dew point feels comfortable? A common comfort scale runs like this. Below 50F the air feels dry, even a touch dry for some. From 50 to 60F is the pleasant range most people prefer. From 60 to 65F it starts getting sticky. From 65 to 70F is unpleasant and muggy. Above 70F is oppressive, and above 75F the air is heavy enough to be dangerous during exertion. The dew point, not the humidity percentage, is what these bands track.
What is dew point depression? Dew point depression is the gap between the air temperature and the dew point, and it is a quick read on how dry the air is. A large spread means dry air with room to cool before condensing, while a spread near zero means the air is saturated, which is why fog forms when the two meet. At 85F with a 70F dew point the depression is 15 degrees; on a foggy morning it is close to zero.