Relative humidity is a percentage of a moving target. It measures how close the air is to saturation, and saturation depends on temperature — so the same air warmed by ten degrees reports a much lower humidity without losing a drop of water.
Dew point measures the water itself: the temperature at which that air would have to be cooled to start condensing. It does not move when the temperature does, which makes it the number that actually predicts whether a day feels sticky.
What does a dew point feel like?
The scale is narrow and reliable.
| Dew point | How it feels |
|---|---|
| Below 10°C | dry, pleasant |
| 13–16°C | comfortable |
| 16–18°C | noticeably humid |
| 18–21°C | uncomfortable |
| 21–24°C | oppressive |
| Above 24°C | genuinely unpleasant |
Those bands hold regardless of the air temperature, which is what makes the measure useful. A dew point of 22 is oppressive at 28 degrees and oppressive at 33.
The mechanism is sweat. Cooling by evaporation works only if the air can accept more water, and a high dew point means it cannot — so the body’s main cooling system stops working just when it is needed.
What is a heat index?
An estimate of how hot the air feels once humidity is accounted for — the temperature that dry air would have to reach to feel the same. It combines temperature and humidity into one apparent figure.
It is only defined over part of the range. Below roughly 27 degrees the correction is negligible and the formula is not meaningful, which is why a heat index quoted for a mild day is an artefact rather than a reading.
It also assumes shade and light wind. Direct sun adds substantially to the felt temperature, so an outdoor figure in full sunlight runs well above the quoted index.
Does wind chill cool objects?
No, and this is the most useful thing to know about it. Wind chill describes how quickly a body loses heat, so it applies to living things and to nothing else.
A pipe exposed to wind at minus two degrees cools to minus two faster than it would in still air, and it does not go below minus two. Water does not freeze at a wind chill temperature it would not freeze at without wind — the wind changes the rate, not the destination.
The formula in common use dates from a revision in 2001, which replaced an older version derived from an experiment freezing water in plastic cylinders rather than measuring people. The current one is based on human heat loss and gives noticeably milder figures.
When does wind chill apply?
Below about 10 degrees, with wind above a light breeze. Outside those bounds the calculation is not defined, which is why a wind chill figure for a summer day does not exist rather than being small.
The practical value is in the frostbite risk it implies. At sufficiently low wind chill values exposed skin freezes in minutes rather than hours, and that timescale is the actionable part of the number.
Clothing changes everything and is not in the formula. The figure describes exposed skin, so it is a warning about what is uncovered rather than a forecast of how you will feel dressed for it.
Why do these numbers differ from a thermometer?
Because they are not temperatures. Both are indices — constructed numbers expressed in degrees so that they can be compared to a temperature, describing a physiological effect rather than a physical state.
That is why they are quoted as "feels like" rather than as a reading. Nothing measures them; they are computed from measurements of temperature, humidity and wind.
Both also assume an average person in average clothing. Individual response varies enough that the index is a guide to conditions rather than a prediction about you.
What should you do with the numbers?
Treat them as thresholds for behaviour rather than as readings to note. A high dew point is the signal to slow down, drink more and rest in shade, because the body’s cooling has less margin than the air temperature suggests.
At the cold end the actionable figure is exposure time. A wind chill low enough to freeze skin in ten minutes is a different instruction from one that merely feels bitter, and the two can occur at the same air temperature with different wind.
Questions people ask
Does a fan help in high humidity? It helps while sweat can still evaporate. Once the dew point is high enough, moving air does much less.
Why does the same temperature feel different in two places? Humidity, wind and sun exposure, in roughly that order of effect.
Can dew point be higher than temperature? No. At equal values the air is saturated, which is fog or rain.
Does air conditioning lower dew point? Yes, by condensing water out. That is the water dripping from the unit.
Is 100 per cent humidity always rain? It means saturation at that temperature. Fog is the usual result.
Why is cold air dry indoors? Cold outdoor air holds little water. Heating it raises capacity, so relative humidity falls even though the water content has not changed.
Use the measure that matches the question. The dew point calculator gives the number that predicts how a day feels, the heat index calculator combines heat with humidity, and the wind chill calculator covers the cold end and the exposure risk that goes with it.