40 °C at 60% humidity — dew point 30.8 °C

Humidity alone tells you little

Relative humidity says what share of the maximum the air is carrying — and that maximum swings hard with temperature. The same 60% means about 18 g of water per cubic metre at 30 °C but only 6 g at 10 °C. The dew point states that quantity as a single temperature, which reads far more directly.

Summer nights are measured in dew points

Below 16 °C it feels pleasant, above 20 °C sticky, above 24 °C oppressive. Air temperature falls at night but the dew point barely does — which is exactly why a humid night stays unbearable.

Nearby cells

Find it by temperature and humidity

Cool the air to the dew point and it condenses

The dew point is the temperature at which water starts to condense. When the pre-dawn air cools to it, dew forms on the grass and fog appears. The same reason beads water on a cold glass: the air touching it fell below its dew point.

How to read this

• The dew point is how far you would have to cool this air before water condenses.
• At 100% humidity the dew point equals the air temperature; lower humidity pulls it down.
• The narrower the gap between air and dew point, the muggier it feels — zero means fog.
• At the same humidity, ten degrees warmer means roughly twice the water in the air.