What is dew point?

The definition, and the part that gets skipped

Air can hold water as invisible vapour, and there is a limit to how much. The limit depends almost entirely on temperature: warm air can hold a great deal, cold air very little.

The dew point is the temperature at which today's air would hit that limit.

Most explanations stop there, and stopping there is why the idea does not stick. The part worth adding is this: because the limit is fixed by temperature, naming the temperature at which air saturates is the same as naming how much water it contains. The dew point is a temperature that is secretly a quantity.

That is the whole trick, and everything useful follows from it.

Why it beats relative humidity

Relative humidity is a percentage of a moving target. It tells you how close the air is to full without telling you how big "full" is — and full varies by a factor of about seven between a freezing morning and a hot afternoon.

Watch what happens to one unchanging parcel of air as the day warms:

The humidity reading falls from 100% to 37% and nothing has happened to the air. The dew point, which is what actually describes it, does not move at all. This is why humidity climbs overnight without a drop of water arriving, and it is the single strongest argument for reading the other number.

The threshold you can act on

The second useful property: the dew point is a real, physical temperature, and anything colder than it gets wet.

Not "might get damp" — gets wet, reliably, as a matter of physics. That covers a car roof at dawn, a drink out of the fridge, a camera lens carried from air conditioning into July, a window on a cold night, and the inside of a tent. Whether each of those fogs is a single comparison: is this surface colder than the dew point?

That is what the condensation calculator does, and it is the mechanism behind most of the situations on this site.

What each range feels like

Because the dew point is a direct measure of water content, it maps onto comfort far better than a percentage does. Forecasters use a rough scale:

• Under 50°F — dry. Pleasant, and things dry quickly.
• 50–60°F — comfortable. Most people never think about the air at all.
• 60–65°F — noticeably sticky. This is where people start describing the humidity rather than the temperature.
• 65–70°F — muggy and uncomfortable.
• Above 70°F — oppressive. Nights stay warm, because the air cannot cool past its own dew point.

The full version, with both units and a printable grid, is on the dew point chart. It is worth being clear that this scale is forecaster convention rather than a formal standard — there is no ASHRAE or ISO document behind it.

Where the word comes from

Literally: the point at which dew forms. On a clear, calm night the ground radiates heat to the sky and cools faster than the air above it. When grass, cars and railings fall below the dew point of the air touching them, water condenses onto them.

Which is why dew appears on a night that never got cold — the air never reached the dew point, but the grass did.

Working it out

From an air temperature and a relative humidity, the standard method is the Magnus-Tetens approximation:

The dew point calculator runs exactly this, and there is a worked example if you want to follow it through by hand.