Dew point

Dew point is the temperature to which air must be cooled for water vapor to reach saturation at its current pressure and moisture content. A higher dew point means more moisture is present in the air. The spread between temperature and dew point helps describe the likelihood of fog, clouds, and reduced visibility.

How it works

When air cools to its dew point, condensation can begin if nuclei and other conditions allow it. The temperature and dew point do not need to be identical for clouds to exist elsewhere, because lifting and mixing can change the local values. Dew point is not the same as relative humidity, although the two are related.

A METAR reports temperature and dew point together, often as 18/12, meaning temperature 18°C and dew point 12°C. The difference is called the temperature dew point spread. A small spread near the surface can indicate that fog or low cloud may form if the air cools further.

Example

A station reports 28/26 in its METAR. The two degree spread indicates moist air near saturation at the observation site. A later temperature decrease could bring the air to saturation, but the report alone does not forecast when or where fog will develop.

Why it matters

Dew point supports weather interpretation, especially when considering low clouds and visibility. It does not independently determine a ceiling or forecast conditions along an entire route. Use the observation time, forecast, terrain, and local effects together.

Review current reports from the NOAA Aviation Weather Center and compare them with the applicable TAF. This glossary entry is a weather reference, not a complete forecast.

Related terms and tools

• Ceiling
• METAR
• TAF