Dewpoint is the temperature at which air becomes fully saturated with water vapor and condensation begins to form. When air cools to its dewpoint, moisture in the atmosphere turns into liquid water, fog, or clouds.
How It Works#
Air holds water vapor, but only up to a limit. That limit depends on temperature. Warmer air holds more moisture. Cooler air holds less.
When air temperature drops to the dewpoint, the air has reached 100% relative humidity. Any further cooling causes water vapor to condense into visible droplets. This is how dew forms on grass, fog settles over an airport, and clouds build overhead.
The gap between air temperature and dewpoint is called the temperature-dewpoint spread. A small spread (3°C or less) means the air is close to saturation. Pilots treat a tight spread as a direct warning: fog or low clouds may form soon.
Dewpoint also helps predict cloud base height. A rough rule of thumb: for every 1°C of temperature-dewpoint spread, the cloud base rises about 400 feet. So a spread of 10°C suggests a cloud base near 4,000 feet AGL (above ground level).
Example in Aviation#
A pilot at a regional airport checks the METAR (routine aviation weather report) before departure. It reads: temperature 12°C, dewpoint 10°C. The spread is only 2°C. The pilot knows fog or low stratus clouds could form quickly, especially around dawn when temperatures typically drop further. She delays the flight and monitors updated METARs before making a go/no-go decision.
This small detail on a weather report directly shapes her safety decision.
Why It Matters#
Dewpoint is one of the most practical weather tools a pilot has. It signals when visibility-reducing fog, low clouds, or precipitation is likely. Ignoring a tight temperature-dewpoint spread has contributed to accidents where pilots flew into instrument conditions without expecting them.
For student pilots, understanding dewpoint builds a foundation for reading weather reports accurately. For instrument-rated pilots, it supports sound decision-making when evaluating approach minimums and alternate planning.
Key Takeaways#
- Dewpoint is the temperature at which air saturates and condensation begins.
- A temperature-dewpoint spread of 3°C or less signals high fog and low cloud risk.
- Dewpoint appears in METARs and is reported in degrees Celsius.
- Use the spread to estimate cloud base: roughly 400 feet per 1°C of spread.
- Monitoring dewpoint trends over time reveals whether conditions are improving or deteriorating.