The dew point is the temperature at which air becomes fully saturated with water vapor and condensation begins to form. When air cools to its dew point, moisture in the air turns into liquid water, producing fog, clouds, or dew on surfaces.
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 cools, it eventually reaches a temperature where it can no longer hold all its water vapor. That temperature is the dew point. At that point, the excess moisture condenses into visible water droplets.
The gap between the current air temperature and the dew point is called the temperature-dew point spread. A small spread, say 2–3°C, means the air is close to saturation. Fog or low clouds are likely. A large spread means the air is dry and clear conditions are more probable.
Dew point is reported in degrees Celsius (°C) in aviation weather. You will see it in a METAR (a routine airport weather observation) listed directly after the current temperature, separated by a forward slash. For example, 18/16 means the temperature is 18°C and the dew point is 16°C.
Example in Aviation#
A pilot checks the METAR for their destination airport before departure. It reads OVC002 12/11, meaning an overcast ceiling at 200 feet and a temperature-dew point spread of just 1°C. The air is nearly saturated, and dense fog has reduced visibility to under a mile. The pilot recognizes this as a classic low-spread, high-moisture situation and holds on the ground until conditions improve.
Later, a second METAR shows the spread has widened to 8°C. The fog has lifted, ceilings have risen, and the flight can proceed safely.
Why It Matters#
Dew point directly affects flight safety. Fog, low clouds, and in-flight icing all trace back to air reaching or approaching its dew point. Pilots who understand dew point can anticipate deteriorating visibility, low ceilings, and carburettor icing risk before those hazards develop.
Student pilots learn to watch the temperature-dew point spread as one of the simplest and most reliable weather indicators available. A narrowing spread during descent or after sunset is a strong signal that fog may form quickly.
Key Takeaways#
- Dew point is the temperature at which air becomes saturated and condensation forms.
- A small temperature-dew point spread (2°C or less) signals high fog or low-cloud risk.
- Dew point appears in METARs right after the temperature, separated by a slash.
- Rising dew points or narrowing spreads during flight planning deserve close attention.
- Carburettor icing can occur even when dew point conditions seem borderline.