Temperature-dew point spread
The difference between air temperature and dew point gives a simple indication of how close the air is to saturation. Pilots often watch the spread when considering the potential for fog, low cloud or changing visibility.
What dew point means
Dew point is the temperature to which air would need to cool, at roughly constant pressure and water-vapor content, to reach saturation. A higher dew point generally indicates more moisture in the air.
Calculate the spread
Subtract dew point from temperature using the same unit. If temperature is 18°C and dew point is 16°C, the spread is 2°C. If the spread narrows toward zero, saturation becomes more likely if other conditions support it.
A small spread is not a fog guarantee
Wind, cloud cover, terrain, moisture source, time of day and temperature trend all affect whether fog or low cloud actually forms. A close temperature-dew point pair is a clue, not a forecast by itself.
Nighttime cooling
On clear nights with light wind, surface temperature may fall toward the dew point. That can favor radiation fog when sufficient moisture is present. Strong mixing or cloud cover can change the outcome.
Cloud-base shortcut
The spread is also used in a rough convective cloud-base estimate. That shortcut assumes a simple vertical temperature and moisture profile and should not be confused with a reported ceiling.
Read two observations together
Suppose one observation reports temperature/dew point 18/16°C, and a later observation reports 17/16°C. The spread has narrowed from 2°C to 1°C because the air cooled; the dew point did not rise. That supports a different explanation from a narrowing spread caused by increasing moisture.
Before drawing a conclusion, compare observation times, visibility, sky layers and wind. Falling visibility or a developing low layer adds information that the temperature pair alone cannot supply. A small spread with good visibility is not itself an IFR report.
The FAA Pilot’s Handbook of Aeronautical Knowledge, Chapters 12 and 13 explains weather processes and observation products. Use the full current report and forecast when assessing conditions.
Read the trend
A sequence of METARs showing a shrinking spread can be more informative than one isolated observation. Pair that trend with visibility, sky condition and forecast information.
Related PilotDesk guides
Cloud-base estimate · Ceiling and visibility · METAR and TAF guide