Asked by Sofia
Can the dew point temperature be greater than the air temperature?
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Short answer: Not for a stable, unsaturated parcel of air. The dew point Td is the temperature to which air must be cooled (at constant pressure) for the water vapor in it to reach saturation. For unsaturated air Td < T, and for saturated air Td = T.
If you see Td > T it means the actual vapor pressure is greater than the saturation vapor pressure at the air temperature — i.e. the air is supersaturated (relative humidity >100%). Supersaturation is usually transient or requires special conditions (very few condensation nuclei, rapid cooling) and normally causes condensation (fog, cloud, dew) which brings the vapor pressure back down so Td ≤ T. Measurement error or reporting differences (instrument location, sensor lag) can also produce an apparent Td > T.
If you see Td > T it means the actual vapor pressure is greater than the saturation vapor pressure at the air temperature — i.e. the air is supersaturated (relative humidity >100%). Supersaturation is usually transient or requires special conditions (very few condensation nuclei, rapid cooling) and normally causes condensation (fog, cloud, dew) which brings the vapor pressure back down so Td ≤ T. Measurement error or reporting differences (instrument location, sensor lag) can also produce an apparent Td > T.
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