WRAL WeatherCenter Blog

Wet bulb temperature vs dew point differences explained

Posted November 3

Is wet bulb temperature the same as dew point?

Mike Moss: In a saturated air parcel (with relative humidity 100%) the wet bulb, dry bulb and dew point temperatures are all the same. In a drier parcel of air, the dry bulb is just the air temperature which will be the warmest of the three variables. The dew point will be the lowest number, and the wet bulb will fall between those two.

If you were to cool that drier air without adding or removing any water vapor, the dew point would remain constant while the dry bulb and wet bulb temperatures would fall (this happens in the atmosphere when air near the surface cools at night to the point that fog or dew develops, or when an air parcel rises and cools via expansion to the point where saturation occurs and clouds form).

If you were to add water vapor vapor (but not by evaporation directly within the air parcel), the dew point and the wet buld would climb, while the dry bulb temperature would stay the same.

If you were to evaporate directly within the air parcel enough water vapor to raise the relative humidity to 100%, the wet bulb would stay the same, while the dew point would climb and the dry bulb temperature would fall (this is a common process in the atmosphere that often causes rapid cooling when precipitation falls into an initially dry layer of air).

Full question from John C: How does wet bulb and dry bulb temperatures relate to dew point?

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