WRAL WeatherCenter Blog

I just read the current conditions @ WRAL studio as temp:55; dewpoint:56; RH:104% How can the dewpoint ever be higher than the actual temperature?

Posted December 21, 2008

MIKE MOSS SAYS:    Grant,     In an airmass lacking surfaces that support condensation of water vapor molecules (called condensation nuclei) relative humidity can rise as high as 400% or so (a considerably higher dew point than temperature) before "homogeneous nucleation" occurs and droplets form.

However, that is a bit of a digression and theoretical abstraction when it comes to the real atmosphere near the roof top here at the station. When condensation nuclei are abundant, in a practical sense the temperature can not fall more than a fraction below the dew point without an almost imediate adjustment of either the dew point upward or the temperature downward to a saturation level where they are essentially equal and the relative humidity is 100%.

Keep in mind, though, that we are measuring these quantities with instruments that have levels of accuracy and precision tolerances that are within +/- a degree or two, so that if the temperature and RH measurements are slightly off or time lagged from each other a bit, there could, for example, be a brief error in which RH is measured or calculated to be slightly over 100 percent and/or the dew point is measured or calculated to be a degree or so above the temperature. Even so, it's a bit surprising that the instrument software doesn't have a "trap" that will not allow the readings you noted.

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