A unique combination of dry and mild

We received a question recently about how low the relative humidity (RH) had ever gotten at the Raleigh-Durham airport, and with some assistance from the State Climate Office found that the record for that particular piece of weather history occurred quite a while back, with the RH dipping to 5% for two hourly observations taken at 1 PM and 2 PM EST on December 4, 1960.

It was interesting to see what kind of weather pattern, and what time of year, that particular record would have been set, since it really requires a bit of an unusual combination of very dry air, most typical of cold wintertime airmasses, together with rather mild temperatures, more typical of spring or fall (when the air usually contains more water vapor). The tricky part of all this, of course, is that relative humidity does not depend only on how much moisture is in the air, but also on how warm or cold the air is. The combination of those two factors for that particular day in December 1960 was such that at 1 PM the temperature was 61 degrees, but the dew point was a very dry -9, while an hour later the temperature and dew point had risen to 63 and -7, respectively. Since the dew point increased a couple of degrees, we see that there was a bit more moisture in the air at 2 PM, but since the temperature went up as well, the RELATIVE humidity did not change.

To get that combination involved a huge surface high pressure center that stretched from New England into the Gulf of Mexico (first image above) and a sharp upper level ridge (2nd image) that was sitting just to our west the same day. The combination produced strong subsidence that produced bright skies, but also left the dew points low even as temperatures recovered to rather mild afternoon values.

It's worth noting that while that day brought the lowest relative humidity so far observed at RDU, the lowest dew point (and therefore the least water vapor in the air, or "absolute humidity") was recorded on January 21, 1985 when the dew point fell to -28 degrees at 2 AM. At that time though, it was a very cold 5 degrees below zero, making the relative humidity 31%.

We've had some rather dry air move into the region over the past couple of days, but nothing approaching those extremes, and it appears we'll turn both warmer and somewhat more humid in the next 2-3 days, with enough moisture for at least a shot at some light rain on Thursday.