There are urban myths and legends involving almost any topic imaginable, and meteorology is no exception. Too often in print and electronic media, and sometimes even from fellow meteorologists, there are misstatements that make me want to throw a shoe across the room. These are three of my “favorite” meteorological pet peeves.
“90 degrees and 90% humidity”
I can't tell you the number of times I have seen or heard a reference to a hot muggy day as having a temperature of 90 degrees and a relative humidity of 90 percent. I have frequently found this in works of fiction, right after “it was a dark and stormy night”, this might fall under the category of literary license but not sound meteorology. 
However, it seems to have become so ingrained in the hot, humid psyche that it’s almost a throwaway line.
In reality 90-90 conditions just do not occur naturally, at least not in the United States. For this to happen, the dew point -- the temperature that moist air must be cooled to for condensation to occur -- would have to be at least 87 degrees, and the highest dewpoints that that are seen in the United States are in the low 80s. And if you were to combine an air temperature 90 degrees with a dew point of 82 degrees the resulting relative humidity would “only” be 77 percent. (Which is incredibly uncomfortable!!).
Probably the only place on the planet where dewpoints exceed 87 degrees is in the vicinity of the Persian Gulf or Red Sea where water temperatures can reach the mid-90s. The world record dew point of 94°F has been measured there on the shore of Eritrea (formerly Ethiopia); but when this is combined with a typical temperature of 110°F the RH there is only 62%.
Jan Null, CCM
SF Weather Examiner
jnull@ggweather.com
Coming tomorrow … “Backward toilets down under”
Yesterday ... "Hurricane force winds"
Do you have a favorite weather-related pet peeve? Let me know.
Comments
I remember once long ago at the weather office in Denver I took a call from a woman who started talking about 90 degrees and 90 percent humidity and she got quite indignant when I said it was virtually impossible for this condition to occur in the US! When I was in the Navy, our ship was in the Gulf of Aden for almost a month in the summer of 1974. We had a thermometer and hygrometer on board and one day I calculated the dewpoint at 85, with a temperature of 101...out of sight of land! The ocean temperature at the time was 92!
Got something to say?
Examiner.com is looking for writers, photographers, and videographers to join the fastest growing group of local insiders. If you are interested in growing your online rep apply to be an Examiner today!