Even before sunrise this morning, we had heat indices in the 80s and 90s across the area while temperatures were uncomfortably warm, but not yet downright hot. It was the other half of that heat index equation, the humidity, that was making things so unbearable. In fact, last night just before sunset we registered some of the highest dew point temperatures I’ve ever seen and certainly the highest we’ve recorded this year. For instance, at 8pm Sunday night, Rochester had a dew point of 81° and Austin had a mind boggling 84°! That is absolutely unheard of! This is more humid than the Gulf Coast usually gets in the summer! Dew points in the 70s are typically the highest most places get in the summer. We get to that level of humidity a handful of days annually, but this spell is extra significant. The main factors going into our additional influx of moisture in the air is recent heavy rains combined with corn and soy bean crops that are reaching maturity and thus giving off moisture through a process known as evapotranspiration. A viewer posed the question “what’s the highest dew point ever recorded?” Good question, but not one that is easily answered. It’s not a statistic that commonly shows up in record books except in certain National Weather Service reporting sites. We do know that Rochester has reached a dew point of 83° in its history (back in the 80s) and came within a degree of tying that mark last July. The summer of 2010, incidentally, was one of the all-time most humid on average in all of Rochester’s history, so it wasn’t just your imagination…it really was uncomfortable! It’ll be interesting to see what this week’s oppressive humidity does to our record books in that regard. I did find this passage on USA Today’s “Weather Extremes” webpage. It’s a Q and A talking about the highest dew point ever measured in the world and it makes me glad I don’t live in the Persian Gulf!
Q: What’s the highest dew point ever recorded?
A: The dew point temperature, which is based on how much water vapor is in the air, is a good measure of how humid it feels. A dew point above 70°F is quite humid. Very high dew points are often found near shallow, subtropical seas. Dhahran, Saudi Arabia, on the Persian Gulf, recorded a dew point of 95°F on July 8, 2003. In the USA, the highest dew points (above 80°F) occur near the Gulf of Mexico and in parts of the upper Mississippi Valley.
See our page on Understanding humidity for more about the dew point temperature.
(Answered by meteorologist Bob Henson, a writer at the University Corporation for Atmospheric Research and the author of The Rough Guide to Weather, June 9, 2005.)
Look at those numbers! The heat index is around 150 in Dhahran on this chart. The highest ever heat index in Dhahran was 172 in that 2003 situation.
At any rate, we’re looking at some incredibly humid (and hot) weather for the next couple of days. Heat indices will commonly be in the 100 to 120 range this week and an Excessive Heast Warning is still in effect. However, the humdity may drop off slightly on Wednesday while temperatures remain quite warm, mainly because of some extra mixing in the atmosphere from gustier winds. Dew points will then be in the 70s instead of the lower 80s, basically. Later in the week, a couple of cold fronts will push through the area, dropping the temperatures a bit, and yes, drawing in some slightly drier, less moisture laden air.
Posted under heat
This post was written by tschmidt on July 18, 2011