Dew Point Records During Current Heat Wave
July 23, 2011 6:28 AM Subscribe
The latest heat wave set dew point records
posted by Seymour Zamboni (114 comments total)
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at Minneapolis-St. Paul, where the dew point reached 82 degrees on July 19th. Similar dew points in the low 80's were observed at numerous east coast locations on July 22. The highest dew point ever recorded was 95 on July 23, 2003, along the Persian Gulf at Dhahran, Saudi Arabia. You will hear people claim on the most torrid days that the temperature is 100 degrees and the humidity is 100%. That has never happened in the USA as it would require a dew point of 100 degrees. Learn more about the difference between dew point and relative humidity here
. And this table
shows the relationship between air temperature, dew point, and relative humidity.
My southern friends will often claim conditions like "it is 100 degrees with 100% humidity. Or similarly, 90 degrees with 90% humidity. These claims are false, at least here in the USA. Don't get me wrong. I lived in the south. I know how uncomfortable hot and humid days can be, day after day after day. But the numbers associated with those most extreme levels of discomfort are not what many people typically claim. This stems from a lack of understanding of dew point and relative humidity. Here is a simple example. Yesterday at 3:00 PM, Wilmington Delaware had an air temperature of 100 degrees and a dew point of 81 degrees. That means the relative humidity was only 54%. Note that a dew point of 81 is extreme! That is approximately the most humid it ever gets anywhere in the USA at any time of year. The 54% tells us that 100 degree air could actually hold much more moisture. Another way to think about it is that if that 100 degree air was cooled to 81, then relative humidity would rise to 100%. So it is possible to have an air temperature of 81 with 100% humidity. But not an air temperature of 100 with 100% humidity.