Over the last few days, I’ve blogged about heat index values of 130 or higher in parts of Iowa and Minnesota and dew points (a measure of humidity) over 80 (any dew point over 70 means really high humidity). Has the District seen heat and humidity readings reach these levels?

It’s established that the hottest temperature on record in D.C. (for any date) is 106 and occurred July 20, 1930. But according to Steve Zubrick, Science Operations Officer at our local National Weather Service Office, dew point and heat index records for Washington, D.C. are incomplete.

The best we can say is we know the heat index has reached at least 122 and that we’ve had a dew point as high as 82. So the heat index of 116 forecast for Friday is not uncharted territory.

Here’s an excerpt from what Zubrick emailed me:

... we REALLY cannot answer the question ‘what is the record heat index is for DC?’

However, going back to 1977...for Washington DC...

Based upon hourly surface observation records, A high HI [heat index] value for Washington DC was 122F recorded July 16, 1980, with the 4 PM local daylight time observations. I am uncertain if this represents a record owing to the incompleteness of our observation database.

When we hit that 122 heat index, the air temperature was 103 and dew point 78. Yikes!

Zubrick also noted that last year (2010) our highest heat index occurred on July 24, when it reached 108F at DCA for 3 consecutive hours between 3 and 5 p.m. That was the highest heat index since a value of 109 on August 8, 2007 (101 air temperature/70 dew point).

As for dew points, Zubrick said that from a limited sample dating back to 1962: “... it appears there are several days the dew point reached at least 82F. I suspect if one were to go back and look through all of the records, 82F dew point would be close to the max in D/C. I’d hazard a guess that 83F or 84F dew points have probably occurred in the past.”