The "Dewpoint Depression" is rarely discussed in weather products for the public, but was the focus for a new record in Las Vegas, Nevada, on Monday. "Dew Point Depression" is the difference between the temperature, and the dewpoint - the measure of the amount of moisture in the air. Relative humidity is directly related to the dewpoint depression. Here's the statement from the National Weather Service in Las Vegas:
RECORD EVENT REPORT...CORRECTED
NATIONAL WEATHER SERVICE LAS VEGAS NV
445 PM PDT MON JUN 27 2011
...ALL-TIME RECORD DEW POINT DEPRESSION SET AT LAS VEGAS TODAY...
AT 432 PM PDT TODAY THE DEW POINT DEPRESSION AT MCCARRAN
INTERNATIONAL AIRPORT IN LAS VEGAS REACHED 129 DEGREES. THE DEW
POINT DEPRESSION IS DEFINED AS THE DIFFERENCE BETWEEN THE AIR
TEMPERATURE AND THE DEW POINT TEMPERATURE. IN THE CASE OF TODAY THE
AIR TEMPERATURE WAS 107 DEGREES AT 432 PM AND THE DEW POINT
TEMPERATURE WAS -22 DEGREES. THIS MADE FOR A RELATIVE HUMIDITY OF 1
PERCENT AT THE TIME. THE PREVIOUS ALL-TIME RECORD DEW POINT
DEPRESSION FOR LAS VEGAS WAS 120 DEGREES SET ON JULY 2ND 2007.
THE ABOVE INFORMATION IS PRELIMINARY AND IS SUBJECT TO A FINAL
REVIEW BEFORE BEING CERTIFIED BY THE NATIONAL CLIMATIC DATA CENTER.