Oppressive Dew Points - July 29-30, 1999

July 29 and 30 will be remembered as two of the most humid days in Minnesota history. Dew point temperatures in the mid to upper 70's were prevalent across the state, with many locations in southern Minnesota reaching values exceeding 80 degrees.

The dew point is the temperature to which air must be cooled to reach saturation. Dew point temperatures are an excellent measure of the amount of water vapor in the air. Dew point temperatures in the 70's are considered tropical and dew points greater than 80 degrees are extremely rare.

At 1100 AM on July 30 the dew point temperature at the Twin Cities International Airport reached 81 degrees. This is the highest dew point temperature ever recorded in the Twin Cities. The previous record was 80 degrees, recorded as recently as July 22, 1999.

The high dew point temperatures, in conjuction with sweltering air temperatures in the mid to upper 90's, combined to produce heat index values exceeding 110 degrees in many southern Minnesota communities. The heat index is a measure of the combined influence of air temperature and dew point temperature on human comfort. Heat index values in excess of 105 degrees are considered to be dangerous or even life threatening. The Twin Cities heat index reached 115 degrees during the afternoon of the 30th. The highest heat index value ever recorded in the Twin Cities is believed to be 119 degrees, occuring on July 11, 1966 (our thanks to climate historian Charles Fisk).

Return to the Climate Journal

mcwg@soils.umn.edu
URL: http://climate.umn.edu/doc/journal/sticky99.htm
Last modified: July 30, 1999