Heat Waves
"Heat Wave": unusually warm air in warm season, as determined by departure from normal regionally and seasonally
Floods and heat are the two leading causes of weather-related deaths in U.S.; more than 1000 people died from heat-related problems in 1995 (over 4700 in 1936; over 9500 in 1901)
Heat Wave requires lack of polar airmasses, with jetstream located well to the north of the affected area; usually an upper level ridge over region; Figure 26.2
Need strong solar heating at the surface in a cloud-free environment; usually associated with a nearly stationary surface high; hottest air on the western flank; Figure 26.3
Need dry ground, which eliminates the need for evaporation and reduces evapotranspiration from trees and vegetation; incoming solar radiation goes primarily into heating the air; creates "feedback" mechanism between drought and extreme heat
Need weak or no vertical mixing of hot (and moist) air near the surface; limits the mixing of cooler (and drier) air to mix down from aloft
Some of the most severe Heat Waves are characterized by strong stability in the form of a persistent shallow (only several hundred meters deep) inversion at altitudes of 1-3 km; warm air subsiding aloft under high pressure creates the strong upper air stability, further limiting vertical mixing of near-surface air
Heat Wave of July 1995 - Chicago and the Midwest; Figures 26.4-6
The Summer of 1980 - Texas and the Southern Plains; Figures 26.7-10
Heat Waves of the 1930s - The Dust Bowl; Figures 26.11-13
"Heat Index Chart:" gives an "apparent temperature" based on ambient temperature and relative humidity; Table 26.1 & Figure 26.1
"Heat Advisory:" issued by the NWS when, for any 24 hour period, the Heat Index is expected to equal or exceed 105 F for at least 3 hours during the day and remain at or above 80 F at night
"Excessive Heat Warning:" issued by the NWS when the Heat Index is expected to equal or exceed 115 F for at least 3 hours during the day and remain at or above 80 F at night
Heat Disorders & Safety Tips