The World Meteorological Organization is specific in its definition by stating that a heat wave is when the daily maximum temperature for more than five consecutive days exceeds the average maximum temperature by 9 degrees Fahrenheit. For Grand Rapids that would be a high of 92° in most of July.
Heat waves are caused by very hot, stagnant air masses. Regions that suffer under intense hot spells are usually dominated by a surface high-pressure system with a mid-tropospheric ridge aloft. Dew points are also high, and to compound matters, wind speeds are often low. Clear or partly cloudy skies allow intense solar energy to further heat the ground and the air mass.
High humidity and stagnant air reduce the body’s ability to cool down through sweating. Lives are endangered when these conditions persist day and night for several days. Each summer in the United States, approximately 175 to 200 deaths are attributable to heat waves. Most of these deaths occur in cities, particularly northern cities.
The National Weather Service issues excessive heat warnings within 12 hours of the onset of extremely dangerous heat conditions. This warning is generally issued when the maximum heat index temperature is expected to be 105 degrees or higher for at least two days and nighttime air temperatures will not drop below 75. These criteria for issuing a warning vary across the country, especially for northern regions which are not used to hot, humid conditions.
Most years Grand Rapids doesn’t have many heat waves as even in the summertime we have cold fronts coming through ever 3 days or so. But there have been years where that is not the case. I will use the temperature of 90 as a breaking point for periods of a Heat Wave for Grand Rapids. Now the most 90 days happened in the summer of 1988 when 37 days reached 90 or better at GRR. Next up is 1931 with 34 then 32 in 2012 and 31 in 1921 all other years so far have had fewer than 30. For Grand Rapids I will use 90 or better as a Heat Wave with 88 and 98 as pre Het Wave. The longest stretch of 90 or better happened way back in 1901 when Grand Rapids recorded 11 days in a row of 90 or better. That Heat Wave started on July 13th with a H/L of 88/59 but the next day it shot up to 94/63 the hottest days were a 97 on the 17th and a 96 on the 20th It broke with a 92/70 on the 24th on the 25th it still reached 88 and then just 82 on the 26th before 2 more 90 days on the 27 and 28th The hottest Heat Wave for Grand Rapids was in July of 1936. July 1936 had a string of 8 days in a row of 90 or better, but what make that stand out is that of the 8 days 6 were 100 or better with one day at 99 and another day at 98. The hottest day was the reported record high for Grand Rapids of 108 on July 13th the day before was 106. During the 1936 Heat Wave there were 7 nights in a row when the temperature did not fall below 76. Now as hot as it was in Grand Rapids just to our south in Kalamazoo it was even hotter. There they had 12 days in a row of 90 or better and of those 9 in a row were 101 or better. That was one heck of a Heat Wave. In more recent times 2012 July started out with a 7 day Heat Wave (including official highs of 101 and 104) then 3 days of highs of 86 then 89 then yet another Heat Wave of 7 days. So that in 2012 of the first 18 days of July 14 were over 90.
The hottest July 4th in GR history is 100 and that happened in 1911 and 1897. In 2012 the high was 99 and that was the 2nd hottest July 4th at GR. The coldest maximum was a cool 64 in 1970 the record low for the date is 45 in 1972. Last year the H/L was 90/71. At this time it is still 76 here at my house. I hope everyone has a safe 4th of July.