Nighttime cooling due to arid climate
The hottest temperatures of the season baked North Idaho this past week with triple digit highs throughout the region (official high temperatures for July 5 were not available at press time). Scorching temperatures, though not common, are usually seen in July and August if they occur. According to the records of local climatologist Cliff Harris, the all-time high temperature reached in Coeur d’Alene was 109 degrees, on Aug. 4, 1961. Coming in a close second, was the high temperature of 108 degrees on July 28, 1939.
Though afternoon temperatures in the triple digits can prove dangerous to those who must be out in the “heat of the day” for extended periods, folks in the Inland Northwest can usually count on some type of relief overnight. The advantage of living in a relatively arid climate, is that we have a pretty generous spread in our daily temperature ranges. In other words, despite temperatures soaring past 100 degrees during the daytime, nighttime lows can still drop into the 50s and 60s. For folks who don’t have air conditioning, this allows those households an opportunity to cool off. In areas where the air is more humid during the summer, such as in the Midwest or the South, nighttime provides much less relief from oppressive afternoon heat.
Daily high/low temperature spreads depend heavily on how much moisture is in the air. Dry/arid climates see the largest diurnal temperature swings. People who live in desert areas may find themselves scorched by day but chilled by night.
The science behind this has to do with something called “specific heat.” This is defined as the amount of heat energy needed to raise the temperature of 1 gram of a substance 1 degree Celsius. Bodies of water have a higher specific heat than land masses, so they heat up more slowly and, conversely, cool more slowly than an equivalent area of land. Coastal cities have temperate climates because the temperatures over the ocean do not change in a drastic fashion.
Unfortunately, in places where very warm temperatures couple with high dew points (air is very moist), the combination can be deadly. The “heat index” or apparent temperature takes the air temperature plus the humidity into account. Because humid conditions hamper the body’s natural cooling mechanisms (sweating), hot temperatures “feel” even hotter.
Folks in these humid climates also have to deal with the smaller temperature spread from day to night, meaning nighttime can also feel uncomfortably warm. Dallas, for example, is a very muggy city, not because it is located next to the ocean, but because strong low level southeasterly winds carry moisture in from the Gulf of Mexico. In June, Dallas had an average high of 90 degrees and an average low of 73 degrees – a 17-degree spread. Compare that with June numbers for Lewiston and Spokane, which had high/low spreads of 26 and 23 degrees, respectively. In the desert of Death Valley, Calif., the spread between the average high and low temperatures was 29 degrees.
During the deadly July 1995 heat wave in Chicago that resulted in more than 500 heat-related deaths, high temperatures were in the 80s, 90s and triple digits. What made the heat so dangerous though, was not just the consistently warm daytime temperatures, but the fact that nighttime temperatures on many occasions dipped only into the 70s or low 80s.
Coeur d’Alene will reach its peak average high temperature of 88 degrees at the end of this month and the beginning of August. During this time, our average low temperature will still be a comfortably cool 55 degrees.