Weather and Climate

Should 92°F in October Be Considered a Heat Wave?

Yesterday we hit 91°F in Atlanta. They day before that we hit 90°F and we’re expected to reach 92°F today. In July these temperatures would hardly be impressive – in fact they would only be one to two degrees above the normal high temperature for the middle of July. But it’s October. The normal high temperature, based on average temperatures observed between 1980 and 2010, is only 77°F for October 1 – 3. That 13 to 15 degree abnormality is similar to what we saw during numerous stretches last month. We hit 95°F on both September 14th and 15th (12°F above average) and hit 94°F on the 19th and 20th (13°F above average). Temperatures in the month of September averaged out to 80.9°F, which was 7.5°F above average. This easily surpassed 1980 as the warmest September on record in Atlanta.

“Heat Wave” is Reserved for Uncomfortable Weather

So why isn’t the current hot streak considered a “heat wave? And why isn’t a string of 75°F days in July considered a “cold snap” or “cold streak”? It turns out the terms are largely used in a subjective manner based on deviations from normal temperatures, geography, and factors like humidity and wind. The terms “heat wave” and “cold snap” are really meant to identify a period of unusually uncomfortable weather, not necessarily abnormally hot or cold temperatures. This makes sense considering a 110°F day in Phoenix in July would be nothing new while the same temperature in Atlanta would likely shut down the city. Similarly a 13°F day in Minneapolis in January would be a minor inconvenience, but the same circumstances in Georgia would be a significant health concern.

It doesn’t appear that NOAA and the National Weather Service actually use the terms “heat wave” or “cold snap” given that there’s no precise definition. Instead they issue excessive heat or cold warnings when temperatures, combined with other factors (wind, humidity), make the weather abnormally uncomfortable and hazardous for people living in a certain place at a certain time of year.  So “heat wave” and “cold snap” are largely used by the news media.

While these terms are great at informing us of weather conditions that pose health hazards, they aren’t great tools for informing us of abnormal temperatures. But maybe we need terms for such situations. Humans aren’t great at recognizing long-term trends. We can hardly remember the weather from last week let alone the weather from 10 years ago. This is particularly true if the weather we’re trying to remember wasn’t particularly uncomfortable. Earlier this year we wrote about why 2017 was the hottest year on record in Atlanta despite there being no “heat waves.” We largely take our cues from the media in determining what is unusual weather, but the National Weather Service generally highlights extreme, uncomfortable weather. The media follows suit. So a 75° day in July ends up being a “nice day” while a 105° day in July ends up being a heat wave with excessive heat warnings. Temperatures on both such days, though, would deviate about 15° from normal.

Given that we had no excessively hot summer days and thus no warnings,, it’s easy to question why last year was the warmest on record. It ended up being the warmest due to abnormally high temperatures in winter months and streaks of days with temperatures a few degrees above normal.

Maybe We Need Advisories for Temperatures That Are Simply Abnormal

An advisory issued by the National Weather Service on the 75° day in July or a 60° in January may draw our attention to abnormal weather. Enough of those advisories may then draw our attention to long-term trends. While the news media has done a much better job of reporting abnormal conditions that aren’t accompanied by official warnings, an official advisory from the National Weather Service would force the issue. Right now it’s debatable as to whether a string of 91° days with low humidity in October in Atlanta is considered a heat wave. This is unusual weather for this time of year, but it’s hardly unusual weather for Atlanta. Perhaps it’s uncomfortable, but it’s not a comfort level we’re unaccustomed to and thus need a warning to take extra precaution. A simple advisory stating that temperature are unusually high would at least prompt residents and the media to take notice. Perhaps there could also be advisories issued not just for several days of significantly abnormal temperatures, but advisories for periods of time where temperatures have consistently been a few degrees above or below normal.

This could go a long way in helping people trust and understand data. Ultimately it comes down to the media pointing out trends that are generally unnoticeable to us in our day-to-day lives, but that point to longer-term changes. WSB Meteorologists Karen Minton and Glenn Burns have done a great job putting the current temperatures into perspective. Karen Minton even refers to the string of days with abnormally high temperatures in October 1954 as a heat wave.




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