The above chart shows the 120-year average high temperatures in Birmingham, AL (black), along with the 1-standard deviation bands (where about 70% of all high temperatures should fall on a given date). Through most of 2019, temperatures stayed within these bands most of the time, so it was a normal year. Even the 100 degree days in August were not too far outside the normal. Note how the bands are tightly packed in the summer, meaning there is not that much variance from the normal in the Summer. The bands get much wider in the cold season. In late December, for example, high temperatures may be as low as the 20s or as high as the 70s!
But then, in September, we went into a very dry pattern, no cold fronts came through, and temperatures stayed very hot. Droughts and heat waves go together…the sun heats the ground which then heats the air, and if there is no water to evaporate, all of the sun’s energy goes into heating.
September 2019 was the 2nd warmest September on record with an average high of 94.5 and average temperature of 82.1. Only Sep 1925 was hotter. Sep 2019 was the 21st warmest month on record, period, beating out dozens of Julys and Augusts.
Using Coleman and Knupp, LLC’s 7-day moving average for all days going back decades, the expected distribution of high temperatures (and low temperatures) can be shown for any day of the year, in any city, with expanded precision. Below is the distribution for Oct 1, yesterday, for Birmingham. It is close to a normal distribution, with a median of 81 degrees, mode 83 degrees, standard deviation 7 degrees. We hit 99 yesterday, something that had never happened during the period Sep 29-Oct 4 before, so obviously it was a record high temperature.
But, using climate data from any site, we at Coleman and Knupp, LLC can custom design the probabilities of exceeding or going below any high or low temperature at any location, and the probability of given amounts of rain. This can be useful for planning weddings and other outdoor events beyond 2 weeks out, when we have essentially no weather predictive capability.
The good news is that a cool front may touch off come clouds and showers this weekend, cooling us off a few degrees, then a more significant cold front will sweep through early next week, making it feel a bit more like October. And, as the sun’s angle and the length of day are both decreasing rapidly now, the Northern Hemisphere has to cool off and more cold fronts will come to the eastern U.S.! Below I show the length of day at Birmingham across the year (solid), and the rate of change (dashed). Right now, our days are just under 12 hours long, and we are losing about 2 minutes of daylight every day. The Geochron-style image shows that the sun is illuminating more of the Southern Hemisphere than the Northern one now, too.
Dr. Timothy A. Coleman
Coleman and Knupp, LLC