Twin Cities Summer Glory Index

Table of Summer Glory Index points and penalties

Summer in Minnesota is precious. The kids are out of school, the snow is off the ground, the lakes are free of ice, the landscape is green—at least it’s supposed to be—and the sun is as high in the sky as it gets. We want to get out and use our incredible resources, and we expect summer to help us make that happen.

The Summer Glory Index (SGI) tells us approximately how wonderful our June through August meteorological summer has been. It combines the effects of temperature, humidity, and precipitation at Minneapolis, which has an unusual, long record of dew points back to the early 20th century. Ideally, we would include a sunlight category, but we simply do not have a good enough record of solar radiation to use it here.

The index is based on the notion that we like to sleep through the night soundly without excessive energy consumption, and we want to be able to get outside and stay outside during the day without having our activities postponed or canceled. Of course some like it hot, some like it muggy, and some like it stormy, but very few people enjoy rain-outs, blasting the AC all night, or wearing winter clothes to the barbeque.

The SGI, therefore, rewards days that are comfortable and dry, and punishes days for being too hot, too cold, too humid, or too wet. In the index, a perfectly glorious day has a high temperature of 73-79, a low of 57-64, a 6 PM dew point of less than 60, and no measurable rain. Such a day earns 10 points for each category, or 40 points total. These conditions are somewhat common in the Twin Cities, so really, the points are ours to lose on a given day. The table above summarizes how points are assigned or penalized in the index.

If this index is to be of any value, then not only should perfect scores of 40 correspond to broadly enjoyable conditions, but the lowest daily scores should also correspond to obnoxiously miserable ones. The lowest score on record is -86.5, on July 23, 1987, when 9.15 inches of rain fell during the infamous “Superstorm” (which extended into the next calendar day as well). The next lowest score is -73, on July 13, 1936, when the high was 105 and the low was an intolerable 86 degrees. Most of the worst scores come from extreme heat, but #5 is attributable to a 7-inch rainfall, and #15 is from June 2, 1945, when the high was 44 and the low was 38—both of which are unacceptable for summer in the Twin Cities. In short, the index appears to penalize our “worst” conditions adequately.

Annual Summer Glory Index scores, 1903-2015

The SGI works best, however, as a tool for assessing the entire summer. The cumulative score for the season tells us how frequently or infrequently we experienced delightful conditions. The finest summer on record is 1922, with a score of 2239.5, and only 13 days with negative scores. By contrast, the most horrendous summer on record was the Dust Bowl year of 1936, with only 852.8 points, and 42 days with negative scores, almost exclusively from extreme heat.

Note: The SGI now gives half-credit for 6PM dew points of 60-64 F, which has resulted in small changes to some of the historical scores reported earlier.

Last modified: Aug 11 , 2017