August 31: ORD

Today is August 31st. It's the 243rd day of the year. 122 days remain. It's week 35 of the year.

Unless noted, the following data is based upon observations collected by the National Weather Service, Chicago, at O'Hare International Airport (ORD) between November 1978 and last year. ORD has been the official site of record for Chicago since Thursday, January 17, 1980.

According to the National Climatic Data Center (NCDC), the normal high is 80 and the normal low is 61. I've calculated the 32-year averages at 80 and 60.

On the hot side, last year, in 2010, the high was 93 with a low of 75. A decade before, in 2000, the high was 92 with a low of 68. The only other 90s high was 90 three years ago in 2008. The only other 70s low was 71 in 1980. The record high of 99 was set in 1953. The record warmest low of 76 was set in 1941. 28 of the past 32 highs (88%) were in the: 70s (13) and 80s (15). Three highs were in the hot 90s and one was in the cool 60s (a high of 68 two years ago in 2009). We've seen extremes the last three years. In '08 the high was 90. Then in '09 the high was 68. And then back up to 93 last year. There was a long string of 70s & 80s highs, 22 years from 1978 through 1999.

In looking back through the years to the first August at O'Hare in 1959, the following instances were notable: In 1975 the high was 72 with a low of 65, a seven degree spread. In 1973 low was a warm 73 degrees. In 1967 the high was 67 with a low of 47, a cool day. That low of 47 is equal to the record low. In 1965 the high was a cool 68 degrees. In 1964 the low was a cold 49 degrees. In 1961 the high was a hot 92 with a low of 62, a 30 degree spread. In 1960 the high was a hot 93 with a low of 70, a hot day.

More recently, there were large spreads of 31 degrees in 1984 (82/51) and 30 degrees three years ago, in 2008 (90/60).

On the cool side, two years ago, in 2009, the high was 68 with a low of 49. That low of 49 was the first 40s low of the past 44 years and just the third 40s low in O'Hare's history (47 in 1967 and 49 in 1964 were the others). In 1994 the high was 71 with a low of 52. In 1987 the high was 72 with a low of 52. The other low 70s high was 70 in 2003. The record low of 47 was set way back in 1872. The record coolest high of 62 occurred way back in 1892. 29 of the last 32 lows (91%) were in the: 50s (14) and 60s (15). One low was in the cold 40s and two were in the warm 70s. The past two years have had radically different lows. In '09 the low was a cold 49 and last year it was a warm 75. There was a string of 28 straight years of 50s & 60s lows from 1981 through 2008.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 5:48 and sunrise is at 6:17. Sunset is at 19:29 and twilight ends at 19:58. There's a total of 13 hours, 12 minutes of daylight today and 14 hours, 10 minutes between twilights. We lost three minutes of daylight from yesterday. Tomorrow twilight begins at 5:50 and sunrise is at 6:18. The moon is waxing crescent. Illumination was 7% at midnight, will be 10% at noon and 14% at midnight tonight. Moonrise is at 9:29 this morning. Moonset is at 20:35 tonight. Moonrise is at 10:45 tomorrow morning. Moonlight time is 11 hours, 6 minutes which is 43 minutes shorter than yesterday.

Summer began June 4 and runs for 110 days through September 21. High temperatures in this period are usually in the 70s and 80s. Highs in the 60s, 90s and 100s occur infrequently. Labor Day is in five days. 9/11 Remembrance Day and Grandparents Day are in ten days.