August 31: ORD

Today is August 31st. It's the 244th 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. Before this, the official site of record was on the south side of Chicago at Midway International Airport (MDW), beginning on Wednesday, July 1, 1942. From Friday, January 1, 1926 through Tuesday, June 30, 1942, the official site was at the University of Chicago. Before that, the official site was at various locations in Chicago going back to Monday, October 16, 1871. Observations taken from October 15, 1870 to October 8, 1871 were lost in the Great Chicago Fire.

According to the National Climatic Data Center (NCDC), the normal high is 80 and the normal low is 61. I've calculated the 33-year averages at 80 and 60.

On the hot side, two years ago, in 2010 (O'Hare's warmest August 31st), the high was 93 (along with a high of 93 in 1960, O'Hare's hottest high of the date) with a low of 75 (O'Hare's warmest low of the date). A decade before, in 2000 (tied with 1969 as O'Hare's fourth warmest August 31st), the high was 92 (along with a high of 92 in 1961, O'Hare's second hottest high of the date) with a low of 68. The record high of 99 was set in 1953 at Midway. The record warmest low of 76 occurred in 1941 at the University of Chicago. 29 of the past 33 highs (88%) were in the: 70s (13) and 80s (16). Three highs were in the hot 90s. One high was in the cool 60s (a high of 68 three years ago in 2009). There was a long string of 70s & 80s highs, 22 years from 1978 through 1999. The following refers to August 31st at O'Hare. The second warmest day was in 1960 (high of 93 (with 2010, the hottest high of the date) and low of 70). Tied for third warmest are the years 1973 (high of 89, low of 73 (second warmest low of the date)) and 1977 (high of 90, low of 72). Tied for fourth warmest with 2000 is the year 1969 (high of 88, low of 72).  

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 in 2008 (90/60). 

On the cool side, three years ago, in 2009 (O'Hare's second coolest August 31st), the high was 68 (along with 1965, O'Hare's second coolest high of the date) with a low of 49 (along with 1964, O'Hare's second coldest low of the date). In 1994 the high was 71 with a low of 52. In 1987 the high was 72 with a low of 52. The record low of 47 was set way back in 1872. The record coolest high of 62 occurred way back in 1892. Both records were set at a time when the official readings were taken adjacent to Lake Michigan. 30 of the last 33 lows (91%) were in the: 50s (14) and 60s (16). One low was in the cold 40s. Two were in the warm 70s. There was a string of 28 straight years of 50s & 60s lows from 1981 through 2008. O'Hare's coolest August 31st was in 1967 as the high was 67 (O'Hare's coolest high of the date) with a low of 47 (O'Hare's coldest low of the date and tied with the record low). O'Hare had a cold low of 49 degrees in 1964 (tied with 2009 as O'Hare's second coldest of the date). In 1965 the high was 68 degrees (tied with 2009 as O'Hare's second coolest high of the date).

There was an instance of back-to-back years in which temperatures were just about as opposite as they can be. In 2009 the high was 68 with a low of 49, overall 11.5 degrees BELOW normal. Then in 2010 the high was 93 with a low of 75, overall 14 degrees ABOVE normal.  

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 5:49 and sunrise is at 6:18. Sunset is at 19:28 and twilight ends at 19:57. There's a total of 13 hours, 10 minutes of daylight today and 14 hours, 8 minutes between twilights. We lost three minutes of daylight from yesterday. Tomorrow twilight begins at 5:50 and sunrise is at 6:19. The moon reaches the full Corn (a.k.a. Green Corn or Grain) moon at 8:58. Since this is the second full moon of the month, it's also Blue moon. Illumination was 100% at midnight, will be 100% at noon and 99% at midnight tonight. Moonset was at 6:25 this morning. Moonrise is at 19:12 tonight. Moonset is at 7:30 tomorrow morning. Moonlight time is 11 hours, 13 minutes which is 37 minutes longer than yesterday.

Summer began on June 4th and runs for 110 days through September 21st. High temperatures in this period are usually in the 70s and 80s. Highs in the 90s and 100s occur infrequently. Labor Day is in three days.