September 1: ORD

Today is September 1st. It's the 245th day of the year. 121 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 this, 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 60. I've calculated the 33-year averages at 81 and 60.

On the hot side, last year, in 2011 (O'Hare's second warmest September 1st), the high was 93 with a low of 71 (*only the fourth 70s low ever at O'Hare for the date). In 2000 the high was 93 with a low of 67. Those highs of 93 degrees are O'Hare's second hottest of the date. In 1990 the high was 89 with a low of 67. O'Hare's hottest high of the date is 96 degrees in 1984. The record high of 101 degrees was set in 1953 at Midway. The record warmest low of 78 degrees was set back in 1912, at a time when the official readings were taken adjacent to Lake Michigan. Another of Chicago's warm lows was 76 degrees in 1973 at Midway. 26 of the past 33 highs (79%) were in the: 70s (9) and 80s (17). O'Hare recorded nine consecutive 70s & 80s highs from 1985 through 1993. Four highs (12%) were in the hot 90s. Three highs (9%) were in the cool 60s. O'Hare's warmest September 1st was in 1960 as the high was 92 (O'Hare's third hottest high of the date) with a low of 73 (tied with the low of 73 degrees in 1973 as O'Hare's warmest lows of the date). *O'Hare's fourth 70s low was 71 degrees two years ago in 2010.

In looking back through the years to the first September at O'Hare in 1959, the following instances were notable: in 1973 the high was 91 with a warm low of 73, a hot day and year of the record warmest low. In 1967 the high was 72 with a cold low of 41 (colder than the record low), a 31 degree spread and cool day. In 1966 the high was 90 with a low of 60, a 30 degree spread. In 1963 the high was 82 with a low of 51, a 31 degree spread. In 1960 the high was 92 with a low of 73, a hot day.
 
More recently, there were large spreads of 39 degrees in 1984 (96/57), 30 degrees in 1997 (85/55) and 30 degrees in 1999 (85/55). There was a small spread of four degrees in 2003 (65/61).

On the cool side, in 1994 the high was 68 (O'Hare's third coolest high of the date) with a low of 50. Three years ago, in 2009, the high was 72 with a low of 47 (O'Hare's second coldest low of the date). In 2001 the high was 67 (O'Hare's second coolest high of the date) with a low of 52. O'Hare's coolest high of the date is 65 degrees in 2003. The record low of 45 degrees was last set in 1949 at Midway after originally occurring in 1901. The record coolest high of 62 degrees occurred back in 1909, at a time when the official readings were taken adjacent to Lake Michigan. 30 of the last 33 lows (91%) were in the: 50s (15) and 60s (15). One low was in the cold 40s. Two lows were in the warm 70s. O'Hare's coolest September 1st was in 1967 as the high was 72 with a low of 41 (O'Hare's coldest low of the date).

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 5:50 and sunrise is at 6:19. Sunset is at 19:26 and twilight ends at 19:55. There's a total of 13 hours, 7 minutes of daylight today and 14 hours, 5 minutes between twilights. We lost three minutes of daylight from yesterday. Tomorrow twilight begins at 5:51 and sunrise is at 6:20. The moon is waning gibbous after reaching the full  Corn (a.k.a. Green Corn or Grain) moon yesterday at 8:58. Since this was the second full moon of August, it's was also the Blue moon. Illumination was 99% at midnight, will be 98% at noon and 97% at midnight tonight. Moonset was at 7:30 this morning. Moonrise is at 19:40 tonight. Moonset is at 8:33 tomorrow morning. Moonlight time is 11 hours, 50 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 two days.