September 23: ORD

Today is September 23rd. It's the 267th day of the year. 99 days remain. It's week 38 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 72 and the normal low is 51. I've calculated the 33-year averages at 69 and 49.

On the hot side, two years ago, in 2010 (O'Hare's warmest September 23rd), the high was 89 (O'Hare's hottest high of the date) with a low of 65 (O'Hare's warmest low of the date). Two years prior, in 2008, the high was 82 with a low of 59. The second hottest high of the date of  85 degrees occurred in 2004 and the third hottest high of 84 degrees occurred in 2007. O'Hare's second warmest low of 64 degrees occurred in 1986. The record high of 91 degrees was set in 1937 at the University of Chicago. The record warmest low of 71 degrees occurred way back in 1891, at a time when the official readings were taken adjacent to Lake Michigan. 23 of the past 33 highs (70%) were in the: 60s (14) and 70s (9). Five highs (15%) were in the warm 80s. Five highs (15%) were in the cool 50s. The five 80s highs occurred more recently, from 1999 through 2010. The five 50s highs occurred longer ago, from 1983 through 1995. Our most recent 50s high is 58 degrees in 1995. O'Hare's third warmest low of the date of 63 degrees occurred in 1968.

In looking back through the years to the first September at O'Hare in 1959, the following instances were notable: in 1976 the high was 68 with a cold low of 36, a 32 degree spread. In 1975 the high was 72 with a low of 42, a 30 degree spread. In 1974 the high was 60 with a cold low of 28, a 32 degree spread and cool day. That low of 28 was colder than the record low. In 1972 the high was a cool 60 degrees. In 1968 the low was a warm 63 degrees. In 1966 the high was 73 with a low of 42, a 31 degree spread. In 1960 the high was 67 with a low of 61, a six degree spread.

More recently, there were large spreads of 34 degrees in 2007 (84/50), 31 degrees in 2004 (85/54), 30 degrees in 2002 (69/39) and 30 degrees in 1979 (73/43). There was a small spread of five degrees in 2005 (67/62).

On the cool side, in 1995 (O'Hare's coolest September 23rd) the high was 58 with the record low of 29 (O'Hare's second coldest low of the date). In 1989 the high was 54 (O'Hare's coolest high of the date) with a low of 37. In 1983 the high was 56 with a low of 35. In 1981 the high was 63 with a low of 35. In 1992 the high was 59 with a low of 40. The record coolest high of 51 degrees occurred back in 1928 at the University of Chicago. 24 of the last 33 lows (73%) were in the: 40s (14) and 50s (10). O'Hare recorded six consecutive 40s and 50s lows from 1996 through 2001. Five lows (15%) were in the cold: 20s (1) and 30s (4). Four lows (12%) were in the warm 60s. O'Hare's second coolest September 23rd was in 1974 as the high was 60 with a low of 28 (O'Hare's coldest low of the date and a low colder than the record low). O'Hare's other cool highs were 59 degrees in 1990 and 60 degrees in 1972. O'Hare's other cold lows were 36 degrees in 1976 and 39 degrees in 2002.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 6:14 and sunrise is at 6:42. Sunset is at 18:48 and twilight ends at 19:16. There's a total of 12 hours, 6 minutes of daylight today and 13 hours, 2 minutes between twilights. We lost three minutes of daylight from yesterday. Tomorrow twilight begins at 6:15 and sunrise is at 6:43. The moon is waxing gibbous, having reached first quarter yesterday at 14:41. Illumination was 55% at midnight, will be 61% at noon and 66% at midnight tonight. Moonrise is at 14:55 this afternoon. Moonset is at 0:54 tomorrow morning. Moonrise is at 15:37 tomorrow afternoon. Moonlight time is 9 hours, 5 minutes which is 37 minutes shorter than yesterday. Moonlight time is at its least today.

Warm Fall began yesterday and runs for 41 days through  November 1st. High temperatures in this period are usually in the 50s, 60s and 70s. Highs in the 30s and 40s occur infrequently. The Autumnal Equinox was yesterday at 9:49. Columbus Day is in 15 days. Standard Time doesn't return until November 4th.