September 20: ORD

Today is September 20th. It's the 264th day of the year. 102 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 73 and the normal low is 52. I've calculated the 33-year averages at 73 and 52.

On the hot side, in 1980 (O'Hare's warmest September 20th) the high was 90 (O'Hare's hottest high of the date) with a low of 67 (O'Hare's second warmest low of the date). In 1998 (tied for fourth warmest 9/20 with 1967 and 1978) the high was 86 with a low of 65. The record high of 91 degrees and record warmest low of 73 degrees were set on the same hot day back in 1931 at the University of Chicago. That record high of 91 degrees was first set adjacent to Lake Michigan way back in 1895. 26 of the past 33 highs (79%) were in the: 60s (12) and 70s (14). O'Hare recorded eight consecutive 60s & 70s highs from 1986 through 1993 and another six consecutive from 1999 through 2004. Six highs (18%) were in the warm: 80s (5) and 90s (1). One high (3%) was in the cool 50s. It's interesting that just two years separated the only 90s high (90 in 1980) from the only 50s high (55 in 1982). O'Hare's second warmest 9/20 occurred in 1965 as the high was 84 with a low of 69 (O'Hare's warmest low of the date). The third warmest 9/20 was in 1959 as the high was 88 (O'Hare's third hottest high of the date) with a low of 64. Tied for fourth warmest 9/20 with 1998 is 1967 (high of 85 and low of 66) and 1978 (high of 89 (O'Hare's second hottest high of the date) and low of 62). O'Hare also had hot highs of 85 degrees in 1970 and 1994.     

In looking back through the years to the first September at O'Hare in 1959, the following instances were notable: in 1978 the high was a hot 89 with a low of 62, a hot day. In 1975 the high was a cool 60 degrees. In 1974 the high was a cool 61 with a low of 55, a six degree spread. In 1970 the high was a hot 85 degrees. In 1967 the high was 85 with a low of 66, a hot day. In 1965 the high was 84 with a warm low of 69, a hot day. In 1962 the high was 60 with a low of 35, a cool day. That low of 35 was colder than the record low of 36. In 1960 the high was 65 with a low of 58, a seven degree spread. In 1959 the high was 88 with a low of 64, a hot day.

More recently, there were large spreads of 34 degrees in 1983 (78/44), 31 degrees in 1979 (79/48) and 30 degrees in 2003 (71/41). There were no small spreads of up to seven degrees.

On the cool side, in 1991 (O'Hare's second coolest September 20th) the high was 60 with the record low of 36 (O'Hare's second coldest low of the date). In 1982 (O'Hare's third coolest September 20th) the high was 55 (O'Hare's coolest high of the date) with a low of 43. In 2006 the high was 62 with a low of 42. The record coolest high of 52 occurred way back in 1901, a time when the official readings were taken adjacent to Lake Michigan. 27 of the last 33 lows (82%) were in the: 40s (10) and 50s (17). For ten consecutive years from 1981 through 1990, lows were in the 40s & 50s and another six consecutive years from 1992 through 1997. One low (3%) was in the cold 30s. Five lows (15%) were in the warm 60s. O'Hare's coolest September 20th was in 1962 as the high was 60 with a low of 35 (O'Hare's coldest low of the date). That low of 35 is colder than the record low. O'Hare was not the official station of record for Chicago at the time. O'Hare's second coolest highs were 60 degrees. These occurred in 1962, 1975, 1991 and 1995. O'Hare's third coolest high of 61 degrees occurred in 1974.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 6:11 and sunrise is at 6:39. Sunset is at 18:53 and twilight ends at 19:21. There's a total of 12 hours, 14 minutes of daylight today and 13 hours, 10 minutes between twilights. We lost three minutes of daylight from yesterday. Tomorrow twilight begins at 6:12 and sunrise is at 6:40. The moon is waxing crescent. Illumination was 22% at midnight, will be 27% at noon and 32% at midnight tonight. Moonrise is at 12:04 this afternoon. Moonset is at 21:48 tonight. Moonrise is at 13:09 tomorrow afternoon. Moonlight time is 9 hours, 44 minutes which is 20 minutes shorter than yesterday.

Summer began on June 4th and runs for 110 days through tomorrow, September 21st. High temperatures in this period are usually in the 70s and 80s. Highs in the 60s, 90s and 100s occur infrequently. The Autumnal Equinox is in two days on Saturday the 22nd at 9:49. Columbus Day is in 18 days. Standard Time doesn't return until November 4th.