September 25: ORD

Today is September 25th. It's the 269th day of the year. 97 days remain. It's week 39 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 71 and the normal low is 50. I've calculated the 33-year averages at 70 and 49.

On the warm side, in 1986 (O'Hare's warmest September 25th) the high was 88 (O'Hare's hottest high of the date) with a low of 68 (with 1972, O'Hare's second warmest low of the date). In 2007 (with 1959, O'Hare's third warmest September 25th) the high was 85 (with 1959, O'Hare's second hottest high of the date) with a low of 63. O'Hare's third warmest low of the date is 67 degrees in 2005. The record high of 90 degrees was set back in 1933 at the University of Chicago. The record warmest low of 72 degrees occurred in 1920, at a time when the official readings were taken adjacent to Lake Michigan. 22 of the past 33 highs (67%) were in the: 60s (9) and 70s (13). For seven consecutive years from 1991 through 1997, highs were in the 60s and 70s. Six highs (18%) were in the warm 80s. Five highs (15%) were in the cool 50s. Our most recent 50s highs were 55 degrees in 2000 and 58 degrees in 2001. This was our second cluster of 50s highs since 1979. The first one occurred from 1982 through 1985 as three of those four years had highs in the 50s with the exception of a high of 74 degrees in 1984. O'Hare's second warmest September 25th was in 1973 as the high was 80 with a low of 69 (O'Hare's warmest low of the date). Tied with 2007 as O'Hare's third warmest 9/25 is 1959 with a high of 85 (O'Hare's second hottest high of the date with 2007) and low of 63. Tying 1986 as O'Hare's second warmest low is a low of 68 degrees in 1972. The other warm low is 63 degrees in 2008.  

In looking back through the years to the first September at O'Hare in 1959, the following instances were notable: in 1975 the high was a cool 59 with a low of 52, a seven degree spread. In 1973 the high was 80 with a warm low of 69, a warm day. In 1972 the high was 74 with a warm low of 68, a six degree spread. In 1971 the high was a cool 58 with a low of 51, a seven degree spread. In 1967 the high was 73 with a low of 40, a 33 degree spread. In 1964 the high was 74 with a low of 40, a 34 degree spread. In 1963 the high was a warm 83 with a low of 52, a 31 degree spread. In 1961 the high was a cool 56 with a low of 42, a cool day. In 1959 the high was 85 with a low of 63, a hot day.

More recently, there were large spreads of 37 degrees in 1999 (80/43), 34 degrees in 1984 (74/40), 32 degrees in 1988 (77/45), 31 degrees in 1989 (70/39), 31 degrees in 2002 (74/43) and 30 degrees in 1998 (81/51). There was an additional small spread of three degrees in 1983 (58/55).

On the cool side, in 1985 (O'Hare's coolest September 25th) the high was 58 with a low of 35 (the only low considered "cold" in O'Hare's history for the date). In 2000 the high was 55 with a low of 43. In 2001 the high was 58 with a low of 40. Along with 1961, 2000 and 2001 are O'Hare's second coolest September 25ths. In 1982 (O'Hare's third coolest September 25th) the high was 52 (O'Hare's coolest high of the date) with a low of 47, a five degree spread. The record low of 30 degrees and record coolest high of 46 degrees were set on the same cold day in 1942 at Midway. 26 of the last 33 lows (79%) were in the: 40s (15) and 50s (11). O'Hare recorded consecutive 40s and 50s lows as follows: 11 from 1974 through 1984 and 15 from 1990 through 2004. Two lows (6%) were in the cold 30s (1985 and 1989). Five lows (15%) were in the warm 60s (four of them bunched together in 2005, 2007, 2008 and 2009 with the other in 1986). Along with 2000 and 2001, O'Hare's second coldest September 25th was in 1961 as the high was 56 with a low of 42. O'Hare's other cool highs of the date were 58 degrees in 1971, 58 degrees in 1983 and 59 degrees in 1975.

There's an instance of back-to-back years in which temperatures were just about as opposite as they can be. In 1985 the high was 58 with a low of 35, overall 14.5 degrees BELOW normal. Then in 1986 the high was 88 with a low of 68, overall 17 degrees ABOVE normal.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 6:16 and sunrise is at 6:44. Sunset is at 18:44 and twilight ends at 19:12. There's a total of 12 hours of daylight today and 12 hours, 56 minutes between twilights. We lost three minutes of daylight from yesterday. Tomorrow twilight begins at 6:18 and sunrise is at 6:45. The moon is waxing gibbous. Illumination was 76% at midnight, will be 80% at noon and 84% at midnight tonight. Moonset was at 2:01 this morning. Moonrise is at 16:13 this afternoon. Moonset is at 3:08 tomorrow morning. Moonlight time is 9 hours, 48 minutes which is 31 minutes longer than yesterday.

Warm Fall began September 22nd 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 three days ago at 9:49. Columbus Day is in 13 days. Standard Time doesn't return until November 4th.