September 26: ORD

Today is September 26th. It's the 270th day of the year. 96 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 51.

On the warm side, in 1998 (O'Hare's warmest September 26th) the high was the record high of 90 (O'Hare's hottest high of the date and latest last 90s high of the past 33 years) with a low of 68 (O'Hare's third warmest low of the date and our most recent 60s low). In 1986 the high was 78 with a low of 69 (O'Hare's second warmest low of the date). In 1999 the high was 86 (O'Hare's second hottest high of the date and our most recent 80s high) with a low of 57. Our third warmest high was 85 degrees in 1987. The record warmest low of 71 degrees occurred in 1973 at Midway. 25 of the last 33 highs (76%) were in the: 60s (16) and 70s (9). For an ongoing 12 consecutive years, from 2000 through last year (2011), highs were in the 60s and 70s. O'Hare recorded 15 consecutive years of highs of at least 65 degrees from 1995 through 2009. Four highs (12%) were in the warm: 80s (3) and 90s (1). Four highs (12%) were in the cool 50s. The post-1978 50s highs occurred from 1980 through 1994 at the rate of about one every five years. O'Hare's second warmest September 26th was in 1973 as the high was 82 with a low of 71 (O'Hare's warmest low of the date). O'Hare also had warm highs of 83 degrees in 1963 and 1979.

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 76 with a low of 46, a 30 degree spread. In 1974 the high was 82 with a low of 50, a 32 degree spread. In 1973 the high was 82 with a warm low of 71, a warm day and year of the record warmest low of 71 at Midway. In 1966 the low was a cold 36. In 1965 the high was a cool 53 with a low of 42, a cool day. In 1963 the high was a warm 83 with a low of 50, a 33 degree spread.

More recently, there were large spreads of 39 degrees in 1987 (85/46), 32 degrees in 1979 (83/51) and 31 degrees in 1995 (76/45). There were small spreads of seven degrees in 1994 (58/51) and seven degrees last year, in 2011 (60/53).

On the cool side, in 1984 (O'Hare's coolest September 26th) the high was 53 (with 1965, O'Hare's coolest high of the date) with the near-record low of 36 (with 1966, O'Hare's coldest low of the date). In 1991 the high was 60 with a low of 40 (O'Hare's second coldest low of the date). In 1980 the high was 58 with a low of 42. The other post-1978 cool highs were 58 degrees in 1989 and 1994. The highs of 58 degrees are O'Hare's second coolest highs of the date. The record low of 35 was set in 1928 at the University of Chicago. The record coolest high of 47 occurred in 1942 at Midway. 30 of the last 33 lows (91%) were in the: 40s (12) and 50s (18). There were two long consecutive stretches of lows in the 40s and 50s. The first was 11 years from 1987 through 1997. The second is an ongoing, 13 year stretch from 1999 through last year, 2011. One low was in the cold 30s. Two lows were in the warm 60s. O'Hare's second coolest September 26th was in 1965 as the high was 53 (with 1984, O'Hare's coolest high of the date) with a low of 42. O'Hare also had a cold low of 36 degrees in 1966 (as it was in 1984, these are O'Hare's coldest lows of the date).

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 6:18 and sunrise is at 6:45. Sunset is at 18:43 and twilight ends at 19:11. There's a total of 11 hours, 58 minutes of daylight today and 12 hours, 53 minutes between twilights. We lost two minutes of daylight from yesterday. Tomorrow twilight begins at 6:19 and sunrise is at 6:47. The moon is waxing gibbous, heading towards the full Harvest moon in three days at 22:19. Illumination was 84% at midnight, will be 88% at noon and 91% at midnight tonight. Moonset was at 3:08 this morning. Moonrise is at 16:45 this afternoon. Moonset is at 4:14 tomorrow morning. Moonlight time is 10 hours, 23 minutes which is 35 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. Columbus Day is in 12 days. Standard Time doesn't return until November 4th.