September 27: ORD

Today is September 27th. It's the 271st day of the year. 95 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 70 and the normal low is 49. I've calculated the 33-year averages at 71 and 50.

On the warm side, in 1986 (O'Hare's second warmest September 27th) the high was 86 (along with 1971 and 1998, O'Hare's second hottest high of the date) with a low of 68 (O'Hare's second warmest low of the date). A year later, in 1987, the high was 89 (O'Hare's hottest high of the date) with a low of 61. In 1998 the high was 86 (along with 1971 and 1986, O'Hare's second hottest high of the date) with a low of 60. The record high of 91 and record warmest low of 71 were set on the same hot day in 1971 at Midway. 29 of the past 33 highs (88%) were in the: 60s (13), 70s (8) and 80s (8). Four highs were in the cool 50s. Our most recent 50s highs were 53 degrees in 1993 and 58 degrees in 1994. For an ongoing 17 consecutive years, from 1995 through 2011, highs were in the 60s, 70s and 80s. 14 of these 17  (82%) featured highs of at least 65 degrees. The exceptions were highs of 61 degrees in 2003, 62 degrees in 2001 and 63 degrees last year, in 2011. O'Hare's warmest September 27th was in 1971 as the high was 86 (along with 1986 and 1998, O'Hare's second hottest high of the date) with a low of 69 (O'Hare's warmest low of the date). O'Hare's other warm highs of 9/27 are 85 degrees in 1963, 84 degrees in 1990 and 83 degrees in 1979. Another warm low of 63 degrees occurred in 1973.  

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 70 with a low of 40, a 30 degree spread. In 1973 the high was 80 with a warm low of 63, a warm day. In 1971 the high was 86 with a low of 69, a hot day. 1971 was also the year of the warm records. In 1967 the high was a cool 55 degrees. In 1964 the high was 61 with a cold low of 35, a cool day. In 1963 the high was a warm 85 with a low of 52, a 33 degree spread. In 1961 the high was 71 with a low of 40, a 31 degree spread.

More recently, there were large spreads of 37 degrees in 1990 (84/47), 31 degrees in 1985 (68/37), 31 degrees in 2004 (76/45) and 30 degrees in 1983 (79/49). There were small spreads of seven degrees in 1993 (53/46) and 1994 (58/51).

On the cool side, in 1984 (O'Hare's coolest September 27th) the high was the near-record coolest high of 50 (O'Hare's coolest high of the date) with a low of 36 (O'Hare's third coldest low of the date). In 1991 (O'Hare's second coolest September 27th) the high was 58 with a low of 34 (O'Hare's coldest low of the date and our most recent 30s low). The other cool highs are 53 degrees in 1993 (O'Hare's second coolest high of the date), 55 degrees in 1967 (O'Hare's third coolest high of the date) and 58 degrees in 1994. The other cold lows are 37 degrees in 1985 and 1989. The record low of 31 degrees and record coolest high of 49 degrees were set on the same cold day in 1942 at Midway. 25 of the last 33 lows (76%) were in the: 40s (14) and 50s (11). For an ongoing 13 consecutive years, from 1999 through 2011, lows were in the 40s and 50s. O'Hare also recorded consecutive 40s and 50s lows as follows: ten from 1974 through 1983 and six from 1992 through 1997. Four lows (12%) were in the cold 30s. Four lows (12%) were in the warm 60s. Our most recent 60s low was 60 degrees in 1998. A great battle between 30s and 60s lows began in 1984. In 1984 and 1985, the 30s won as 36 and 37 degrees. Then from 1986 through 1988, the 60s took over as 68, 61 and 60 degrees. Then in 1989, a low of 37 occurred.  The 30s returned in 1991 as 34 degrees. But more recently, in 1998, a low of 60 occurred. The score over the past 33 years is now tied at four a piece! O'Hare's third coolest September 27th was in 1964 as the high was 61 with a low of 35 (O'Hare's second coldest low of the date).

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 6:19 and sunrise is at 6:47. Sunset is at 18:41 and twilight ends at 19:09. There's a total of 11 hours, 54 minutes of daylight today and 12 hours, 50 minutes between twilights. We lost four minutes of daylight from yesterday. Tomorrow twilight begins at 6:20 and sunrise is at 6:48. The moon is waxing gibbous, heading towards the full Harvest moon in two days at 22:19. Illumination was 91% at midnight, will be 94% at noon and 96% at midnight tonight. Moonset was at 4:14 this morning. Moonrise is at 17:14 this afternoon. Moonset is at 5:18 tomorrow morning. Moonlight time is 11 hours which is 37 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 11 days. Standard Time doesn't return until November 4th.