September 29: ORD

Today is September 29th. It's the 273rd day of the year. 93 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 69 and the normal low is 48. I've calculated the 33-year averages at 70 and 49.

On the warm side, in 1982 (O'Hare's warmest September 29th) the high was 83 with a low of 63. In 2002 the high was 84 (O'Hare's second hottest high of the date) with a low of 60. In 1979 the high was 85 (O'Hare's hottest high of the date) with a low of 58. In 1986 the high was 77 with a low of 64. The other warm high was 81 degrees in 1983. The record high of 99 degrees was set in 1953 at Midway. The record warmest low of 69 degrees occurred way back in 1904, at a time when the official readings were taken adjacent to Lake Michigan. 23 of the past 33 highs (70%) were in the: 60s (13) and 70s (10). There's been two notable streaks of 60s and 70s highs. The first was eight years from 1985 through 1992. The second was six years from 1996 through 2001. Six highs (18%) were in the warm 80s. Four highs (12%) were in the cool 50s. Although there is not a rule, on three occasions the 50s and 80s have occurred in successive years. In 1983 the high of 81 was followed by a high of 55 in 1984. In 2002 the high of 84 was followed by a high of 58 in 2003. In 2006 the high of 59 was followed by a high of 80 in 2007. Another occurrence, although not perfect, was the high of 54 in 1993 which was followed by highs of 70 in 1994 and 80 in 1995. O'Hare's second warmest September 29th was in 1971 as the high was 80 with a low of 65 (O'Hare's warmest low of the date).

In looking back through the years to the first September at O'Hare in 1959, the following instances were notable: in 1971 the high was 80 with a warm low of 65, a warm day. In 1970 the high was 72 with a low of 39, a 33 degree spread. In 1967 the high was a cold 47 with a low of 41, a six degree spread and cold day. In 1964 the high was 67 with a cold low of 32, a 35 degree spread. In 1962 the high was 66 with a cold low of 36, a 30 degree spread. In 1961 the high was 66 with a cold low of 32, a 34 degree spread. The lows of 32 in 1961 and 1964 are equal to the record.

More recently, there were large spreads of 31 degrees in 1983 (81/50) and 30 degrees in 2007 (80/50). There were no small spreads of up to seven degrees.

On the cool side, in 1984 (O'Hare's coolest September 29th) the high was 55 (O'Hare's third coolest high of the date) with the record low of 32. In 1993 the high was 54 (O'Hare's second coolest high of the date) with a low of 37. The record coldest high of 44 degrees was set way back in 1899, at a time when the official readings were taken adjacent to Lake Michigan. 25 of the last 33 lows (76%) were in the: 40s (11) and 50s (14). For eight consecutive years, from 1994 through 2001, lows were in the 40s and 50s. Five lows (15%) were in the cold 30s. Three lows (9%) were in the warm 60s. O'Hare's second coolest September 29th was in 1967 as the high was 47 (O'Hare's coolest high of the date) with a low of 41. Along with the current record low of 32 degrees in 1984, O'Hare also had lows of 32 degrees in 1961 and 1964. O'Hare's second coldest lows of the date of 36 degrees occurred in 1962 and 1992. Today is also the runner-up date of the earliest first freeze of fall which occurred in 1984. The earliest first freeze of fall was on September 22 in 1995. 

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 6:21 and sunrise is at 6:49. Sunset is at 18:37 and twilight ends at 19:05. There's a total of 11 hours, 48 minutes of daylight today and 12 hours, 44 minutes between twilights. We lost three minutes of daylight from yesterday. Tomorrow twilight begins at 6:22 and sunrise is at 6:50. The moon is reaches the full Harvest moon tonight at 22:19. Illumination was 99% at midnight, will be 100% at noon and 100% at midnight tonight. Moonset was at 6:21 this morning. Moonrise is at 18:09 tonight. Moonset is at 7:23 tomorrow morning. Moonlight time is 12 hours, 12 minutes which is 36 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 nine days. Standard Time doesn't return until November 4th.