September 30: ORD

Today is September 30th. It's the 274th day of the year. 92 days remain. It's week 40 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 72 and 48.

On the warm side, in 2002 (O'Hare's second warmest September 30th) the high was 85 (one of O'Hare's co-second hottest highs of the date) with a low of 68 (O'Hare's warmest low of the date). In 1995 the high was 84 with a low of 65 (along with a low of 65 degrees in 1971, O'Hare's second warmest lows of the date). In 1982 the high was 85 (the other of O'Hare's co-second hottest highs of the date) with a low of 58. In 2007 the high was 83 with a low of 59. The other warm highs were 83 degrees in both 1981 and 1994, 82 degrees in 1991 and 81 degrees in 1983. The record high of 92 degrees and record warmest low of 69 degrees were set on the same hot day in 1971 at Midway. 22 of the past 33 highs (67%) were in the: 60s (10) and 70s (12). There were two long stretches of highs in the 60s and 70s. The first was seven years from 1984 through 1990 and the second was six years from 1996 through 2001. Eight highs (24%) were in the warm 80s. Three highs (9%) were in the cool 50s. Two of the 50s highs were preceded by highs in the 80s: 85 in 2002 followed by 56 in 2003 and 83 in 2007 followed by 59 in 2008. The 50s highs occurred more recently (from 2003 through last year, 2011) and then much longer ago in 1960, 1972 and 1974. O'Hare's warmest September 30th was in 1971 as the high was 92 (O'Hare's hottest high of the date and only 90s high) with a low of 65 (with 1995, O'Hare's second 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 1976 the high was 77 with a low of 47, a 30 degree spread. In 1973 the high was 65 with a warm low of 60, a five degree spread. In 1972 the high was 53 with a low of 36, a cool day. In 1971 the high was 92 with a low of 65, a hot day. 1971 was the year of the record high of 92 and record warmest low of 69, both set at Midway. In 1968 the high was 80 with a low of 50, a 30 degree spread. In 1964 the low was a cold 36 degrees. In 1963 the high was 77 with a low of 45, a 32 degree spread. In 1962 the high was 70 with a cold low of 36, a 34 degree spread. In 1961 the high was 75 with a low of 44, a 31 degree spread. In 1960 the high was a cool 57 degrees.

More recently, there were additional large spreads of 36 degrees in 2004 (72/36), 34 degrees in 1989 (77/43), 33 degrees in 1983 (81/48), 31 degrees in 1994 (83/52), 31 degrees in 1996 (78/47) and 30 degrees in 1991 (82/52). There were no small spreads of up to seven degrees.

On the cool side, in 1984 (with 1972, O'Hare's coolest September 30th) the high was 60 with the record low of 29, a 31 degree spread. In 1993 the high was 61 with a low of 31 (O'Hare's second coldest low of the date), a 30 degree spread. O'Hare's second coolest high 56 degrees occurred in 2003. Tied with 1960 as O'Hare's third coolest high of the date is a high of 57 degrees last year, in 2011. O'Hare's other cold lows were 36 degrees in 1962, 1964 and 2004. The record coolest high of 44 degrees occurred way back in 1899, at a time when the official readings were taken adjacent to Lake Michigan. 26 of the last 33 lows (79%) were in the: 40s (14) and 50s (12). There were four periods of consecutive 40s and 50s lows. The first was nine years from 1975 through 1983. The second was seven years from 1985 through 1991. The third was six years from 1996 through 2001. The fourth, and ongoing period is currently seven years and began in 2005. Five lows (15%) were in the cold: 20s (1) and 30s (4). Two lows (6%) were in the warm 60s. Tied with 1984 as O'Hare's coolest September 30th is 1972 as the high was 53 (O'Hare's coolest high of the date) and low of 36. Tied with 2011 as a co-third coolest high of the date is the high of 57 degrees in 1960.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 6:22 and sunrise is at 6:50. Sunset is at 18:36 and twilight ends at 19:04. There's a total of 11 hours, 46 minutes of daylight today and 12 hours, 42 minutes between twilights. We lost two minutes of daylight from yesterday. Tomorrow twilight begins at 6:23 and sunrise is at 6:51. The moon is waning gibbous, having passed the full Harvest moon last night at 22:19. Illumination was 100% at midnight, will be 100% at noon and 99% at midnight tonight. Moonset was at 7:23 this morning. Moonrise is at 18:37 tonight. Moonset is at 8:23 tomorrow morning. Moonlight time is 12 hours, 46 minutes which is 34 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 eight days. Standard Time doesn't return until November 4th.