September 16: ORD

Today is September 16th. It's the 260th day of the year. 106 days remain. It's week 38 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 75 and the normal low is 54. I've calculated the 33-year averages at 72 and 53.

On the warm side, in 1992 (O'Hare's warmest September 16th) the high was 84 with a low of 70 (O'Hare's warmest low of the date). Five years later, in 1997, the high was 85 with a low of 65. Three years before, in 1994, the high was 86 (along with 1962, O'Hare's hottest high of the date) with a low of 63.  The other warm lows were 66 in both 1987 and 1998. The record high of 92 degrees was originally set in 1931 and tied at Midway in 1955. The record warmest low of 72 occurred back in 1907, a time when the official readings were taken adjacent to Lake Michigan. 25 of the past 33 highs (76%) were in the: 60s (9) and 70s (16). O'Hare recorded consecutive highs in the 60s & 70s as follows: ten from 1982 through 1991 and seven from 2004 through 2010. Five highs were in the warm 80s. Three highs were in the cool 50s.  

In looking back through the years to the first September at O'Hare in 1959, the following instances were notable: in 1973 the high was a cool 59 with a low of 52, a seven degree spread. Also in 1973, there was a mere 0.04 inches of rain but a big 1.32 inches on the following day, on the 17th. There were also ten straight days of highs in the 70s (the 6th to the 15th). The month began with three 90s highs. In 1970 the high was a cool 63 degrees. In 1969 the high was 73 with a warm low of 66 degrees, a seven degree spread. In 1966 the high was 73 with a cold low of 40, a 33 degree spread. In 1965 the high was a cool 61 degrees. In 1963 the high was 85 with a low of 55, a 30 degree spread. In 1962 the high was 86 with a low of 48, a 38 degree spread. In 1959 the high was a cool 56 with a low of 47, a cool day and year of the record coolest high of 56.

More recently, there were large spreads of 34 degrees in 1985 (76/42), 32 degrees in 2003 (82/50) and 30 degrees in 1979 (77/47). There was a small spread of seven degrees in 1987 (73/66).

On the cool side, in 1984 (O'Hare's coolest September 16th) the high was 64 with the record low of 37. Last year, in 2011, the high was 58 with a low of 46. In 1981 the high was 59 with a low of 46. The other post-1978 cool highs were 59 degrees in 1993, 61 degrees in 1982 and 63 degrees in 1986 and 1990. The other cold low was 42 degrees in 1985. The record coolest high of 56 degrees occurred in 1959 at Midway. 26 of the last 33 lows (79%) were in the: 40s (13) and 50s (13). O'Hare recorded 13 consecutive lows in the 40s & 50s from 1999 through 2011, last year. One low (3%) was in the cold 30s. Six lows (18%) were in the warm: 60s (5) and 70s (1). O'Hare's second coolest September 16th was in 1959 as the high was 56 (O'Hare's coolest high of the date) with a low of 47. O'Hare's second coldest low of the date, 40 degrees, occurred in 1966. O'Hare's other pre-1979 cool highs are 59 degrees in 1973,  61 degrees in 1965 and 63 degrees in 1970. 

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 6:07 and sunrise is at 6:35. Sunset is at 19:00 and twilight ends at 19:28. There's a total of 12 hours, 25 minutes of daylight today and 13 hours, 21 minutes between twilights. We lost three minutes of daylight from yesterday. Tomorrow twilight begins at 6:08 and sunrise is at 6:36. The moon is waxing crescent, having passed new last night at 21:11. Illumination was 0% at midnight, will be 1% at noon and 2% at midnight tonight. Moonrise was at 7:16 this morning. Moonset is at 19:02 tonight. Moonrise is at 8:29 tomorrow morning. Moonlight time is 11 hours, 46 minutes which is 39 minutes shorter than yesterday.

Summer began on June 4th and runs for 110 days through September 21st. High temperatures in this period are usually in the 70s and 80s. Highs in the 60s, 90s and 100s occur infrequently. Citizenship Day is tomorrow. Columbus Day is in 22 days.