September 11: ORD

Today is September 11th. It's the 255th day of the year. 111 days remain. It's week 37 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 77 and the normal low is 56. I've calculated the 33-year averages at 78 and 56.

On the hot side, in 2005 (O'Hare's warmest September 11th) the high was 90 (tied with 1978 as O'Hare's hottest high of the date) with a low of 69 (tied with 1974 as O'Hare's warmest low of the date). In 1979 the high was 87 with a low of 65. Three years later, in 1982, the high was 88 with a low of 64. The record high of 95 degrees was set in 1952. The record warmest low of 73 degrees occurred in 1961. Both records were set at Midway. 28 of the past 33 highs (85%) were in the: 70s (14) and 80s (14). One high was in the hot 90s. Four highs were in the cool 60s. The 70s & 80s have the day as far as consecutive appearances: six from 1979 through 1984, six from 1986 through 1991, 11 from 1994 through 2004 and six from 2007 through this year, 2012. O'Hare's second warmest 9/11 was in 1978 as the high was 90 (tied with 2005 as O'Hare's hottest high of the date) with a low of 68 (tied with 1961 as O'Hare's second warmest low of the date). O'Hare's third warmest 9/11 was in 1961 as the high was 89 (O'Hare's second hottest high of the date) with a low of 68 (tied with 1978 as O'Hare's second warmest low of the date). Another warm low is 69 degrees in 1974 (tied with 2005 as 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 1978 the high was 90 with a low of 68, a hot day. In 1976 the high was 84 with a low of 53, a 31 degree spread. In 1974 the low was a warm 69 degrees. In 1967 the high was 71 with a cold low of 41, a 30 degree spread. That low of 41 was colder than the record low. In 1966 the high was 86 with a low of 49, a 37 degree spread. In 1964 the high was a cool 65 degrees. In 1962 the high was 80 with a low of 47, a 33 degree spread. In 1961 the high was 89 with a low of 68, a hot day and year of the record warmest low of 73 at Midway. In 1959 the high was a cool 65 degrees.

More recently, there were large spreads of 35 degrees in 1999 (81/46) and 30 degrees in 1988 (87/57). There was a small spread of seven degrees in 1991 (71/64). 

On the cool side, in 1993 (O'Hare's coolest September 11th) the high was 69 with the record low of 42. In the prior year of 1992, the high was 68 with a low of 47. In 1985 the high was 67 with a low of 49. The other cool high was 67 degrees in 2006. Those highs of 67 degrees are O'Hare's second coolest of the date. The record coolest high of 61 degrees occurred in 1940, a time when the official readings were taken at the University of Chicago. 28 of the last 33 lows (85%) were in the: 50s (17) and 60s (11). O'Hare recorded consecutive lows in the 50s & 60s as follows: seven from 1978 through 1984, six from 1986 through 1991 and 12 from 2000 through 2011 (last year). Five lows were in the cold 40s. No lows were in the warm 70s.  O'Hare's second coolest 9/11 was in 1967 as the high was 71 with a low of 41 (O'Hare's coldest low of the date). O'Hare's co-coolest highs of the date are 65 degrees in 1959 and again in 1964. 

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 6:01 and sunrise is at 6:30. Sunset is at 19:09 and twilight ends at 19:37. There's a total of 12 hours, 39 minutes of daylight today and 13 hours, 36 minutes between twilights. We lost three minutes of daylight from yesterday. Tomorrow twilight begins at 6:02 and sunrise is at 6:31. The moon is waning crescent. Illumination was 25% at midnight, will be 21% at noon and 17% at midnight tonight. Moonrise was at 1:43 this morning. Moonset is at 16:23 this afternoon. Moonrise is at 2:45 tomorrow morning. Moonlight time is 14 hours, 40 minutes which is 20 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. 9/11 Remembrance Day is today. Citizenship Day is in six days.