September 12: ORD

Today is September 12th. It's the 256th day of the year. 110 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 76 and the normal low is 56. I've calculated the 33-year averages at 79 and 57.

On the hot side, in 2005 (O'Hare's warmest September 12th) the high was 90 (O'Hare's second hottest high of the date) with a low of 68 (with 1961, O'Hare's warmest low of the date). In 1988 the high was 85 with a low of 67 (with 1978, O'Hare's second warmest low of the date). In 1994 the high was 87 with a low of 65. The other warm high of 88 degrees occurred in 1998. The record high of 96 degrees was set in 1952 at Midway. The record warmest low of 72 degrees occurred back in 1931, a time when the official readings were taken adjacent to Lake Michigan. 28 of the past 33 highs (85%) were in the: 70s (13) and 80s (15). O'Hare recorded consecutive 70s & 80s highs as: seven from 1978 through 1984, six from 1990 through 1995 and eight from 1997 through 2004. One high was in the hot 90s. Four highs were in the cool 60s. O'Hare's second warmest 9/12 was in 1962 as the high was 91 (O'Hare's hottest high of the date) with a low of 62. Tied as O'Hare's warmest low of the date with 2005 is 1961's 68 degrees. Tied as O'Hare's second warmest low of the date with 1988 is 1978's 67 degrees.  

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 61 with a low of 43, a cool day. That low of 43 is colder than the record low of 44. In 1973 the high was 76 with a low of 46, a 30 degree spread. In 1972 the high was 70 with a low of 64, a six degree spread. In 1967 the high was 79 with a cold low of 41, a 38 degree spread. That low of 41 is colder than the record low of 44. In 1966 the high was 84 with a low of 52, a 32 degree spread. In 1964 the high was 65 with a cold low of 38, a cool day. That low of 38 is colder than the record low of 44. In 1962 the high was a hot 91 degrees. In 1961 the low was a warm 68 degrees. In 1960 the high was a cool 62 degrees.

More recently, there were no large spreads of at least 30 degrees or more. There was a small spread of four degrees in 2008 (73/69). 

On the cool side, in 1985 the high was 66 with a low of 46. In 2007 the high was 67 with a low of 46. The record low of 44 degrees was set in 1955 at Midway. The record coolest high of 53 degrees occurred in 1902, a time when the official readings were taken adjacent to Lake Michigan. 28 of the last 33 lows (85%) were in the: 50s (14) and 60s (14). O'Hare recorded consecutive lows in the 50s & 60s as follows: nine from 1976 through 1984 and six from 1986 through 1991. Five lows were in the cold 40s. No lows were in the warm 70s. O'Hare's coolest September 12th was in 1964 as the high was 65 with a low of 38 (O'Hare's coldest low of the date). O'Hare's second coolest 9/12 was in 1975 as the high was 61 (O'Hare's coolest high of the date) with a low of 43 (O'Hare's third coldest low of the date). O'Hare's second coldest low of the date is 41 degrees in 1967. O'Hare's second coolest high of 62 degrees occurred in 1960.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 6:02 and sunrise is at 6:31. Sunset is at 19:07 and twilight ends at 19:35. There's a total of 12 hours, 36 minutes of daylight today and 13 hours, 33 minutes between twilights. We lost three minutes of daylight from yesterday. Tomorrow twilight begins at 6:04 and sunrise is at 6:32. The moon is waning crescent. Illumination was 17% at midnight, will be 13% at noon and 10% at midnight tonight. Moonrise was at 2:45 this morning. Moonset is at 16:57 this afternoon. Moonrise is at 3:50 tomorrow morning. Moonlight time is 14 hours, 12 minutes which is 28 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 in five days.