September 13: ORD

Today is September 13th. It's the 257th day of the year. 109 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 77 and 58.

On the hot side, in 2005 (O'Hare's warmest September 13th) the high was 94 (O'Hare's hottest high of the date) with a low of 71 (tied with 1962 as O'Hare's warmest low of the date). In 1994 the high was 88 with a low of 66. The other hot high (post-1978) was 90 degrees in 1998. The other warm lows (post-1978) are 70 degrees in 1982 (O'Hare's second warmest low of the date), and 68 degrees in 1993, 2003 and 2008. The record high of 98 degrees was set back in 1939. The record warmest low of 75 degrees occurred back in 1927. Both records were set at the University of Chicago. 24 of the past 33 highs (73%) were in the: 70s (12) and 80s (12). O'Hare recorded six consecutive highs in the 70s & 80s from 1990 through 1995. Two highs (6%) were in the hot 90s. Seven highs (21%) were in the cool 60s. O'Hare's second warmest September 13th was in 1962 as the high was 91 (O'Hare's second hottest high of the date) with a low of 71 (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 1975 the high was 63 with a low of 40, a cool day. In 1974 the high was 59 with a low of 44, a cool day. In 1970 the high was a cool 59 with a low of 52, a seven degree spread. In 1964 the high was 70 with a cold low of 37, a 33 degree spread and cool day. That low of 37 was colder than the record low. In 1963 the high was 62 with a low of 43, a cool day. In 1962 the high was 91 with a low of 71, a hot day.

More recently, there were large spreads of 33 degrees in 1998 (90/57) and 32 degrees in 2000 (79/47). There were small spreads of five degrees in 2006 (65/60) and seven degrees in 2008 (75/68). In 2008 we were getting lots of rain from the remnants of Hurricane Ike. O'Hare recorded 6.64 inches of rain, a new record for September 13th. 

On the cool side, in 1985 (O'Hare's third coolest September 13th) the high was 64 with a low of 42. In 1996 the high was 64 with a low of 48. In 1989 the high was 60 (O'Hare's second coolest high of the date) with a low of 53, a seven degree spread. The record low of 39 degrees was set in 1890. The record coolest high of 55 occurred way back in 1880. Both records were set at a time when the official readings were taken adjacent to Lake Michigan. 26 of the last 33 lows (79%) were in the: 50s (14) and 60s (12). Five lows (15%) were in the cold 40s. Two lows (6%) were in the warm 70s (71 in 2005 and 70 in 1982). O'Hare recorded consecutive lows in the 50s and 60s as follows: eight  from 1988 through 1995 and six from 2005 through last year, 2011. O'Hare's coolest September 13th is shared by two years: 1974 (co-coolest high of 59 with a low of 44) and 1975 (high of 63 with O'Hare's second coldest low of the date of 40). O'Hare's second coolest September 13th was in 1963 as the high was 62 with a low of 43. O'Hare's coldest low of 37 degrees occurred in 1964 (a low colder than the record low). O'Hare's other co-coolest high of the date of 59 degrees occurred in 1970. 

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 6:04 and sunrise is at 6:32. Sunset is at 19:06 and twilight ends at 19:34. There's a total of 12 hours, 34 minutes of daylight today and 13 hours, 30 minutes between twilights. We lost two minutes of daylight from yesterday. Tomorrow twilight begins at 6:05 and sunrise is at 6:33. The moon is waning crescent. Illumination was 10% at midnight, will be 7% at noon and 4% at midnight tonight. Moonrise was at 3:50 this morning. Moonset is at 17:29 this afternoon. Moonrise is at 4:57 tomorrow morning. Moonlight time is 13 hours, 39 minutes which is 33 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 four days.