September 2: ORD

Today is September 2nd. It's the 246th day of the year. 120 days remain. It's week 36 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 80 and the normal low is 60. I've calculated the 33-year averages at 81 and 60.

On the hot side, last year, in 2011 (O'Hare's warmest September 2nd) the high was 92 (O'Hare's second hottest high of the date) with a low of 76 (O'Hare's warmest low of the date). Three years earlier, in 2008, the high was 94 (O'Hare's hottest high of the date) with a low of 65. In 2000 the high was 91 with a low of 67. In 1979 the high was 86 with a low of 70. The record high of 101 degrees was set in 1953 at Midway. The record warmest low of 78 degrees was set back in 1913, at a time when the official readings were taken adjacent to Lake Michigan. 28 of the past 33 highs (85%) were in the: 70s (12) and 80s (16). O'Hare recorded 19 consecutive years of highs in the 70s & 80s from 1975 through 1993. Four highs were in the hot 90s. One high (68 degrees in 1994) was in the cool 60s. O'Hare's second warmest September 2nd was in 1973 as the high was 91 with a low of 73 (O'Hare's second warmest low of the date). O'Hare's third warmest low of 72 degrees occurred in 1960. 

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 91 with a warm low of 73, a hot day and year of the record warmest low. In 1967 the high was 72 with a cold low of 41 (colder than the record low), a 31 degree spread and cool day. In 1966 the high was 90 with a low of 60, a 30 degree spread. In 1963 the high was 82 with a low of 51, a 31 degree spread. In 1960 the high was 92 with a low of 73, a hot day.
 
More recently, there were large spreads of 39 degrees in 1984 (96/57), 30 degrees in 1997 (85/55) and 30 degrees in 1999 (85/55). There was a small spread of four degrees in 2003 (65/61).

On the cool side, in 1994 (O'Hare's third coolest September 2nd) the high was 68 (with 1972, O'Hare's second coolest high of the date) with a low of 49. In 1987 the high was 70 with a low of 51. Three years ago, in 2009, the high was 73 with a low of 50. Lows of 49 degrees also occurred in 1964 and 2001. The record low of 44 degrees was set in 1946. The record coolest high of 61 degrees occurred in 1974. Both records were set at Midway. Another of Chicago's cool highs of 63 degrees occurred back in 1935 at the University of Chicago. 29 of the last 33 lows (88%) were in the: 50s (12) and 60s (17). O'Hare recorded 14 consecutive lows in the 50s & 60s from 1980 through 1993 and another nine consecutive from 2002 through 2010. O'Hare's coolest September 2nd was in 1974 as the high was 58 (O'Hare's: coolest high of the date, only sub-60 degree high of the date and a high colder than the record coolest high) with a low of 48 (O'Hare's second coldest low of the date). O'Hare's second coolest September 2nd was in 1967 as the high was 73 with a low of 40 (O'Hare's coldest low of the date and a low colder than the record low). O'Hare's high of 68 degrees in 1972 is tied with the same high in 1994 as O'Hare's second coolest high of the date.   

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 5:51 and sunrise is at 6:20. Sunset is at 19:25 and twilight ends at 19:53. There's a total of 13 hours, 5 minutes of daylight today and 14 hours, 2 minutes between twilights. We lost two minutes of daylight from yesterday. Tomorrow twilight begins at 5:53 and sunrise is at 6:21. The moon is waning gibbous after reaching the full Corn (a.k.a. Green Corn or Grain) moon two days ago at 8:58. Since this was the second full moon of August, it was also the Blue moon. Illumination was 97% at midnight, will be 95% at noon and 95% at midnight tonight. Moonset was at 8:33 this morning. Moonrise is at 20:08 tonight. Moonset is at 9:35 tomorrow morning. Moonlight time is 12 hours, 25 minutes which is 35 minutes longer 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 90s and 100s occur infrequently. Labor Day is tomorrow.