August 11: ORD

Today is August 11th. It's the 224th day of the year. 142 days remain. It's week 32 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 that, 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 82 and the normal low is 64. I've calculated the 33-year averages at 80 and 62.

On the hot side, in 1995 the high was 93 with a low of 72 (O'Hare's co-second warmest low of the date). Two years ago, in 2010, the high was 90 with a low of 75 (O'Hare's warmest low of the date). In 1988 the high was 92 (O'Hare's second hottest high of the date) with a low of 72 (O'Hare's other co-second warmest low of the date). O'Hare's highs of 93 degrees in 1995 and 2002 were O'Hare's hottest highs of the date. The record high of 98 was last set in 1941 at the University of Chicago after originally being set way back in 1874. The record warmest low of 77 occurred in 1944 at Midway. 27 of the past 33 highs (82%) were in the: 70s (15) and 80s (12). Four highs were in the hot 90s. Two highs were in the cool 60s.

In looking back through the years to the first August at O'Hare in 1959, the following instances were notable: In 1967 the high was 70 with a low of 47, a cool day. That low of 47 is equal to the current record low. In 1965 the high was 79 with a cold low of 45, a 34 degree spread (or diurnal). That low of 45 is colder than the current record low.

More recently, there were no additional large spreads of 30 degrees or more. The small spreads were six degrees in 1997 (70/64) and seven degrees in 1994 (67/60).
 
On the cool side, in 2004 (O'Hare's coolest August 11th) the high was the record coolest high of 62 with a low of 53. The previous record coolest high of 65 degrees occurred way back in 1903. In 1982 the high was 77 with the record low of 47, a 30 degree spread. In 1979 the high was 71 with a low of 54. The other cool highs were 67 degrees in 1994 and 70 degrees in 1997. 20 of the last 33 lows (61%) were in the 60s. There was a stretch of eight consecutive years of 60s lows from 1996 through 2003. This is 40% of the 60s lows since 1979 (8 of 20). Ten lows (30%) were in the cool: 40s (1) and 50s (9). Three lows (9%) were in the warm 70s. O'Hare's second coolest August 11th was in 1967 as the high was 70 with a low of 47 (a low equal to the record low). O'Hare's coldest low of the date was 45 degrees in 1965 (a low colder than the official record low).

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 5:26 and sunrise is at 5:57. Sunset is at 19:59 and twilight ends at 20:20. There's a total of 14 hours, 2 minutes of daylight today and 15 hours, 2 minutes between twilights. We lost two minutes of daylight from yesterday. Tomorrow twilight begins at 5:28 and sunrise is at 5:58. The moon is waning crescent, having reached last quarter two days ago at 13:55. Illumination was 36% at midnight, will be 32% at noon and 28% at midnight tonight. Moonrise was at 0:23 this morning. Moonset is at 15:36 this afternoon. Moonrise is at 1:09 tomorrow morning. Moonlight time is 15 hours, 13 minutes which is 30 minutes longer than yesterday. Moonlight time is at its peak today and tomorrow.

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 in 23 days.