July 11: ORD

Today is July 11th. It's the 193rd day of the year. 173 days remain. It's week 28 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 85 and the normal low is 64. I've calculated the 33-year averages at 84 and 63.

On the hot side, in 1980 (O'Hare's warmest July 11th) the high was 95 (O'Hare's hottest high of the date) with a low of 72. In 1987 the high was 91 with a low of 69. Our most recent 90s highs were 91 in 1987 and 90 in 1988. The record high of 97 and record warmest low of 80 were set on the same hot day back in 1936 at the University of Chicago. 28 of the past 33 highs (85%) were in the: 70s (10) and 80s (18). Five highs were in the hot 90s. All of the 90s highs (1979-2011) occurred from 1979 through 1988. O'Hare's second warmest July 11th was in 1972 as the high was 91 with a low of 75 (O'Hare's warmest low of the date).  

In looking back through the years to the first July at O'Hare in 1959, the following instances were notable: In 1975 today (the 11th) and tomorrow (the 12th) the temperatures were the same: a high of 76 with a low of 56. In 1968 the high was 80 with a cold low of 50, a 30 degree spread. In 1965 the low was a cold 51 degrees. In 1963 the high was 88 with a low of 53, a 35 degree spread. In 1961 the high was 86 with a low of 55, a 31 degree spread.

More recently, there were additional large spreads of 30 degrees in 1994 (84/54) and 30 degrees in 1997 (84/54).

On the cool side, in 1996 (O'Hare's coolest July 11th)  the high was 82 with the record low of 50, a 32 degree spread. The former record low was 51 in 1978 at Midway. In 1999 the high was 81 with a low of 52 (our most recent 50s low). O'Hare's coolest high of the date was 73 in 1990. The record coolest high of 65 occurred way back in 1873, at a time when the official readings were taken adjacent to Lake Michigan. 25 of the last 33 lows (76%) were in the 60s. There have been two amazing streaks of 60 degree lows: an eight year streak from 1986 through 1993 and then an 11 year streak from 2000 through 2010. 76% of the 60s lows (19/25) occurred in these two streaks. Six lows (18%) were in the cool 50s. Two lows (6%) were in the warm 70s.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 4:54 and sunrise is at 5:28. Sunset is at 20:30 and twilight ends at 21:04. There's a total of 15 hours, 2 minutes of daylight today and 16 hours, 10 minutes between twilights. We lost a minute of daylight from yesterday. Tomorrow twilight begins at 4:55 and sunrise is at 5:28. The moon is waning crescent, having reached last quarter last night at 20:48. Illumination was 48% at midnight, will be 44% at noon and 39% at midnight tonight. Moonrise was at 0:05 this morning. Moonset is at 14:02 this afternoon. Moonrise is at 0:35 tomorrow morning. Moonlight time is 13 hours, 57 minutes which is 55 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. Parent's Day is in 11 days.