July 23: ORD

Today is July 23rd. It's the 205th day of the year. 161 days remain. It's week 30 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 84 and the normal low is 64. I've calculated the 33-year averages at 83 and 64.

On the hot side, in 2001 the high was 92 with a low of 73. Two years prior, in 1999, the high was 94 with a low of 71. The record high of 99 was set way back in 1914, at a time when the official readings were taken adjacent to Lake Michigan. The record warmest low of 78 occurred in 1965 at Midway. 18 of the past 33 highs (55%) were in the 80s. Five highs (15%) were in the hot 90s. Ten highs (30%) were in the cool: 60s (1) and 70s (9). Our most recent 70s highs occurred three years in a row (76 in 2002, 75 in 2003 and 73 in 2004). O'Hare's warmest July 23rds were: 1965 (high of 94, low of 72) and 1976 (high of 92, low of 74). O'Hare's second warmest July 23rds are: 1972 (high of 88, low of 77 (O'Hare's warmest low of the date)), 1999 (high of 94, low of 71) and 2001 (high of 92, low of 73). O'Hare's hottest highs of the date are 94 degrees in: 1965, 1979, 1999 and 2010. 
 
In looking back through the years to the first July at O'Hare in 1959, the following instance was notable: In 1972 the low was a warm 77 degrees.

More recently, there were no other large or small daily temperature spreads.

On the cool side, in 1992 the high was the record coolest high (and only 60s high) of 64 with a low of 60, a small four degree spread. In 2000 the high was 73 with a low of 52 (O'Hare's coldest low of the date). The record low of 51 was set in 1947 at Midway. 19 of the last 33 lows (58%) were in the 60s. There were eight consecutive 60s lows from 1991 through 1998. Seven lows (21%) were in the cold 50s. Our most recent 50s lows were 59 in 2003 and 56 in 2004. Seven lows (21%) were in the warm 70s.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 5:05 and sunrise is at 5:38. Sunset is at 20:21 and twilight ends at 20:54. There's a total of 14 hours, 43 minutes of daylight today and 15 hours, 49 minutes between twilights. We lost two minutes of daylight from yesterday. Tomorrow twilight begins at 5:06 and sunrise is at 5:39. The moon is waxing crescent. Illumination was 18% at midnight, will be 22% at noon and 27% at midnight tonight. Moonrise is at 10:35 this morning. Moonset is at 22:28 tonight. Moonrise is at 11:43 tomorrow morning. Moonlight time is 11 hours, 53 minutes which is 38 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 90s and 100s occur infrequently. Labor Day is in 42 days.