July 24: ORD

Today is July 24th. It's the 206th day of the year. 160 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 85 and 64.

On the hot side, in 2005 (O'Hare's warmest July 24th) the high was 102 (O'Hare's hottest high of the date) with the record warmest low of 78. The record warmest low of 78 was first set in 1940. In 1987 the high was 92 with a low of 75. The record high of 105 was set back in 1934, the hottest official temperature ever recorded in Chicago's history. 21 of the past 33 highs (64%) were in the 80s. O'Hare recorded consecutive 80s highs as follows: six from 1993 through 1998 and six from 2006 through last year, 2011. Eight highs (24%) were in the hot: 90s (7) and 100s (1). Our most recent 90s high was 90 in 2001. Four highs (12%) were in the cool 70s.

In looking back through the years to the first July at O'Hare in 1959, there were no notable instances of significantly hot or cool temperatures from 1959-1978.

More recently, there was a large temperature spread (diurnal) of 32 degrees in 1988 as the high was 91 with a low of 59.

On the cool side, in 2004 (O'Hare's coolest July 24th) the high was 71 (O'Hare's coolest high of the date) with a low of 55. Four years earlier, in 2000, the high was 79 with the record low of 53. The record coolest high of 66 was set way back in 1896, at a time when the official readings were taken adjacent to Lake Michigan. 18 of the last 33 lows (55%) were in the 60s. Eight lows (24%) were in the cold 50s. 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 5:06 and sunrise is at 5:39. Sunset is at 20:20 and twilight ends at 20:53. There's a total of 14 hours, 41 minutes of daylight today and 15 hours, 47 minutes between twilights. We lost two minutes of daylight from yesterday. Tomorrow twilight begins at 5:07 and sunrise is at 5:40. The moon is waxing crescent, heading towards first quarter in two days at 3:56. Illumination was 27% at midnight, will be 32% at noon and 38% at midnight tonight. Moonrise is at 11:43 this morning. Moonset is at 22:59 tonight. Moonrise is at 12:53 tomorrow afternoon. Moonlight time is 11 hours, 16 minutes which is 37 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 41 days.