July 31: ORD

Today is July 31st. It's the 213th day of the year. 153 days remain. It's week 31 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 83 and the normal low is 64. I've calculated the 33-year averages at 85 and 65.

On the hot side, in 2006 (O'Hare's warmest July 31st) the high was the record high of 99 with a low of 76. The record high of 99 originally occurred in 1945. In 1995 the high was 95 (O'Hare's second hottest high of the date) with a low of 74. The other warm low was 76 in 1999. O'Hare's warmest lows of the date were those lows of 76 in 1999 and 2006. The record warmest low of 79 occurred back in 1917, at a time when the official readings were taken adjacent to Lake Michigan. 17 of the past 33 highs (52%) were in the 80s. Nine highs (27%) were in the hot 90s. Seven highs (21%) were in the cool: 60s (1) and 70s (6). Our most recent 70s high was 74 in 2000. From 1979-2011, today is the average eighth hottest day of the year. Unlike the past few days, I can't say that the year following a hot year is a lot colder. The two references: in 2006, the high was 99. Then in 2007 the high was 91, only an eight degree drop. However, in 1995 the high was 95 and in 1996 it was only 75, a 20 degree drop. So we'll have to see what happens in future years with this.

In looking back through the years to the first July at O'Hare in 1959, the following instances were notable: In 1978 the low was a cold 52 degrees. In 1971 the low was a cold 49 degrees, a temperature colder than the official record low. In 1966 the high was 82 with a cold low of 52, a 30 degree spread. In 1965 the high was a cool 70 degrees.

More recently, there were large spreads of 35 degrees in 1984 (86/51) and 33 degrees in 1991 (92/59). There was an additional small spread of seven degrees in 1989 (75/68).

On the cool side, in 1990 (O'Hare's coolest July 31st) the high was 70 with a low of 52. Five years earlier, in 1985, the high was 69 (O'Hare's only 60s high and coolest high of the date) with a low of 62, a small seven degree spread. The record low of 51 was set in 1984. The record coolest high of 66 occurred back in 1912, 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 cool 50s. Seven lows (21%) were in the warm 70s. O'Hare's pre-1979 cold lows were 49 in 1971 (O'Hare's coldest low of July 31st and a low colder than the record low), 52 in 1966 and 52 in 1978.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 5:14 and sunrise is at 5:46. Sunset is at 20:13 and twilight ends at 20:45. There's a total of 14 hours, 27 minutes of daylight today and 15 hours, 31 minutes between twilights. We lost two minutes of daylight from yesterday. Tomorrow twilight begins at 5:15 and sunrise is at 5:47. The moon is waxing gibbous, heading towards full tomorrow night at 22:27. Illumination was 95% at midnight, will be 97% at noon and 99% at midnight tonight. Moonset was at 4:06 this morning. Moonrise is at 19:02 tonight. Moonset is at 5:16 tomorrow morning. Moonlight time is 9 hours, 4 minutes which is 22 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 in 34 days.