July 31: ORD

Today is July 31st. It's the 212th 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.

According to the National Climatic Data Center (NCDC), the normal high is 83 and the normal low is 64. I've calculated the 31-year averages at 85 and 65.

On the hot side, in 2006 the high was the co-record high of 99 with a low of 76. The other co-record high (of 99) occurred in 1945. Another hot high was 95 in 1995. Another warm low was 76 in 1999. The record warmest low of 79 occurred back in 1917. The official thermometer was located near Lake Michigan at the time. 16 of the last 31 highs were 80s. Eight highs were in the warmer 90s. Seven highs were in the cooler: 60s (1) and 70s (6). Through 2010, this is the average seventh hottest day of the year.

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 cool 52. In 1971 the low was a cool 49, a temperature cooler than the official record low. In 1966 the high was 82 with a cool low of 52, a 30 degree spread. In 1965 the high was a cool 70.

More recently, there were large spreads of 35 degrees in 1984 (86/51) and 33 degrees in 1991 (92/59). There were small spreads of seven degrees in both 1985 (69/62) and 1989 (75/68).

On the cool side, in 1990 the high was 70 with a low of 52. The coolest low was the record low of 51, set in 1984. Another cool high was 69 in 1985. The record coolest high of 66 occurred back in 1912. The official thermometer was located near Lake Michigan at the time. 16 of the last 31 lows were 60s. Eight lows were in the cooler 50s and seven were in the warmer 70s. Last year (81/58) was the coolest July 31st since 1997 (82/56). The last 70s high was 74 in 2000.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 5:13 and sunrise is at 5:45. Sunset is at 20:14 and civil twilight ends at 20:45. Nautical twilight ends at 21:25 and astronomical twilight ends at 22:09. There's a total of 14 hours, 29 minutes of daylight today and 15 hours, 32 minutes between twilights. We are losing about two minutes of daylight a day. Tomorrow twilight begins at 5:14 and sunrise is at 5:46. The moon is waning gibbous. Illumination was 77% at midnight, will be 73% at noon and 69% at midnight tonight. Moonset is at 11:15 this morning. Moonrise is at 22:24 tonight. Moonset is at 12:17 tomorrow afternoon. Moonlight time is 12 hours, 51 minutes which is 37 minutes longer than yesterday. -Bernie-