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 32-year averages at 85 and 65.

On the hot side, in 2006 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 with a low of 74. Highs of 94 occurred in the back-to-back years of 2001 and 2002. Other warm lows were 76 in 1999 and 75 in 1980. The record warmest low of 79 occurred back in 1917. The official thermometer was located near Lake Michigan at the time. 17 of the past 32 highs (53%) were in the 80s. Eight highs (25%) were in the hot 90s. Seven highs (22%) were in the cool: 60s (1) and 70s (6). Our most recent 70s high was 74 in 2000. From 1979-2010, today is the average seventh 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 cool 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 cool 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 a small spread of seven degrees in 1989 (75/68).

On the cool side, in 1990 the high was 70 with a low of 52. Five years earlier, in 1985, the high was 69 (our only 60s high) 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. The official thermometer was located near Lake Michigan at the time. 17 of the last 32 lows (53%) were in the 60s. Eight lows (25%) were in the cool 50s. Seven lows (22%) were in the warm 70s. Two years ago, in 2009, (high of 81, low of 58) was the coolest July 31st since 1997's high of 82 and low of 56.

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 twilight ends at 20:46. There's a total of 14 hours, 29 minutes of daylight today and 15 hours, 33 minutes between twilights. We lost two minutes of daylight from yesterday. Tomorrow twilight begins at 5:14 and sunrise is at 5:46. The moon is waxing crescent, having become new yesterday at 13:40. Illumination was 0% at midnight, will be 1% at noon and 3% at midnight tonight. Although you won't be able to see it, moonrise is at 6:51 this morning. Moonset is at 20:32 tonight. Moonrise is at 8:05 tomorrow morning. Moonlight time is 13 hours, 41 minutes which is 41 minutes shorter than yesterday.

Summer began June 4 and runs for 110 days through September 21. High temperatures in this period are usually in the 70s and 80s. Highs in the 60s, 90s and 100s occur infrequently. Labor Day is in 36 days.