August 17: ORD

Today is August 17th. It's the 229th day of the year. 136 days remain. It's week 33 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 82 and the normal low is 63. I've calculated the 32-year averages at 82 and 63.

On the hot side, in 1988 the high was the record high of 98 with a low of 77. That record high of 98 was originally set in 1947. In 1995 the high was 93 (our most recent 90s high) with a low of 72. In 1983 the high was 94 with a low of 68. There were two other 70s lows: 71 in 2003 and 70 two years ago in 2009. The record warmest low of 78 occurred in 1972. 29 of the past 32 highs (91%) were in the: 70s (12) and 80s (17). Three highs were in the hot 90s.

In looking back through the years to the first August at O'Hare in 1959, the following instances were notable: In 1976 the high was 80 with a cold low of 47, a 33 degree spread. That low of 47 was colder than the record low and 1976 is the year of the record low (48 at Midway). In 1972 the high was 92 with a warm low of 77, a hot day. 1972 is the year of the record warmest low (78 at Midway). In 1966 the high was 88 with a low of 57, a 31 degree spread. In 1963 the high was 69 with a cold low of 46, a cool day. That low of 46 is colder than the record low. In 1962 the high was 81 with a cool low of 51, a 30 degree spread.

More recently, there was a small spread of four degrees in 1990 (high of 74, low of 70).

On the cool side, in 1981 the high was 73 with a low of 51. In 1992 the high was 77 with a low of 49, our only 40s low. The record low of 48 was set in 1976. The record coolest high of 66 occurred back in 1924. The official thermometer was located near Lake Michigan at the time. 18 of the last 32 lows (56%) were in the 60s. Nine lows (28%) were in the cool: 40s (1) and 50s (8). Five lows (16%) were in the warm 70s.

As you can see, this is a day dominated by highs in the 70s and 80s. There's an ongoing long stretch of such highs. This is 15 years from 1996 through last year, 2010.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 5:33 and sunrise is at 6:03. Sunset is at 19:51 and twilight ends at 20:21. There's a total of 13 hours, 48 minutes of daylight today and 14 hours, 48 minutes between twilights. We lost three minutes of daylight from yesterday. Tomorrow twilight begins at 5:34 and sunrise is at 6:04. The moon is waning gibbous. Illumination was 89% at midnight, will be 86% at noon and 82% at midnight tonight. Moonset is at 9:57 this morning. Moonrise is at 21:15 tonight. Moonset is at 10:56 tomorrow morning. Moonlight time is 12 hours, 42 minutes which is 36 minutes longer 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 19 days.