August 20: ORD

Today is August 20th. It's the 232nd day of the year. 133 days remain. It's week 34 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 81 and 62.

On the hot side, in 1980 the high was 93 with a low of 74. Last year, in 2010, the high was 92 with a low of 71. The two 90s highs prior to 1980 were both very hot: 94 in 1971 and 95 in 1968. In 1968, today was the second of six straight days of 90 degrees or higher (from the 19th through the 24th). Three of these days (today, the 21st and the 23rd) recorded the warmest highs, that of 95 degrees. The record high of 98 was set in 1947. The record warmest low of 80 occurred back in 1916. 30 of the past 32 highs (94%) were in the: 70s (15) and 80s (15). Two high 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 88 with a low of 57, a 31 degree spread. In 1971 the high was a hot 94 degrees. In 1968 the high was a hot 95 degrees. In 1967 the low was a cool 50 degrees. In 1962 the high was 91 with a low of 61, a 30 degree spread. In 1961 the low was a cool 51 degrees.

More recently, there was a small spread of five degrees in 1990 (74/69). There were two large spreads: 32 degrees in 1986 (86/54) and 30 degrees in 1987 (85/55).

On the cool side, in 1992 the high was 76 with our lone 40s low of 49 degrees. In 1985 the high was 71, our coolest high, with a low of 57. Another cool low was 51 in 1984. The record low of 47 was set in 1950. The record coolest high of 66 occurred in 1940. 29 of the last 32 lows (91%) were in the: 50s (13) and 60s (16). One low was in the cold 40s and two were in the warm 70s.

As you can see, this is a day dominated by highs in the 70s and 80s and lows in the 50s and 60s. There was a long 29 year stretch of such highs. This was from 1981 through 2009. There were two long stretches of lows in the 50s and 60s. The first was 11 years from 1981 through 1991. The second was 17 years from 1993 through 2009.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 5:36 and sunrise is at 6:06. Sunset is at 19:47 and twilight ends at 20:17. There's a total of 13 hours, 41 minutes of daylight today and 14 hours, 41 minutes between twilights. We lost two minutes of daylight from yesterday. Tomorrow twilight begins at 5:37 and sunrise is at 6:07. The moon is waning gibbous, heading towards last quarter tomorrow at 16:54. Illumination was 66% at midnight, will be 61% at noon and 57% at midnight tonight. Moonset is at 12:55 this afternoon. Moonrise is at 22:44 tonight. Moonset is at 13:54 tomorrow afternoon. Moonlight time is 14 hours, 11 minutes which is 26 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 16 days.