August 21: ORD

Today is August 21st. It's the 233rd day of the year. 132 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 82 and 61.

On the hot side, in 2003 the high was 95 with a low of 72. Two decades earlier, in 1983, the high was 95 with a low of 67. Our only other 90s high was 90 in 1996. The record high of 99 was set in 1955. The other 70s lows were 70 in 1998 and 71 last year, in 2010. The record warmest low of 82 occurred back in 1916. 29 of the past 32 highs (91%) were in the: 70s (11) and 80s (18). 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 1975 the high was 94 with a low of 76, a hot day. In 1968 the high was 95 with a low of 77, a hot day. In 1962 the high was 90 with a low of 60, a 30 degree spread. In 1961 the low was a cold 48 degrees. In 1959 the high was 94 with a low of 75, a hot day.

More recently, there was a small spread of four degrees in 1990 (71/67). The large spreads were 32 degrees in 1991 (85/53) and 32 degrees in 1992 (81/49).

On the cool side, in both 1985 and 2004, the high was 74 with a low of 52. Our coldest low temperature was 49 in 1992. The record low of 46 was set back in 1950. Our coolest high was 71 in 1990. The record coolest high of 66 occurred back in 1939. 28 of the last 32 lows (88%) were in the: 50s (14) and 60s (14). One low was in the cold 40s and three lows were in the warm 70s, including last year's 71.

As you can see, this is a day dominated by highs in the 70s and 80s and lows in the 50s and 60s. There were three long stretches of such highs. The first was 12 years from 1984 through 1995. The second was six years from 1997 through 2002. The most recent, ongoing seven year stretch began in 2004. There were two long stretches of lows in the 50s and 60s. The first was 13 years from 1979 through 1991. The second was six years from 2004 through 2009.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 5:37 and sunrise is at 6:07. Sunset is at 19:45 and twilight ends at 20:15. There's a total of 13 hours, 38 minutes of daylight today and 14 hours, 38 minutes between twilights. We lost three minutes of daylight from yesterday. Tomorrow twilight begins at 5:38 and sunrise is at 6:08. The moon reaches last quarter today at 16:54. Illumination was 57% at midnight, will be 52% at noon and 47% at midnight tonight. Moonset is at 13:54 this afternoon. Moonrise is at 23:24 tonight. Moonset is at 14:51 tomorrow afternoon. Moonlight time is 14 hours, 30 minutes which is 19 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 15 days.