August 21: ORD

Today is August 21st. It's the 234th 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. Before this, the official site of record was on the south side of Chicago at Midway International Airport (MDW), beginning on Wednesday, July 1, 1942. From Friday, January 1, 1926 through Tuesday, June 30, 1942, the official site was at the University of Chicago. Before that, the official site was at various locations in Chicago going back to Monday, October 16, 1871. Observations taken from October 15, 1870 to October 8, 1871 were lost in the Great Chicago Fire.

According to the National Climatic Data Center (NCDC), the normal high is 82 and the normal low is 63. I've calculated the 33-year averages at 82 and 61.

On the hot side, in 2003 (O'Hare's fourth warmest August 21st) the high was 95 with a low of 72. Two decades earlier, in 1983, the high was 95 with a low of 67. Those highs of 95 degrees are O'Hare's hottest of the date along with a high of 95 in 1968. The record high of 99 was set in 1955 at Midway. The record warmest low of 82 occurred back in 1916, at a time when the official readings were taken adjacent to Lake Michigan. 30 of the past 33 highs (91%) were in the: 70s (11) and 80s (19). Three highs were in the hot 90s. O'Hare's warmest August 21st was in 1968 as the high was 95 (O'Hare's hottest high of the date along with 95 degrees in 1983 and 2003) with a low of 77 (O'Hare's warmest low of the date). The second warmest 8/21 was in 1975 as the high was 94 with a low of 76 (O'Hare's second warmest low of the date). The third warmest 8/21 was in 1959 as the high was 94 with a low of 75 (O'Hare's third warmest low of the date). These highs of 94 degrees were O'Hare's co-second hottest highs of the date.

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 (O'Hare's co-second coolest August 21st days), the highs were 74 with lows of 52. The coldest low since 1979 and  second coldest low of the date is 49 degrees in 1992. The record low of 46 was set back in 1950 at Midway. O'Hare's coolest high of the date is 71 degrees in 1990. The record coolest high of 66 occurred in 1939 at the University of Chicago. 29 of the last 33 lows (88%) were in the: 50s (14) and 60s (15). One low was in the cold 40s. Three lows were in the warm 70s. O'Hare's coolest August 21st was in 1961 as the high was 75 with a low of 48 (O'Hare's coldest low of the date). 

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 eight year stretch began in 2004. There were two long stretches of lows in the 50s and 60s. The first was 16 years from 1976 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 5:38 and sunrise is at 6:08. Sunset is at 19:44 and twilight ends at 20:14. There's a total of 13 hours, 36 minutes of daylight today and 14 hours, 36 minutes between twilights. We lost three minutes of daylight from yesterday. Tomorrow twilight begins at 5:39 and sunrise is at 6:09. The moon is waxing crescent. Illumination was 16% at midnight, will be 20% at noon and 25% at midnight tonight. Moonrise is at 10:44 this morning. Moonset is at 21:37 tonight. Moonrise is at 11:55 tomorrow morning. Moonlight time is 10 hours, 53 minutes which is 36 minutes shorter than yesterday.

Summer began on June 4th and runs for 110 days through September 21st. High temperatures in this period are usually in the 70s and 80s. Highs in the 90s and 100s occur infrequently. Labor Day is in 13 days.