August 20: ORD

Today is August 20th. It's the 233rd 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. 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 81 and 62.

On the hot side, in 1980 (O'Hare's warmest August 20th) the high was 93 with a low of 74 (with a low of 74 degrees in 1959, O'Hare's warmest lows of the date). Two years ago, in 2010, the high was 92 with a low of 71. The record high of 98 degrees was set in 1947 at Midway. The record warmest low of 80 degrees occurred back in 1916, at a time when the official readings were taken adjacent to Lake Michigan. 31 of the past 33 highs (94%) were in the: 70s (15) and 80s (16). Two highs were in the hot 90s. O'Hare's second warmest August 20th was in 1968 as the high was 95 (O'Hare's hottest high of the date) with a low of 71. O'Hare's third warmest August 20th was in 1959 as the high was 91 with a low of 74 (with a low of 74 in 1980, O'Hare's warmest lows of the date). O'Hare's second hottest high of the date was 94 degrees in 1971. 

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 cold 50 degrees. In 1962 the high was 91 with a low of 61, a 30 degree spread. In 1961 the low was a cold 51 degrees.

More recently, there was a small spread of five degrees in 1990 (74/69). There were large spreads of: 32 degrees in 1986 (86/54) and 30 degrees in 1987 (85/55).
 
On the cool side, in 1992 (O'Hare's co-second coolest August 20th) the high was 76 with O'Hare's lone 40s low of the date (thus, O'Hare's coldest low of the date) of 49 degrees. In 1985 the high was 71 (O'Hare's coolest high of the date) with a low of 57. The record low of 47 degrees was set in 1950 at Midway. The record coolest high of 66 degrees occurred in 1940 at the University of Chicago. 30 of the last 33 lows (91%) were in the: 50s (13) and 60s (17). One low was in the cold 40s. Two lows were in the warm 70s.  O'Hare's coolest August 20th was in 1967 as the high was 74 with a low of 50 (O'Hare's second coldest low of the date). O'Hare's other co-second coolest August 20th was in 1961 as the high was 74 with a low of 51 (along with a low of 51 in 1984, O'Hare's third coldest readings).

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 5:37 and sunrise is at 6:07. Sunset is at 19:46 and twilight ends at 20:15. There's a total of 13 hours, 39 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 is waxing crescent. Illumination was 8% at midnight, will be 12% at noon and 16% at midnight tonight. Moonrise was at 9:33 this morning. Moonset is at 21:02 tonight. Moonrise is at 10:44 tomorrow morning. Moonlight time is 11 hours, 29 minutes which is 38 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 14 days.