August 17: ORD

Today is August 17th. It's the 230th 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. 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 63.

On the hot side, in 1988 (O'Hare's warmest August 17th) the high was the record high of 98 with a low of 77. That record high of 98 was originally set in 1947. The low of 77 is tied with a low of 77 in 1972 as O'Hare's warmest lows of the date. In 1995 the high was 93 (O'Hare's third hottest high of the date) with a low of 72. In 1983 the high was 94 (O'Hare's second hottest high of the date) with a low of 68. The record warmest low of 78 occurred in 1972 at Midway. 30 of the past 33 highs (91%) were in the: 70s (12) and 80s (18). Three highs were in the hot 90s. O'Hare's second warmest August 17th was in 1972 as the high was 92 with a low of 77.

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 cold 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 (O'Hare's second coolest August 17th) the high was 73 with a low of 51. In 1992 (O'Hare's third coolest August 17th) the high was 77 with a low of 49. The record low of 48 was set in 1976 at Midway. The record coolest high of 66 occurred back in 1924, at a time when the official readings were taken adjacent to Lake Michigan. 19 of the last 33 lows (58%) were in the 60s. There were six consecutive 60s lows at O'Hare from 1997 through 2002. Nine lows (27%) were in the cool: 40s (1) and 50s (8). Five lows (15%) were in the warm 70s. O'Hare's coolest August 17th was in 1963 as the high was 69 (O'Hare's coolest high of the date) with a low of 46 (O'Hare's coldest low of the date). The other cold lows were 47 in 1976 and 51 in 1962.
 
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 16 years from 1996 through last year, 2011.

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 19:50 and twilight ends at 20:20. There's a total of 13 hours, 47 minutes of daylight today and 14 hours, 47 minutes between twilights. We lost three minutes of daylight from yesterday. Tomorrow twilight begins at 5:35 and sunrise is at 6:04. The moon is new today at 10:54. Illumination was 0% at midnight, will be 0% at noon and 1% at midnight tonight. Moonrise was at 6:08 this morning. Moonset is at 19:31 tonight. Moonrise is at 7:15 tomorrow morning. Moonlight time is 13 hours, 23 minutes which is 35 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 17 days.