August 19: ORD

Today is August 19th. It's the 232nd day of the year. 134 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 80 and 62.

On the hot side, in 1983 (O'Hare's warmest August 19th) the high was the record high of 99 with a low of 71. The record high of 99 was originally set in 1947. In 1995 the high was 89 with a low of 72. In 1980 the high was 91 (O'Hare's co-third hottest high of the date) with a low of 69. 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 (13) and 80s (17). Our most recent 70s high was 73 degrees in 2004. Two highs were in the hot 90s. One high was in the cool 60s. O'Hare's second warmest August 19th was in 1968 as the high was 91 with a low of 75 (O'Hare's warmest low of the date). O'Hare's second hottest high of the date is 92 degrees in 1959. O'Hare's other co-third hottest high of the date is 91 degrees in 1968. Rivaling 1995 as O'Hare's second warmest low of 72 degrees are lows of 72 degrees in 1970 and 1972. 

In looking back through the years to the first August at O'Hare in 1959, the following instances were notable: In 1977 the low was a cold 51 degrees. In 1968 the low was a warm 75 degrees. In 1967 the high was a cool 64 with a low of 57, a seven degree spread and cool day. In 1964 the high was 82 with a cold low of 49, a 33 degree spread. That low of 49 is colder than the record low.

More recently, there were small spreads of four degrees in 1988 (74/70) and six degrees in 2007 (68/62).
 
On the cool side, in 1992 (O'Hare's third coolest August 19th) the high was 74 with the record low of 51. In 1985 the high was 72 with a low of 55. The record coolest high of 62 occurred way back in 1897, at a time when the official readings were taken adjacent to Lake Michigan. After 1978, O'Hare's other cool highs are 70 degrees in 1991 and 68 degrees in 2007. You have to go back 45 years to 1967 to find the previous sub-70 degree high of 64 degrees at O'Hare (O'Hare's coolest high of the date). 29 of the last 33 highs (88%) were in the: 50s (13) and 60s (16). Four lows were in the warm 70s. O'Hare's coldest low of the date is 49 degrees in 1964 (a low colder than the record low). Remarkably, the high climbed 33 degrees in 1964 to a high of 82 degrees, O'Hare's largest spread of the date! O'Hare's coolest August 19th was in 1967 as the high was 64 with a low of 57. O'Hare's second coolest August 19th was in 1977 as the high was 73 with a low of 51.  
 
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 23 year stretch of such highs. This was from 1984 through 2006. There's an ongoing, 16 year stretch of lows in the 50s and 60s. This began in 1996 and continues as of last year, 2011.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 5:36 and sunrise is at 6:05. Sunset is at 19:47 and twilight ends at 20:17. There's a total of 13 hours, 42 minutes of daylight today and 14 hours, 41 minutes between twilights. We lost three minutes of daylight from yesterday. Tomorrow twilight begins at 5:37 and sunrise is at 6:07. The moon is waxing crescent, having passed new two days ago at 10:54. Illumination was 3% at midnight, will be 5% at noon and 8% at midnight tonight. Moonrise was at 8:24 this morning. Moonset is at 20:31 tonight. Moonrise is at 9:33 tomorrow morning. Moonlight time is 12 hours, 7 minutes which is 39 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 15 days.