August 13: ORD

Today is August 13th. It's the 226th day of the year. 140 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 64. I've calculated the 33-year averages at 81 and 63.

On the hot side, in 1995 (O'Hare's warmest August 13th) the high was 96 (O'Hare's hottest high of the date) with the co-record warmest low of 77. In 1988 the high was 91 with the other co-record warmest low of 77. Two years ago, in 2010, the high was 92 with a low of 73 (O'Hare's second warmest low of the date). The record high of 98 was set in 1944 at Midway. 19 of the past 33 highs (58%) were in the 80s. Three highs (9%) were in the hot 90s. 11 highs (33%) were in the cool: 60s (1) and 70s (10).

In looking back through the years to the first August at O'Hare in 1959, the following instances were notable: In 1974 on the 13th & 14th, the temperatures were the same: a high of 78 with a low of 64. In 1967 the high was 77 with a cold low of 42, a 35 degree spread and cool day. That low of 42 was colder than the record low. In 1964 the high was 70 with a low of 45, a cool day. That low of 45 was also colder than the record low. In 1961 the high was 81 with a cold low of 48, a 33 degree spread. That low of 48 was also colder than the record low.

More recently, there was a small spread of seven degrees in 2005 (75/68).
 
On the cool side, in 2004 the high was 71 with the co-record low of 51. In 1992 the high was 69 (O'Hare's coolest high of the date) with a low of 54. The other co-record low of 51 occurred in 1982. The record coolest high of 64 occurred way back in 1883, at a time when the official readings were taken adjacent to Lake Michigan. 18 of the last 33 lows (55%) were in the 60s. Ten lows (30%) were in the cool 50s. Five lows (15%) were in the warm 70s. O'Hare's coolest August 13th was in 1964 as the high was 70 with a low of 45 (O'Hare's second coldest low of the date). O'Hare's second coldest August 13th was in 1967 as the high was 77 with a low of 42 (O'Hare's coldest low of the date). O'Hare's third coldest low of the date was 48 degrees in 1961. All of these 40s lows are colder than the official record low! 

As you can see, this is a day dominated by highs in the 70s or 80s and lows in the 50s or 60s. There were two long stretches of such occurrences. The first was nine years from 1979 through 1987. The second was 14 years from 1996 through 2009. The stretches were interrupted by highs in the 90s and lows in the 70s.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 5:29 and sunrise is at 5:59. Sunset is at 19:56 and twilight ends at 20:27. There's a total of 13 hours, 57 minutes of daylight today and 14 hours, 58 minutes between twilights. We lost three minutes of daylight from yesterday. Tomorrow twilight begins at 5:30 and sunrise is at 6:00. The moon is waning crescent. Illumination was 19% at midnight, will be 16% at noon and 12% at midnight tonight. Moonrise was at 2:00 this morning. Moonset is at 17:10 this afternoon. Moonrise is at 2:57 tomorrow morning. Moonlight time is 15 hours, 10 minutes which is six 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 21 days.