August 24: ORD

Today is August 24th. It's the 237th day of the year. 129 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 81 and the normal low is 62. I've calculated the 33-year averages at 81 and 62.

On the hot side, last year, in 2011, the high was 92 (O'Hare's co-second hottest high of the date) with a low of 69. in 1992 the high was 88 with a low of 69. Two years later, in 1994, the high was 87 with a low of 69. The prior year of  1993 saw a high of 87 with a low of 68. The record high of 100 was set in 1947 at Midway. The record warmest low of 78 occurred in 1975 at Midway. 31 the past 33 highs (94%) were in the: 70s (12) and 80s (19). Two highs were in the hot 90s. There was an incredible 28 years between 90s highs (1983 to 2011). O'Hare's warmest August 24th was in 1975 as the high was 91 with a low of 79 (O'Hare's warmest low of the date). O'Hare's hottest high of 94 degrees occurred in 1962 with a low of 71. O'Hare's other co-second hottest high of 92 degrees occurred in 1959 with a low of 71. 

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 91 with a warm low of 79, a hot day and year of the record warmest low. That low of 79 is warmer than the record warmest low of 78 and is our most recent 70s low. In 1971 the high was 90 with a low of 59, a 31 degree spread. In 1969 the high was 87 with a low of 57, a 30 degree spread. In 1967 the low was a cold 48 degrees, equal to the record low. In 1966 the low was a cold 49 degrees, just above the record low. In 1965 the high was 82 with a cold low of 45, a 37 degree spread. That low of 45 is colder than the record low. In 1964 the high was 78 with a cold low of 45, a 33 degree spread. That low of 45 is also colder than the record low. In 1963 the high was 70 with a low of 66, a four degree spread. In 1962 the high was a hot 94 degrees.

More recently, there was a large spread of 31 degrees in 1983 (90/59). There were small spreads of six degrees in 1989 (73/67) and seven degrees in 1999 (74/67).

On the cool side, in 1987 (O'Hare's co-coolest August 24th) the high was 72  with a low of 51. Three years earlier, in 1984, the high was 76 with a low of 51. The record low of 48 was set in 1942 at Midway. The record coolest high of 63 occurred way back in 1891, at a time when the official readings were taken adjacent to Lake Michigan. 21 of the last 33 lows (64%) were in the 60s. O'Hare recorded consecutive 60s lows as follows: six from 1982 through 1987, six from 1988 through 1995 and six from 1997 through 2002. 12 lows (36%) were in the cool 50s. There have been no lows in the 70s over the last 33 years. O'Hare's most recent 70s low was 79 degrees in 1975. O'Hare's other co-coolest August 24th was in 1964 as the high was 78 with a low of 45 (O'Hare's co-coldest low of the date). O'Hare's second coolest August 24ths were in: 1966 (high of 76, low of 49) and 1967 (high of 77, low of 48). O'Hare's other co-coldest low is 45 degrees in 1965. O'Hare's coolest high of 70 degrees occurred in 1963. 

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 5:41 and sunrise is at 6:11. Sunset is at 19:40 and twilight ends at 20:09. There's a total of 13 hours, 29 minutes of daylight today and 14 hours, 28 minutes between twilights. We lost two minutes of daylight from yesterday. Tomorrow twilight begins at 5:42 and sunrise is at 6:12. The moon reaches first quarter today at 8:54. Illumination was 46% at midnight, will be 52% at noon and 58% at midnight tonight. Moonrise is at 14:13 this afternoon. Moonset is at 23:51 tonight. Moonrise is at 15:15 tomorrow afternoon. Moonlight time is 9 hours, 38 minutes which is 16 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 ten days.