August 23: ORD

Today is August 23rd. It's the 236th day of the year. 130 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 80 and 62.

On the hot side, in 1998 the high was 90 with a low of 71. Five years earlier, in 1993, the high was 91 with a low of 69. The record high of 97 was set in 1947 at Midway. The record warmest low of 76 occurred in 1968 at Midway. 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 warmest August 23rd was in 1968 as the high was 95 (O'Hare's hottest high of the date) with a low of 77 (O'Hare's warmest low of the date). O'Hare's second warmest 8/23 was in 1975 (high of 92 and low of 74 (O'Hare's second warmest low of the date)). O'Hare's third warmest 8/23 was in 1959 (high of 91 and low of 73 (O'Hare's third warmest low of the date)). Another very warm day was in 1962 as the high was 94 (O'Hare's second hottest high of the date) and low of 67.

In looking back through the years to the first August at O'Hare in 1959, the following instances were notable: In 1975 the low was a warm 74 degrees. In 1973 the high was a cool 67 with a low of 63, a four degree spread. In 1972 the high was 75 with a low of 71, a four degree spread. In 1969 the high was 86 with a low of 53, a 33 degree spread. In 1968 the high was 95 with a low of 77, a hot day and year of the record warmest low. In 1965 the high was 73 with a cold low of 47 (colder than the record low) and a cool day. In 1964 the high was a cool 68 with a low of 52, a cool day. In 1962 the high was a hot 94 degrees. In 1961 the high was a cool 68 degrees.

More recently, there was a small spread of five degrees in 2001 (high of 74, low of 69).
 
On the cool side, in 1987 the high was 73 with a low of 54. Three years earlier, in 1984, the high was 74 with a low of 55. The record low is somewhat in dispute. The National Weather Service/Chicago (website) says that it's 51 degrees, set way back in 1888. However, in the daily summary for O'Hare, it says 48 degrees set way back in 1904. The record coolest high of 62 occurred way back in 1890. In any event, both the record low and record coolest highs were set at a time when the official readings were taken adjacent to Lake Michigan. 22 of the last 33 lows (67%) were in the 60s. O'Hare recorded six consecutive 60s lows from 1988 through 1993. Ten lows (30%) were in the cool 50s. One low (3%) was in the warm 70s (71 in 1998). There's a tie for O'Hare's coolest August 23rd and they were in back-to-back years. In 1964 the high was 68 (O'Hare's co-second coolest high of the date) with a low of 52. In 1965 the high was 73 with a low of 47 (O'Hare's coldest low of the date). The other co-second coolest high of 68 degrees occurred in 1961. O'Hare's coolest high of the date is 67 degrees in 1973. O'Hare's second coolest August 23rd was in 1966 as the high was 71 with a low of 51.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 5:40 and sunrise is at 6:10. Sunset is at 19:41 and twilight ends at 20:10. There's a total of 13 hours, 31 minutes of daylight today and 14 hours, 30 minutes between twilights. We lost three minutes of daylight from yesterday. Tomorrow twilight begins at 5:41 and sunrise is at 6:11. The moon is waxing crescent, heading towards first quarter tomorrow at 8:54. Illumination was 35% at midnight, will be 41% at noon and 46% at midnight tonight. Moonrise is at 13:06 this afternoon. Moonset is at 23:00 tonight. Moonrise is at 14:13 tomorrow afternoon. Moonlight time is 9 hours, 54 minutes which is 26 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 11 days.