August 27: ORD

Today is August 27th. It's the 239th day of the year. 126 days remain. It's week 35 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 63.

On the hot side, in 1990 the high was 93 (O'Hare's co-third hottest high of the date) with a low of 72. Three years later, in 1993, the high was 94 (O'Hare's second hottest high of the date and our most recent 90s high) with a low of 70. The record high of 97 and record warmest low of 77 were set on the same hot day in 1973 at Midway. The record high of 97 was originally set in 1948 and tied five years later (and two decades before the latest incarnation) in 1953. 17 of the past 33 highs (52%) were in the 80s. Five highs (15%) were in the hot 90s. 11 highs (33%) were in the cool: 60s (5) and 70s (6). O'Hare's warmest August 27th days occurred in: 1973 (high of 95 (O'Hare's co-hottest high of the date) and low of 75 (O'Hare's second warmest low of the date)) and 1977 (high of 92 and low of 78 (O'Hare's warmest low of the date)). O'Hare's second warmest 8/27 was in 1976 as the high was 95 (O'Hare's other co-hottest high of the date) with a low of 74 (O'Hare's third warmest low of the date). O'Hare's other co-third hottest high of the date is 93 degrees in 1961. 

In looking back through the years to the first August at O'Hare in 1959, the following instances were notable: in 1977 the high was 92 with a warm low of 78, a hot day. In 1976 the high was 95 with a low of 74, a hot day. In 1973 the high was 95 with a low of 75, a hot day. 1973 was the year that the records were set at Midway Airport. In 1968 the high was 67 with a low of 50, a cool day. In 1967 the high was a cool 64 with a low of 55, a cool day. In 1964 the high was 85 with a low of 55, a 30 degree spread. In 1962 the high was 88 with a low of 51, a 37 degree spread. In 1961 the high was a hot 93 degrees.

More recently, there were small spreads of five degrees in 1987 (66/61) and six degrees three years ago, in 2009 (67/61).
 
On the cool side, in 1986 (O'Hare's coolest August 27th) the high was 67 with a low of 48 (O'Hare's second coldest low of the date). In 1992 the high was 63 (O'Hare's coolest high of the date) with a low of 55. A decade earlier, in 1982, the high was 71 with the record low of 47. The other cool highs were 66 in 1987, 67 three years ago (in 2009) and 69 in 1988. The record coolest high of 61 occurred way back in 1915, at a time when the official readings were taken adjacent to Lake Michigan. 23 of the last 33 lows (70%) were in the 60s. O'Hare recorded 12 consecutive 60s lows from 1994 through 2005. Five lows (15%) were in the cool: 40s (2) and 50s (3). As you can see, the two 40s lows occurred quite some time ago. Five lows (15%) were in the warm 70s. O'Hare's second coolest August 27th occurred in 1968 as the high was 67 with a low of 50. O'Hare's second coolest high of 64 degrees occurred in 1967. 

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 5:45 and sunrise is at 6:14. Sunset is at 19:35 and twilight ends at 20:04. There's a total of 13 hours, 21 minutes of daylight today and 14 hours, 19 minutes between twilights. We lost two minutes of daylight from yesterday. Tomorrow twilight begins at 5:46 and sunrise is at 6:15. The moon is waxing gibbous. Illumination was 79% at midnight, will be 83% at noon and 87% at midnight tonight. Moonset was at 1:54 this morning. Moonrise is at 16:57 this afternoon. Moonset is at 3:02 tomorrow morning. Moonlight time is 8 hours, 57 minutes which is 17 minutes longer 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 seven days.