August 15: ORD

Today is August 15th. It's the 228th day of the year. 138 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 63. I've calculated the 33-year averages at 82 and 63.

On the hot side, in 1988 (O'Hare's warmest August 15th) the high was 94 (O'Hare's hottest high of the date) with a low of 74 (O'Hare's warmest low of the date). In 2000 (tied with 1978 as O'Hare's second warmest August 15th) the high was 92 (O'Hare's co-second hottest high of the date) with a low of 71. In 1983 (O'Hare's third warmest August 15th) the high was 90 with a low of 72 (O'Hare's second warmest low of the date). The record high of 98 was set in 1944 at Midway. The record warmest low of 76 occurred in 1938 at the University of Chicago. 28 of the past 33 highs (85%) were in the: 70s (10) and 80s (18). Four highs were in the hot 90s. One high was in the cool 60s (69 in 1979). In 1978 (tied with 2000 as O'Hare's second warmest August 15th) the high was 92 (O'Hare's other co-second hottest high of the date) with a low of 71. O'Hare's fourth warmest August 15th was in 1965 as the high was 91 with a low of 70.

In looking back through the years to the first August at O'Hare in 1959, the following instances were notable: In 1964 the high was 78 with a cold low of 41, a 37 degree spread and cool day. That low of 41 is colder than the record low. In 1963 the low was a cool 50 degrees.

More recently, there was a large spread of 33 degrees in 1984 (87/54).
 
On the cool side, in 1979 (O'Hare's coolest August 15th) the high was 69 (O'Hare's coolest high of the date) with a low of 46 (a low colder than the record low). In 2004 the high was 75 with the record low of 48. The former record low of 51 was set at Midway in 1964. In 1992 the high was 71 with a low of 52. The record coolest high of 64 occurred way back in 1894, at a time when the official readings were taken adjacent to Lake Michigan. 18 of the last 33 lows (55%) were in the 60s. Eight lows (24%) were in the cool: 40s (2) and 50s (6). Seven lows (21%) were in the warm 70s. O'Hare's second coolest August 15th was in 1964 as the high was 78 with a low of 41 (O'Hare's coldest temperature of the date).
 
As you can see, this is a day dominated by highs in the 70s or 80s. There's an ongoing long stretch of such highs. This is 11 years from 2001 through last year, 2011. There was also an 11 year stretch from 1989 through 1999.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 5:31 and sunrise is at 6:01. Sunset is at 19:53 and twilight ends at 20:23. There's a total of 13 hours, 52 minutes of daylight today and 14 hours, 52 minutes between twilights. We lost three minutes of daylight from yesterday. Tomorrow twilight begins at 5:32 and sunrise is at 6:02. The moon is waning crescent. Illumination was 7% at midnight, will be 4% at noon and 2% at midnight tonight. Moonrise was at 3:58 this morning. Moonset is at 18:27 tonight. Moonrise is at 5:02 tomorrow morning. Moonlight time is 14 hours, 29 minutes which is 25 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 19 days.