August 7: ORD

Today is August 7th. It's the 220th day of the year. 146 days remain. It's week 32 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 83 and the normal low is 64. I've calculated the 33-year averages at 83 and 64.

On the hot side, in 2001 (O'Hare's warmest August 7th) the high was 95 (O'Hare's hottest high of the date) with a low of 75. In 1979 the high was 93 (O'Hare's third hottest high of the date) with a low of 75. Those lows of 75 degrees were O'Hare's warmest of the date. O'Hare's second hottest high of the date was 94 degrees in 1988. O'Hare's third hottest high of the date was 93 degrees in 1979. O'Hare's second warmest lows were 74 degrees in 1968 and 1996. The record high of 99 and record warmest low of 79 were set on the same hot day back in 1918, at a time when the official readings were taken adjacent to Lake Michigan. 14 of the past 33 highs (43%) were in the 80s. Ten highs (30%) were in the cool 70s. Nine highs (27%) were in the hot 90s.

In looking back through the years to the first August at O'Hare in 1959, the following instances were notable: In 1972 the high was a cool 68 degrees. In 1966 the high was 86 with a low of 56, a 30 degree spread. In 1964 the high was 92 with a low of 56, a 36 degree spread.

More recently, there were large spreads of 31 degrees in 1988 (94/63), 31 degrees in 2005 (89/58) and 30 degrees in 1994 (80/50). The small spreads were six degrees in 1999 (75/69) and seven degrees in 1998 (78/71).
 
On the cool side, in 1989 (O'Hare's coolest August 7th) the high was 72 with a low of 52. Four years later, in 1993, the high was 76 with a low of 53. Three years prior, in 1990, the high was 79 with the co-record low of 50. The record low of 50 was most recently set in 1994. The record coolest high of 65 was set way back in 1903, at a time when the official readings were taken adjacent to  Lake Michigan. 15 of the last 33 lows (46%) were in the 60s. Nine lows (27%) were in the cool 50s. Our most recent 50s lows were 53 in 2004 and 58 in 2005. Another nine lows (27%) were in the warm 70s. O'Hare's second coolest August 7th was in 1972 as the high was 68 (O'Hare's coolest high of the date) with a low of 58. O'Hare's third coolest August 7th was in 1976 as the high was 74 with a low of 53.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 5:22 and sunrise is at 5:53. Sunset is at 20:04 and twilight ends at 20:35. There's a total of 14 hours, 11 minutes of daylight today and 15 hours, 13 minutes between twilights. We lost three minutes of daylight from yesterday. Tomorrow twilight begins at 5:23 and sunrise is at 5:54. The moon is waning gibbous, heading towards last quarter in two days at 13:55. Illumination was 74% at midnight, will be 69% at noon and 65% at midnight tonight. Moonset is at 11:49 this morning. Moonrise is at 22:36 tonight. Moonset is at 12:48 tomorrow afternoon. Moonlight time is 13 hours, 13 minutes which is 32 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 27 days.