August 4: ORD

Today is August 4th. It's the 217th day of the year. 149 days remain. It's week 31 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 66.

On the hot side, in 1988 (O'Hare's warmest August 4th) the high was 97 (O'Hare's hottest high of the date) with the record warmest low of 78. In 2005 the high was 88 with a low of 76. All of the 90s highs occurred in the 1980s: 93 in 1980, 90 in 1984 and most recently 97 in 1988. The record high of 100 was set in 1947. 22 of the past 33 highs (67%) were in the 80s. O'Hare recorded seven consecutive 80s highs from 2005 through last year, 2011. Three highs (9%) were in the hot 90s. Eight highs (24%) were in the cool 70s. Our most recent 70s highs were 76 in 2003 and 74 in 2004. For the past 23 years (1989-2011), there have been no highs in the 90s. It did hit 89 three times during this period (in 1989, 2002 and 2006).

In looking back through the years to the first August at O'Hare in 1959, the following instances were notable: In 1978 the high was 75 with a cold low of 49, a temperature colder than the record low. In 1974 the high was a cool 70 degrees. 1970 and 1971 had cool highs of 71 degrees. In 1966 the high was 81 with a cold low of 48, a 33 degree spread. That low of 48 was colder than the record low.

More recently, there was a small spread of four degrees in 1998 (72/68).
 
On the cool side, in 1992 (O'Hare's second coolest August 4th) the high was 72 with the record low of 53. That low of 53 was originally set way back in 1880 and tied 14 years later in 1894. The next year, 1993, saw a high of 73 with a low of 54 (just a degree above the record). Our most recent 50s low (54) occurred in 2000. In 1985 the high was 72 with a low of 56. The record coolest high of 62 was set back in 1915, at a time when the official readings were taken adjacent to Lake Michigan. 20 of the last 33 lows (61%) were in the 60s. Six lows (18%) were in the cold 50s. Seven lows (21%) were in the warm 70s. O'Hare's coolest August 4th was in 1978 as the high was 75 with a low of 49 (colder than the record low). O'Hare's coldest low was 48 degrees in 1966. O'Hare's coolest high of 70 degrees occurred in 1974. O'Hare's second coolest highs of 71 degrees occurred in 1970 and 1971. Also of note, one of O'Hare's pre-1979 coolest days occurred in 1972 as the high was 73 with a low of 53.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 5:18 and sunrise is at 5:50. Sunset is at 20:08 and twilight ends at 20:39. There's a total of 14 hours, 18 minutes of daylight today and 15 hours, 21 minutes between twilights. We lost three minutes of daylight from yesterday. Tomorrow twilight begins at 5:20 and sunrise is at 5:51. The moon is waning gibbous, having reached the full Sturgeon (a.k.a. Green Corn) moon three nights ago at 22:27. Illumination was 95% at midnight, will be 92% at noon and 89% at midnight tonight. Moonset was at 8:42 this morning. Moonrise is at 21:12 tonight. Moonset is at 9:46 tomorrow morning. Moonlight time is 11 hours, 30 minutes which is 39 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 30 days.