August 12: ORD

Today is August 12th. It's the 225th day of the year. 141 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 64. I've calculated the 33-year averages at 81 and 62.

On the hot side, in 1995 (O'Hare's warmest August 12th) the high was 98 (O'Hare's hottest high of the date) with a low of 75. In 1988 (O'Hare's second warmest August 12th) the high was 93 with a low of 75. Those lows of  75 are O'Hare's warmest lows of the date. Two years ago, in 2010 (O'Hare's third warmest August 12th), the high was 92 with a low of 74 (O'Hare's second warmest low of the date). The record high of 101 was set back in 1918. The record warmest low of 78 occurred way back in 1881. Both records were set at a time when the official readings were taken adjacent to Lake Michigan. 28 of the past 33 highs (85%) were in the: 70s (12) and 80s (16). Three highs were in the hot 90s and two were in the cool 60s.

In looking back through the years to the first August at O'Hare in 1959, the following instances were notable: In 1977 the low was a cold 52 degrees. In 1967 the high was 74 with a cold low of 44, a 30 degree spread and cool day. That low of 44 was colder than the current record low. In 1964 the high was a cool 66 with a low of 53, a cool day.

More recently, there were no large spreads of at least 30 degrees or small spreads of up to seven degrees diurnal.
 
On the cool side, in 2004 (O'Hare's coolest August 12th) the high was the record coolest high of 62 with a low of 49. The previous record coolest high of 64 occurred way back in 1903. In 1982 the high was 74 with the record low of 47. In 1992 the high was 67 with a low of 56. The other cold lows were 50 degrees in 1979 and 1986. 26 of the last 33 lows (79%) were in the: 50s (9) and 60s (17). O'Hare recorded eight consecutive 50s & 60s lows from 1996 through 2003. Five lows (15%) were in the warm 70s. Two lows (6%) were in the cold 40s. O'Hare's second coolest August 12th was in 1967 as the high was 74 with a low of 44 (O'Hare's coldest low of the date and colder than the official record low).

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 5:28 and sunrise is at 5:58. Sunset is at 19:58 and twilight ends at 20:28. There's a total of 14 hours of daylight today and 15 hours between twilights. We lost two minutes of daylight from yesterday. Tomorrow twilight begins at 5:29 and sunrise is at 5:59. The moon is waning crescent, having reached last quarter three days ago at 13:55. Illumination was 28% at midnight, will be 23% at noon and 19% at midnight tonight. Moonrise was at 1:09 this morning. Moonset was at 16:25 this afternoon. Moonrise is at 2:00 tomorrow morning. Moonlight time is 15 hours, 16 minutes which is three minutes longer than yesterday. Moonlight time is at its peak today as it was 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 22 days.