August 29: ORD

Today is August 29th. It's the 242nd day of the year. 124 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 80 and the normal low is 61. I've calculated the 33-year averages at 81 and 61.

On the hot side, the following four days are tied as O'Hare's warmest August 29ths: 1973 (high of 92 with a low of 73), 1984 (high of 94 with a low of 71), 1991 (high of 92 with a low of 73), and two years ago, in 2010 (high of 94 with a low of 71). The other warm highs were 94 degrees in 1983, 92 degrees in 1962 and 92 degrees in 1980. The highs of 94 degrees are O'Hare's hottest of the date. The other warm low of 73 degrees occurred in 1995. The lows of 73 degrees are O'Hare's warmest lows of the date. The record high of 98 was set in 1953. The record warmest low of 74 was set in 1973. Both records were set at Midway. 26 of the past 33 highs (79%) were in the: 70s (11) and 80s (15). O'Hare recorded 12 consecutive years of highs in the 70s and 80s from 1992 through 2003. Five highs (15%) were in the hot 90s. Two highs (6%) were in the cool 60s (69 degrees three years ago (in 2009) and 69 degrees in 2004.

In looking back through the years to the first August at O'Hare in 1959, the following instances were notable: In 1973 the high was 92 with a low of 73, a hot day. 1973 was the year of the record warmest low. In 1971 the high was 84 with a low of 53, a 31 degree spread. In 1965 the high was 66 with a low of 41, a cool day. That low of 41 was colder than the record low. In 1964 the high was 83 with a low of 52, a 31 degree spread. In 1962 the high was a hot 92 with a low of 59, a 33 degree spread. In 1959 the temperatures for today (the 29th) and tomorrow (the 30th) were the same: a high of 86 with a low of 69.

More recently, there was a large spread of 30 degrees in 1983 (high of 94, low of 64).
 
On the cool side, in 1982 the high was 72 with the co-record low of 43. That record low of 43 degrees was first set in 1965 at Midway. In 1986 the high was 71 with a low of 44. 1982 and 1986 are O'Hare's co-second coolest August 29ths. The other cold low was 48 degrees in 1988. As you can see, those 40s lows occurred quite some time ago in the 1980s, from 1982 through 1988. The record coolest high of 62 degrees occurred way back in 1893, at a time when the official readings were taken adjacent to Lake Michigan. 25 of the last 33 lows (76%) were in the: 50s (10) and 60s (15). O'Hare recorded nine consecutive 50s and 60s lows from 2001 through 2009. Three lows (9%) were in the cold 40s. Five lows (15%) were in the warm 70s. O'Hare's coolest August 29th was in 1965 as the high was 66 (O'Hare's coolest high of the date) with a low of 41 (a low colder than the record low and O'Hare's coldest low of the date). 

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 5:47 and sunrise is at 6:16. Sunset is at 19:31 and twilight ends at 20:00. There's a total of 13 hours, 15 minutes of daylight today and 14 hours, 13 minutes between twilights. We lost three minutes of daylight from yesterday. Tomorrow twilight begins at 5:48 and sunrise is at 6:17. The moon is waxing gibbous, two days from the full Corn (a.k.a. Green Corn or Grain) moon at 8:58. Since this is also the second full moon of the month, it's also Blue moon. Illumination was 94% at midnight, will be 96% at noon and 98% at midnight tonight. Moonset was at 4:11 this morning. Moonrise is at 18:12 tonight. Moonset is at 5:19 tomorrow morning. Moonlight time is 9 hours, 59 minutes which is 34 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 five days.