August 18: ORD

Today is August 18th. It's the 231st day of the year. 135 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 81 and 62.

On the hot side, in 1995 (O'Hare's second warmest August 18th) the high was 90 with a low of 73. In 1988 the high was 89 with a low of 72. Two years later, in 1990, the high was 91 with a low of 70. The record high of 98 was set in 1947 at Midway. The record warmest low of 79 occurred in 1972 at Midway. 30 of the past 33 highs (91%) were in the: 70s (14) and 80s (16). Our most recent 70s high was 78 degrees in 2004. Two highs were in the hot 90s. One high was in the cool 60s. O'Hare's warmest August 18th was in 1972 as the high was 93 (O'Hare's hottest high of the date) with a low of 76 (O'Hare's warmest low of the date). O'Hare's second hottest high was 92 degrees in 1960. In addition to the low of 73 degrees in 1995, O'Hare also recorded a low of 73 degrees in 1978. 

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 51 degrees. In 1972 the high was 93 with a low of 76, a hot day. 1972 was the year of the record warmest low (79 at Midway). In 1963 the high was 69 with a low of 42, a cool day and year of the record low (49 at Midway). That low of 42 is colder than the record low. In 1962 the high was 82 with a cold low of 48, a 34 degree spread. That low of 48 is colder than the record low. In 1961 the high was 87 with a low of 56, a 31 degree spread.

More recently, there was a large spread of 30 degrees in 1982 (high of 83 and low of 53).
 
On the cool side, in 1981 the high was 75 with a low of 51. In 2007 the high was 68 (our lone 60s high) with a low of 59. You have to go all the way back to 1963 to find the previous sub-70 degree high of 69 degrees. On that day in 1963 the low was 42, the lowest temperature of August 1963. The high was also 69 on the prior day, August 17, 1963. In 1997 the high was 70 with a low of 58. The record low of 49 was set in 1963 at Midway. The record coolest high of 65 occurred in 1950 at Midway. 28 of the last 33 lows (84%) were in the: 50s (11) and 60s (17). Five lows were in the warm 70s. 

As you can see, this is a day dominated by highs in the 70s and 80s and lows in the 50s and 60s. There were two 11 year stretches of such highs. The first was from 1979 through 1989. The second was from 1996 through 2006. There was a ten year stretch of lows in the 50s and 60s. This was from 1996 through 2005.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 5:35 and sunrise is at 6:04. Sunset is at 19:49 and twilight ends at 20:19. There's a total of 13 hours, 45 minutes of daylight today and 14 hours, 44 minutes between twilights. We lost two minutes of daylight from yesterday. Tomorrow twilight begins at 5:36 and sunrise is at 6:05. The moon is waxing crescent, having passed new yesterday at 10:54. Illumination was 1% at midnight, will be 2% at noon and 3% at midnight tonight. Moonrise was at 7:15 this morning. Moonset is at 20:01 tonight. Moonrise is at 8:24 tomorrow morning. Moonlight time is 12 hours, 46 minutes which is 37 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 16 days.