July 18: ORD

Today is July 18th. It's the 200th day of the year. 166 days remain. It's week 29 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 84 and the normal low is 64. I've calculated the 33-year averages at 85 and 66.

On the hot side, last year, in 2011 (O'Hare's warmest July 18th), the high was 92 with the new record warmest low of 80 (at 6:04). The former record warmest low of 78 occurred in 1942. In 1986 (O'Hare's second warmest July 18th) the high was 93 with a low of 75. O'Hare's hottest high of 96 occurred in 1991. The record high of 100 was set in 1946 at Midway. 25 of the past 33 highs (76%) were in the: 80s (14) and 90s (11). O'Hare recorded consecutive highs in the 80s & 90s as follows: 13 from 1971 through 1983 and seven from 1993 through 1999. Eight highs (24%) were in the cool 70s. From 1979-2011, this is the average fourth hottest day of the year.

In looking back through the years to the first July at O'Hare in 1959, the following instances were notable: In 1966 the high was 95 with a low of 67. The day falls short of being exceptionally hot. However, we don't reach 95 degrees all that often.

More recently, there were no additional small spreads of up to 7 degrees or large spreads of at least 30 degrees.

On the cool side, in 1979 (O'Hare's coolest July 18th) the high was 82 with a low of 45, O'Hare's coldest low of the date and a low colder than the record low. Three decades later, in 2009, the high was 72 with a low of 57. The record low of 53 was set in 1979 at Midway. O'Hare's coolest high of the date was 71 degrees in 2000. The record coolest high of 62 occurred back in 1924, at a time when the official readings were taken adjacent to Lake Michigan. 26 of the last 33 lows (79%) were in the: 60s (16) and 70s (10). O'Hare recorded consecutive lows in the 60s & 70s as follows: six from 1986 through 1991 and ten from 1993 through 2002. Six lows (18%) were in the cool: 40s (1) and 50s (5). One low (3%) was in the hot 80s (the record of 80 set last year).

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 5:00 and sunrise is at 5:33. Sunset is at 20:25 and twilight ends at 20:58. There's a total of 14 hours, 52 minutes of daylight today and 15 hours, 58 minutes between twilights. We lost two minutes of daylight from yesterday. Tomorrow twilight begins at 5:01 and sunrise is at 5:34. The moon becomes new tonight at 23:24. Illumination was 1% at midnight, will be 0% at noon and 0% at midnight tonight. Moonrise was at 5:08 this morning. Moonset is at 19:54 tonight. Moonrise is at 6:11 tomorrow morning. Moonlight time is 14 hours, 46 minutes which is 19 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. Parent's Day is in four days.