July 17: ORD

Today is July 17th. It's the 199th day of the year. 167 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 86 and 66.

On the hot side, in 1982 (O'Hare's warmest July 17th) the high was 93 with a low of 78 (O'Hare's warmest low of the date). Four years later, in 1986 (O'Hare's second warmest July 17th), the high was 94 with a low of 74. O'Hare's hottest temperature of the date was 97 in 2005. O'Hare's second hottest temperature of the date was 95 in 2006. The record high of 100 was set in 1942 at Midway after first occurring in 1887. The record warmest low of 80 occurred way back in 1878, at a time when the official readings were taken adjacent to Lake Michigan. 28 of the past 33 highs (85%) were in the: 80s (18) and 90s (10). Five highs were in the cool 70s, including a high three years ago (in 2009) of 70 which, had it not made it to 70, would've been the first 60s high in the history of O'Hare on July 17. From 1979-2011, today ranks as the average hottest day of the year. O'Hare's second warmest low of the date is 75 degrees in 1968.

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 88 with a cold low of 51, a 37 degree spread. That low of 51 is equal to the record low, set in 1985.

More recently, there was a large spread of 34 degrees in 1989 (89/55).

On the cool side, in 1979 (O'Hare's coolest July 17th) the high was 71 with a low of 50 (O'Hare's coldest low of the date), a reading colder than the record low of 51 which was set in 1985. In 2009 the high was 70 (O'Hare's coolest high of the date) with a low of 57. The record coolest high of 64 occurred back in 1924, at a time when the official readings were taken adjacent to Lake Michigan. 28 of the last 33 lows (85%) were in the: 60s (18) and 70s (10). Five lows were in the cool 50s. O'Hare had a cold low of 51 in 1966 which is tied with the record low.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 4:59 and sunrise is at 5:32. Sunset is at 20:26 and twilight ends at 20:59. There's a total of 14 hours, 54 minutes of daylight today and 16 hours between twilights. We lost a minute of daylight from yesterday. Tomorrow twilight begins at 5:00 and sunrise is at 5:33. The moon is waning crescent, heading towards new tomorrow night at 23:24. Illumination was 4% at midnight, will be 2% at noon and 1% at midnight tonight. Moonrise was at 4:10 this morning. Moonset is at 19:15 tonight. Moonrise is at 5:08 tomorrow morning. Moonlight time is 15 hours, 5 minutes which is 11 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 five days.