July 17: ORD

Today is July 17th. It's the 198th 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.

According to the National Climatic Data Center (NCDC), the normal high is 84 and the normal low is 64. I've calculated the 32-year averages at 86 and 66.

On the hot side, in 1982 the high was 93 with a low of 78. Four years later, in 1986, the high was 94 with a low of 74. The other hot highs were 97 in 2005, 95 in 2006, 93 in 1983 and 93 last year (in 2010). The record high of 100 was set in 1942 after first occurring in 1887. The record warmest low of 80 occurred way back in 1878. 18 of the past 32 highs (56%) were in the 80s. Nine highs (28%) were in the hot 90s and five highs (16%) were in the cool 70s, including a high two 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 1979-2010, this is the average 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 88 with a cool 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 the high was 71 with a low of 50, a reading colder than the record low of 51, set in 1985. In 2009 the high was 70 with a low of 57. The record coolest high of 64 occurred back in 1924. The other cool 70s highs were 75 in 2004, 78 in 1984 and 79 in 1985. 27 of the last 32 lows (84%) were in the: 60s (17) and 70s (10). Five lows were in the cool 50s. The other 50s lows were 55 in 1989 and 57 in 1984.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 4:58 and sunrise is at 5:32. Sunset is at 20:27 and twilight ends at 21:00. There's a total of 14 hours, 55 minutes of daylight today and 16 hours, 2 minutes between twilights. We lost a minute of daylight from yesterday. Tomorrow twilight begins at 4:59 and sunrise is at 5:33. The moon is waning gibbous, having passed the full Buck or Thunder moon two days ago at 1:40. Illumination was 96% at midnight, will be 93% at noon and 91% at midnight tonight. Moonset is at 8:06 this morning. Moonrise is at 21:36 tonight. Moonset is at 9:08 tomorrow morning. Moonlight time is 10 hours, 30 minutes which is 38 minutes longer than yesterday.

Summer began June 4 and runs for 110 days through September 21. High temperatures in this period are usually in the 70s and 80s. Highs in the 60s, 90s and 100s occur infrequently. Parent's Day is in seven days.