October 17: ORD

Today is October 17th. It's the 291st day of the year. 75 days remain. It's week 42 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 this, 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 62 and the normal low is 42. I've calculated the 33-year averages at 61 and 42.
 
On the warm side, in 1994 (O'Hare's second warmest October 17th) the high was 78 with a low of 61 (O'Hare's third warmest low of the date). Four years later, in 1998, the high was 70 with the record warmest low of 63, a seven degree spread. As you can see, two of O'Hare's three lows in the 60s on October 17th occurred in the past 33 years. In 1990 the high was 78 with a low of 51. The other warm highs were 80 degrees in 1964 (O'Hare's second hottest high of the date), 75 degrees in 2005, 74 degrees in 1991 and 74 degrees in 1995. The record high of 86 degrees was set in 1950 at Midway. 17 of the past 33 highs (52%) were in the: 40s (3) and 50s (14). 16 of the past 33 highs (48%) were in the: 60s (6) and 70s (10). O'Hare's warmest October 17th was in 1965 as the high was 82 (O'Hare's warmest high of the date) with a low of 62 (O'Hare's second warmest low of the date). The other warm days are: 1963 (high of 76, low of 57), 1961 (high of 78, low of 53), 1968 (high of 77, low of 54) and 1971 (high of 76, low of 51).   

In looking back through the years to the first October at O'Hare in 1959, the following instances were notable: in 1977 the high was 67 with a low of 35, a 32 degree spread. In 1976 the high was 50 with a low of 29, a cold day. In 1972 the high was a cool 47 with a low of 31, a cool day. In 1971 the high was a warm 76 with a low of 51, a warm day. In 1968 the high was 77 with a low of 54, a warm day. In 1965 the high was 82 with a low of 62, a warm day. In 1964 the high was a warm 80 with a low of 43, a 37 degree spread. In 1963 the high was 76 with a low of 57, a warm day. In 1962 the high was 69 with a low of 37, a 32 degree spread. In 1961 the high was a warm 78 with a low of 53, a warm day. 

More recently, there were large spreads of 34 degrees in 2005 (75/41) and 30 degrees two decades earlier in 1985 (72/42). There were no additional small spreads of up to seven degrees.

On the cool side, in 1992 (O'Hare's coolest October 17th) the high was 49 with a low of 28 (O'Hare's coldest low of the date). A decade later, in 2002, the high was 45 (O'Hare's coolest high of the date) with a low of 36. The other cool high of 47 degrees (tied with 1972 as O'Hare's second coolest high of the date) occurred in 1989. The record low of 26 degrees was set in 1948 at Midway. The record coolest high of 40 degrees occurred way back in 1880, a time when the official readings were taken adjacent to Lake Michigan. 27 of the last 33 lows (82%) were in the: 30s (10) and 40s (17). One low was in the cold 20s. Five lows were in the warm: 50s (3) and 60s (2). O'Hare's second coolest October 17th was in 1972 as the high was 47 (tied with 1989 as O'Hare's second coolest high of the date) with a low of 31. O'Hare's third coolest October 17th was in 1976 as the high was 50 with a low of 29 (O'Hare's second coldest low of the date). 

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 6:41 and sunrise is at 7:09. Sunset is at 18:08 and twilight ends at 18:36. There's a total of 10 hours, 59 minutes of daylight today and 11 hours, 55 minutes between twilights. We lost two minutes of daylight from yesterday. Tomorrow twilight begins at 6:42 and sunrise is at 7:10. The moon is waxing crescent and was new two days ago at 7:02. Illumination was 4% at midnight, will be 7% at noon and 11% at midnight tonight. Moonrise was at 9:47 this morning. Moonset is at 19:40 tonight. Moonset is at 10:57 tomorrow morning. Moonlight time is 9 hours, 53 minutes which is 24 minutes shorter than yesterday.

Warm Fall began September 22nd and runs for 41 days through November 1st. High temperatures in this period are usually in the 50s, 60s and 70s. Highs in the 30s and 40s occur infrequently. United Nations Day is in seven days. Standard Time doesn't return until November 4th.