October 20: ORD

Today is October 20th. It's the 294th day of the year. 72 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 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 61 and the normal low is 42. I've calculated the 33-year averages at 60 and 42.

On the warm side, in 1979 (O'Hare's warmest October 20th) the high was 80 with a low of 68 (O'Hare's warmest low of the date). The record warmest low of 68 degrees also occurred in 1979 at Midway. This was also the only 60s low in O'Hare's history on October 20th. In 2000 the high was 77 with a low of 55. Three years later, in 2003, the high was 85 (O'Hare's hottest high of the date) with a low of 46, a 39 degree spread. The other warm highs were 74 degrees in 1986, 73 degrees in 1978 and 73 degrees in 2007. The record high of 86 degrees was set in 1953 at Midway. 18 of the past 33 highs (55%) were in the 50s. 12 highs (36%) were in the warm: 60s (5), 70s (5) and 80s (2). Three highs (9%) were in the cold 40s. O'Hare's second warmest October 20th was in 1971 as the high was 81 (O'Hare's second warmest high of the date) and low of 59 (O'Hare's co-second warmest low of the date). The other warm day was in 1965 as the high was 68 with a low of 59 (the other co-second warmest low of the date).  

In looking back through the years to the first October at O'Hare in 1959, the following instances were notable: in 1978 the high was a warm 73 with a low of 35, a 38 degree spread. In 1976 the high was a cold 47 degrees. In 1975 the high was 72 with a low of 40, a 32 degree spread. In 1974 the high was 47 with a low of 30, a cold day. In 1972 the high was 49 with a low of 25, a cold day. In 1971 the high was 81 with a low of 59, a warm day. In 1970 the high was 55 with a low of 51, a four degree spread. In 1966 the high was 63 with a low of 32, a 31 degree spread. In 1965 the high was 68 with a warm low of 59, a warm day. In 1964 the high was 45 with a low of 23, a cold day. That low of 23 was colder than the record low. In 1961 the high was a cold 48 with a low of 45, a three degree spread. In 1960 the high was 45 with a low of 29, a cold day. Tom Skilling reported on his blog that back in 1952, three inches of lake-effect snow fell in Chicago but melted quickly. The high that day was just 41 with a low of 29.

More recently, there was an additional large spread of 35 degrees in 1986 (74/39). There were additional small spreads of three degrees in 2004 (53/50), four degrees in 1983 (52/48), four degrees last year, in 2011 (47/43) and six degrees in 1985 (57/51).

On the cool side, in 1989 (O'Hare's third coolest October 20th) the high was 41 (O'Hare's coolest high of the date) with a low of 34. In 2002 the high was 52 with a low of 29. The other cool highs were 47 degrees in 1976, 47 degrees last year (in 2011), 48 degrees in 1961 and 49 degrees in 1995. The other cold low of 29 degrees occurred in 1991. The record low of 24 degrees was last set in 1930 but originally set way back in 1895. The record coolest high of 36 degrees occurred way back in 1895. As you can see, 1895 was by far the coldest of all October 20ths. The official readings were taken adjacent to Lake Michigan in 1895 and at the University of Chicago in 1930. 27 of the last 33 lows (82%) were in the: 30s (11) and 40s (16). Two lows were in the cold 20s. Four lows were in the warm: 50s (3) and 60s (1). O'Hare's coolest October 20th was in 1964 as the high was 45 (O'Hare's co-second coolest high of the date) and low of 23 (O'Hare's coldest low of the date and a low colder than the record low). There's a tie for O'Hare's second coolest October 20ths between 1960 and 1972. In 1960 the high was 45 (O'Hare's other co-second coolest high of the date) and low of 29. In 1972 the high was 49 with a low of 25 (O'Hare's second coldest low of the date). The other cool day was in 1974 as the high was 47 with a low of 30.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 6:44 and sunrise is at 7:12. Sunset is at 18:03 and twilight ends at 18:31. There's a total of 10 hours, 51 minutes of daylight today and 11 hours, 47 minutes between twilights. We lost two minutes of daylight from yesterday. Tomorrow twilight begins at 6:45 and sunrise is at 7:14. The moon is waxing crescent. Illumination was 29% at midnight, will be 35% at noon and 40% at midnight tonight. Moonrise is at 12:52 this afternoon. Moonset is at 22:46 tonight. Moonrise is at 13:36 tomorrow afternoon. Moonlight time is 9 hours, 54 minutes which is 13 minutes longer 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 four days. Halloween is in 11 days. Election Day is in 17 days. Standard Time doesn't return until November 4th.