October 21: ORD

Today is October 21st. It's the 295th day of the year. 71 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 60 and the normal low is 41. I've calculated the 33-year averages at 60 and 43.

On the warm side, in 1979 (O'Hare's warmest October 21st) the high was 84 (O'Hare's hottest high of the date) with a low of 70 (O'Hare's warmest low of the date). The record warmest low of 70 degrees was set in 1979 at Midway. In 2007 (O'Hare's second warmest October 21st) the high was 80 (O'Hare's second hottest high of the date with 1978) and a low of 60. In 1986 the high was 77 with a low of 52. The other warm highs were 74 degrees in 1977, 73 degrees in 1975 and 72 degrees in 1994. The record high of 87 degrees was set in 1953 at Midway. The other warm low of 55 degrees occurred in 2009. 26 of the past 33 highs (79%) were in the: 50s (16) and 60s (10). There were three long periods of 50s and 60s highs. The first was six years from 1980 through 1985. The second was six years from 1988 through 1993. The third was nine years from 1998 through 2006. Four highs (12%) were in the warm: 70s (2) and 80s (2). Three highs (9%) were in the cold 40s. The other warm days were in: 1971 (high of 73, low of 66 (O'Hare's second warmest low of the date)), 1978 (high of 80 (O'Hare's second hottest high of the date with 2007), low of 53) and 1963 (high of 69, low of 59).

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 80 with a low of 53, a warm day. In 1977 the high was a warm 74 degrees. In 1976 the high was a cold 46 with a low of 30, a cold day. In 1975 the high was a warm 73 degrees. In 1974 the low was a cold 27 degrees. In 1973 the high was 67 with a low of 37, a 30 degree spread. In 1971 the high was 73 with a low of 66, a seven degree spread and warm day. In 1968 the high was 69 with a low of 38, a 31 degree spread. In 1963 the high was 69 with a warm low of 59, a warm day. In 1961 the high was 49 with a low of 44, a five degree spread. In 1960 the high was 49 with a cold low of 27, a cold day. 

More recently, there were large spreads of 32 degrees in 1991 (67/35), 32 degrees in 1999 (68/36) and 31 degrees in 1994 (72/41). There was a small spread of five degrees in 1983 (56/51). 

On the cool side, in 1987 (O'Hare's coolest October 21st) the high was 42 (O'Hare's coolest high of the date) with a low of 32. In 1995 the high was 43 (O'Hare's second coolest high of the date) with a low of 36, a seven degree spread. In 1982 the high was 52 with a low of 27. The record low of 26 degrees was set in 1952 at Midway. The record coolest high of 39 degrees occurred back in 1913, a time when the official readings were taken adjacent to Lake Michigan. 26 of the last 33 lows (79%) were in the: 30s (15) and 40s (11). There were 14 consecutive 30s and 40s lows from 1987 through 2000. One low (3%) was in the cold 20s. Six lows (18%) were in the warm: 50s (4), 60s (1) and 70s (1). 1960 and 1976 share O'Hare's second coolest October 21st days. In 1960 the high was 49 with a low of 27. In 1976 the high was 46 with the second coldest low of 30. O'Hare's coldest lows of the date were 27 degrees in 1960, 1974 and 1982.

There's an instance of back-to-back years in which temperatures were just about as opposite as they can be. In 1986 the high was 77 with a low of 52, overall 13.5 degrees ABOVE normal. Then in 1987 the high was 42 with a low of 32, overall 14 degrees BELOW normal.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 6:45 and sunrise is at 7:14. Sunset is at 18:01 and twilight ends at 18:30. There's a total of 10 hours, 47 minutes of daylight today and 11 hours, 45 minutes between twilights. We lost four minutes of daylight from yesterday. Tomorrow twilight begins at 6:46 and sunrise is at 7:15. The moon reaches first quarter today at 22:32. Illumination was 40% at midnight, will be 46% at noon and 51% at midnight tonight. Moonrise is at 13:36 this afternoon. Moonset is at 23:54 tonight. Moonrise is at 14:14 tomorrow afternoon. Moonlight time is 10 hours, 18 minutes which is 24 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 three days. Halloween is in ten days. Election Day is in 16 days. Standard Time doesn't return until November 4th.