September 24: ORD

Today is September 24th. It's the 268th day of the year. 98 days remain. It's week 39 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 72 and the normal low is 51. I've calculated the 33-year averages at 69 and 49.

On the warm side, in 2007 (O'Hare's warmest September 24th) the high was 90 (O'Hare's hottest high of the date) with a low of 68 (O'Hare's warmest low of the date). In 1984 (O'Hare's third warmest September 24th) the high was 84 (along with 1960, O'Hare's second hottest high of the date) with a low of 61. O'Hare's second warmest lows of the date are 64 degrees three years ago (in 2009) and  64 degrees in 1986. The record high of 91 degrees was set way back in 1891, at a time when the official readings were taken adjacent to Lake Michigan. The record warmest low of 71 degrees occurred in 1930 at the University of Chicago. 17 of the past 33 highs (52%) were in the 60s. 13 highs (39%) were in the warm: 70s (8), 80s (4) and 90s (1). Three highs (9%) were in the cool 50s. O'Hare's second warmest September 24th was in 1960 as the high was 84 (with 1984, O'Hare's second hottest high of the date) with a low of 63. The other warm low of 63 degrees occurred in 2005. 

In looking back through the years to the first September at O'Hare in 1959, the following instances were notable: in 1978 the high was 79 with a low of 49, a 30 degree spread. In 1976 the high was 69 with a cold low of 34, a 35 degree spread. That low of 34 was colder than the record low. In 1966 the high was 72 with a cold low of 39, a 33 degree spread. In 1965 the high was a cool 54 with a low of 42, a cool day. In 1960 the high was 84 with a low of 63, a warm day.

More recently, there were large spreads of 31 degrees in 1993 (69/38) and 30 degrees in 1983 (66/36). There was a small spread of seven degrees in 2000 (56/49). 

On the cool side, in both 1985 and 1989, the highs were 61 with co-record lows of 35. Those lows of 35 degrees are actually O'Hare's second coldest lows of the date (see 1976 below)! The record low of 35 degrees was originally set in 1942. 1985 and 1989 are tied with 1965 (O'Hare's coolest high of 54 and low of 42) as O'Hare's coolest September 24ths. In 1991 the high was 57 with a low of 40. In 1995 the high was 62 with a low of 37 (our most recent 30s low). The other cool highs were 55 degrees in 2001 and 56 degrees in 2000 (our most recent 50s highs). The record coolest high of 51 degrees occurred in 1942 at Midway. 15 of the last 33 lows (46%) were in the 40s. Five lows (15%) were in the cold 30s. 13 lows (39%) were in the warm: 50s (8) and 60s (5). The other cold lows are 34 degrees in 1976 (O'Hare's coldest low of the date and a low that's colder than the record lows!), 36 degrees in 1983 (a degree above the record lows), 38 degrees in 1993 and 39 degrees in 1966.

There was an instance of back-to-back years in which temperatures were just about as opposite as they can be. In 1984 the high was 84 with a low of 61, overall 11.5 degrees ABOVE normal. Then in 1985 the high was 61 with a low of 35, overall 13 degrees BELOW normal. 

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 6:15 and sunrise is at 6:43. Sunset is at 18:46 and twilight ends at 19:14. There's a total of 12 hours, 3 minutes of daylight today and 12 hours, 59 minutes between twilights. We lost three minutes of daylight from yesterday. Tomorrow twilight begins at 6:16 and sunrise is at 6:44. The moon is waxing gibbous, having passed first quarter two days ago at 14:41. Illumination was 66% at midnight, will be 71% at noon and 76% at midnight tonight. Moonset was at 0:54 this morning. Moonrise is at 15:37 this afternoon. Moonset is at 2:01 tomorrow morning. Moonlight time is 9 hours, 17 minutes which is 12 minutes longer than yesterday.
 
Warm Fall began two days ago 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. The Autumnal Equinox was two days ago at 9:49. Columbus Day is in 14 days. Standard Time doesn't return until November 4th.