October 31: ORD

Today is October 31st. It's the 305th day of the year. 61 days remain. It's week 44 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 that, 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 56 and the normal low is 39. I've calculated the 33-year averages at 59 and 41.

On the warm side, in 1979 (O'Hare's second warmest October 31st) the high was 75 with a low of 53. Two years later, in 1981 (O'Hare's third warmest October 31st), the high was 72 with a low of 54. In 1997 the high was 65 with a low of 54. Four years later, in 2001, the high was 68 with a low of 50. In 2008 the high was 70 with a low of 48. In 2000 the high was 70 with a low of 47. The other warm highs were 73 degrees in 1990 and 69 degrees in 1999. The other warm lows were 54 degrees in 1998 and 51 degrees in 1982. The record high of 84 degrees was set in 1950 at Midway. The record warmest low of 65 degrees occurred in 1974 at Midway. 24 of the past 33 highs (73%) were in the: 50s (14) and 60s (10). There were eight consecutive 50s and 60s highs from 1982 through 1989. Additionally, five highs (15%) were in the warm 70s and four (12%) were in the cold 40s. Extremes of the highs happened recently as the high in 2008 was 70 degrees while the high in 2009 was 46 degrees. O'Hare's warmest October 31st was in 1974 as the high was 77 (O'Hare's second hottest high of the date) with a low of 65 (O'Hare's warmest low of the date). The other warm day was in 1968 as the high was 79 (O'Hare's hottest high of the date) with a low of 42. In summary, O'Hare's second warmest lows of the date were 54 degrees in 1981, 1997 and 1998. 

In looking back through the years to the first October at O'Hare in 1959, the following instances were notable: in 1974 the high was 77 with a low of 65, a warm day and year of the record warmest low, also 65 at Midway. In 1973 the high was 49 with a low of 42, a seven degree spread. In 1972 the high was 46 with a low of 44, a two degree spread. In 1969 the high was 50 with a low of 47, a three degree spread. In 1968 the high was a warm 79 with a low of 42, a 37 degree spread and warm day. In 1967 the high was 51 with a low of 49, a two degree spread. In 1964 the high was 67 with a low of 29, a 38 degree spread. In 1959 the high was 50 with a low of 46, a four degree spread.

More recently, the large spread was 31 degrees in 1990 (73/42). There were small spreads of five degrees in 1998 (59/54) and seven degrees in 2005 (57/50).

On the cold side, in 1996 (O'Hare's coolest October 31st) the high was 40 (O'Hare's coolest high of the date) with a low of 26 (O'Hare's co-second coldest low of the date). Three years earlier, in 1993 (O'Hare's second coolest October 31st), the high was 44 (O'Hare's second coolest high of the date) with a low of 25 (O'Hare's coldest low of the date). The other co-second coldest low was 26 degrees in 1988. The record low of 23 degrees and record coldest high of 31 degrees occurred on the same cold, winter-like day way back in 1873, at a time when the official readings were taken adjacent to Lake Michigan. 23 of the last 33 lows (70%) were in the: 30s (11) and 40s (12). There were six consecutive 30s and 40s lows from 2006 through last year, 2011. Three lows (9%) were in the cold 20s. The 20s lows occurred in a nine year span from 1988 through 1996. Seven lows (21%) were in the warm 50s. The most recent 50s low was 50 degrees in 2005. For 14 years (from 1983 through 1996) lows were below 50 degrees. The past six years (from 2006 to 2011) have seen lows below 50 degrees.   

There's an instance of back-to-back years in which temperatures were just about as opposite as they can be. In 1996 the high was 40 with a low of 26, overall 14 degrees BELOW normal. Then in 1997 the high was 65 with a low of 54, overall 12.5 degrees ABOVE normal. 

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 6:57 and sunrise is at 7:26. Sunset is at 17:48 and twilight ends at 18:17. There's a total of 10 hours, 22 minutes of daylight today and 11 hours, 20 minutes between twilights. We lost two minutes of daylight from yesterday. Tomorrow twilight begins at 6:58 and sunrise is at 7:27. The moon is waning gibbous, having passed the full Hunter's moon two days ago at 14:49. Illumination was 98% at midnight, will be 97% at noon and 95% at midnight tonight. Moonset was at 9:09 this morning. Moonrise is at 18:56 tonight. Moonset is at 10:02 tomorrow morning. Moonlight time is 14 hours, 13 minutes which is 16 minutes longer than yesterday. Moonlight time is near its peak today.

Warm Fall began September 22nd and runs for 41 days through tomorrow, November 1st. High temperatures in this period are usually in the 50s, 60s and 70s. Highs in the 30s and 40s occur infrequently. Halloween is today. Standard Time returns in four days. Election Day is in six days.