September 15: ORD

Today is September 15th. It's the 259th day of the year. 107 days remain. It's week 37 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 75 and the normal low is 55. I've calculated the 33-year averages at 72 and 54.

On the hot side, in 1994 (O'Hare's warmest September 15th) the high was 91  with a low of 71 (O'Hare's warmest low of the date). Three years earlier, in 1991, the high was 91 with a low of 68. Those highs of 91 degrees are O'Hare's hottest of the date. In 2004 the high was 86 with a low of 68. In 1992 the high was 85 with a low of 68. The other post-1978 warm low is 67 degrees in 1998. The record high of 99 was set back in 1939. The record warmest low of 75 occurred back in 1927. Both records were set at the University of Chicago. 23 of the past 33 highs (70%) were in the: 60s (11) and 70s (12). O'Hare recorded consecutive 60s and 70s highs as follows: seven from 1979 through 1985 and six from 1998 through 2003. Eight highs (24%) were in the warm: 80s (6) and 90s (2). Two highs (6%) were in the cool 50s, this being the record coolest high of 55 degrees in 1993 and a high of 59 degrees last year, in 2011. O'Hare's second warmest low of 69 degrees occurred in 1969.  

In looking back through the years to the first September at O'Hare in 1959, the following instances were notable: in 1969 the low was a warm 69 degrees. In 1966 the high was 65 with a cold low of 40, a cool day. In 1964 the high was a cool 62 degrees. In 1963 the high was 78 with a low of 46, a 32 degree spread. In 1961 the low was a cold 41 degrees. In 1959 the high was a cool 62 degrees.

More recently, there was a large spread of 31 degrees in 1985 (71/40). There were small spreads of five degrees in 1998 (72/67) and seven degrees in 1980 (64/57). 

On the cool side, in 2007 (O'Hare's coolest September 15th) the high was 61 with the record low of 39. The former record low (and second coldest low along with 1966) of 40 degrees occurred in 1985. Last year, in 2011, the high was 59 with a low of 42. In 1984 the high was 62 with a low of 41. In 1993 the high was the record coolest high of 55 with a low of 51, a four degree spread. The other cold low of 43 degrees occurred in 1999. The other cool highs are 62 degrees (1959, 1964, 1983 and 2000) and 63 degrees in 2008. The other co-record coolest high of 55 degrees occurred in 1916, a time when the official thermometer was located adjacent to Lake Michigan. 18 of the last 33 lows (55%) were in the 50s. Eight lows (21%) were in the cold: 30s (1) and 40s (7). Seven lows (21%) were in the warm: 60s (6) and 70s (1). So in back-to-back years, we had our hottest and coolest highs (55 in 1993 and 91 in 1994). 

There was an instance of back-to-back-to-back years in which temperatures were just about as opposite as they can be. It started in 1992 as the high was 85 with a low of 68, overall 11.5 degrees above normal. Then in 1993 the high was 55 with a low of 51, overall 12 degrees below normal. Then in 1994 the high was 91 with a low of 71, overall 16 degrees above normal.  

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 6:06 and sunrise is at 6:34. Sunset is at 19:02 and twilight ends at 19:30. There's a total of 12 hours, 28 minutes of daylight today and 13 hours, 24 minutes between twilights. We lost three minutes of daylight from yesterday. Tomorrow twilight begins at 6:07 and sunrise is at 6:35. The moon is new tonight at 21:11. Illumination was 1% at midnight, will be 0% at noon and 0% at midnight tonight. Moonrise was at 6:06 this morning. Moonset is at 18:31 tonight. Moonrise is at 7:16 tomorrow morning. Moonlight time is 12 hours, 25 minutes which is 38 minutes shorter than yesterday.

Summer began on June 4th and runs for 110 days through September 21st. High temperatures in this period are usually in the 70s and 80s. Highs in the 60s, 90s and 100s occur infrequently. Citizenship Day is in two days.