September 15: ORD

Today is September 15th. It's the 258th 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.

According to the National Climatic Data Center (NCDC), the normal high is 75 and the normal low is 55. I've calculated the 32-year averages at 72 and 54.

On the hot side, in 1994 the high was 91 (our most recent 90s high) with a low of 71 (our only 70s low). Three years earlier, in 1991, the high was 91 with a low of 68. In 2004 the high was 86 with a low of 68. In 1992 the high was 85 with a low of 68. Another warm low was 67 in 1998. The record high of 99 was set back in 1939. The record warmest low of 75 occurred back in 1927. Both the record high and record warmest low were set in the city, closer to Lake Michigan. 23 of the past 32 highs (72%) were in the: 60s (11) and 70s (12). Eight highs (25%) were in the warm: 80s (6) and 90s (2). One high (3%) was in the cool 50s, this being the record coolest high of 55 in 1993.

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 the high was 61 with the record low of 39. The former record low of 40 occurred in 1985. 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. Other cool highs were 62 in 1983, 62 in 2000 and 63 three years ago in 2008. The other co-record coolest high of 55 occurred in 1916, a time when the official thermometer was located adjacent to Lake Michigan. 18 of the last 32 lows (56%) were in the 50s. Seven lows (22%) were in the cold: 30s (1) and 40s (6). Seven lows (22%) 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).

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 6:05 and sunrise is at 6:33. Sunset is at 19:03 and twilight ends at 19:31. There's a total of 12 hours, 30 minutes of daylight today and 13 hours, 26 minutes between twilights. We lost three minutes of daylight from yesterday. Tomorrow twilight begins at 6:06 and sunrise is at 6:34. The moon is waning gibbous and was full three days ago as the full Harvest (a.k.a. Barley) moon at 4:27. Illumination was 93% at midnight, will be 90% at noon and 87% at midnight tonight. Moonset is at 9:47 this morning. Moonrise is at 20:13 tonight. Moonset is at 10:46 tomorrow morning. Moonlight time is 13 hours, 34 minutes which is 32 minutes longer than yesterday.

Summer began June 4 and runs for 110 days through September 21. 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.