September 21: ORD

Today is September 21st. It's the 265th day of the year. 101 days remain. It's week 38 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 73 and the normal low is 52. I've calculated the 33-year averages at 71 and 52.

On the warm side, two years ago, in 2010 (tied with 1970 as O'Hare's warmest September 21st), the high was 88 with a low of 68 (along with 1965 and 1970, O'Hare's second warmest low of the date). Three years earlier, in 2007 (O'Hare's second warmest September 21st), the high was 88 with a low of 64. Those highs of 88 degrees in 2007 and 2010 are O'Hare's hottest of the date along with a high of 88 degrees in 1970. In 1980 (O'Hare's fourth warmest September 21st with 1959 and 1965) the high was 79 with a low of 69 (O'Hare's warmest low of the date). Post-1978, O'Hare's other warm high was 87 degrees in 2005. The record high of 92 degrees was set at Midway in 1970. The record warmest low of 76 degrees occurred in 1931 at the University of Chicago. 14 of the past 33 highs (42%) were in the 60s. 17 highs (52%) were in the warm: 70s (8) and 80s (9). 5 of our last 8 highs (62%) and 6 of our last 10 highs (60%) were in the 80s. Two highs (6%) were in the cool 50s. Tied with 2010 as O'Hare's warmest September 21st is 1970 as the high was 88 (tied with 2007 and 2010 as the hottest high of the date) and low of 68. O'Hare's third warmest 9/21 was in 1964 as the high was 85 with a low of 65. The other two years that are tied with 1980 as O'Hare's fourth warmest 9/21 is 1959 (high of 84, low of 64) and 1965 (high of 80, low of 68).  

In looking back through the years to the first September at O'Hare in 1959, the following instances were notable: in 1976 the low was a cold 40 degrees. In 1975 the high was a cool 58 with a low of 51, a seven degree spread. In 1971 the high was 72 with a low of 42, a 30 degree spread. In 1970 the high was 88 with a low of 68, a hot day and year of the record high. In 1965 the high was 80 with a warm low of 68, a warm day. In 1964 the high was 85 with a low of 65, a hot day. In 1962 the high was 71 with a cold low of 33, a 38 degree spread. That low of 33 was colder than the record low. In 1959 the high was 84 with a warm low of 64, a warm day.

More recently, there were large spreads of 34 degrees in 2005 (87/53), 32 degrees in 1984 (80/48) and 31 degrees in 2004 (82/51). There was a small spread of seven degrees three years ago in 2009 (67/60).

On the cool side, in 1995 (O'Hare's coolest September 21st) the high was the record coolest high of 50 with a low of 41. In 1983 (O'Hare's second coolest September 21st) the high was 53 (O'Hare's second coolest high of the date) with the co-record low of 39. The other co-record low of 39 degrees occurred in 1991. Those lows of 39 degrees are actually the second coldest lows of the date. The other post-1978 cool highs were 61 degrees in both 1982 and 1999. The other post-1978 cold low of 40 degrees occurred in 2006. 17 of the last 33 lows (52%) were in the 50s. Nine lows (27%) were in the 40s. Two lows (6%) were in the cold 30s. Five lows (15%) were in the warm 60s. O'Hare's coldest low of 33 degrees (colder than the record low) occurred in 1962, at a time when O'Hare was not the official station of record. O'Hare's third coolest high of the date of 58 degrees occurred in 1975.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 6:12 and sunrise is at 6:40. Sunset is at 18:51 and twilight ends at 19:19. There's a total of 12 hours, 11 minutes of daylight today and 13 hours, 7 minutes between twilights. We lost three minutes of daylight from yesterday. Tomorrow twilight begins at 6:13 and sunrise is at 6:41. The moon is waxing crescent and reaches first quarter tomorrow morning at 9:49. Illumination was 32% at midnight, will be 38% at noon and 44% at midnight tonight. Moonrise is at 13:09 this afternoon. Moonset is at 22:45 tonight. Moonrise is at 14:06 tomorrow afternoon. Moonlight time is 9 hours, 36 minutes which is eight minutes shorter than yesterday.

Summer began on June 4th and runs for 110 days through today, September 21st. High temperatures in this period are usually in the 70s and 80s. Highs in the 60s, 90s and 100s occur infrequently. The Autumnal Equinox is tomorrow at 9:49. Columbus Day is in 17 days. Standard Time doesn't return until November 4th.