July 21: ORD

Today is July 21st. It's the 203rd day of the year. 163 days remain. It's week 29 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 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 84 and the normal low is 64. I've calculated the 33-year averages at 84 and 66.

On the hot side, in 2002 the high was 96 (O'Hare's co-second hottest high of the date) with a low of 78 (O'Hare's second warmest low of the date). Last year, in 2011, the high was 99 (O'Hare's hottest high of the date) with a low of 75. 2002 and 2011 are tied as O'Hare's warmest July 21sts. The other co-second hottest high of the date was 96 degrees in 1983. The other warm low of 76 degrees occurred in 1998. The record high of 103 was set way back in 1901, at a time when the official readings were taken adjacent to Lake Michigan. The record warmest low of 80 occurred in 1972 at Midway. 15 of the past 33 highs (46%) were in the 80s. Nine highs (27%) were in the hot 90s. Nine highs (27%) were in the cool: 60s (2) and 70s (7). 1972 was O'Hare's second warmest July 21st as the high was 93 with a low of 79 (O'Hare's warmest low of the date).

In looking back through the years to the first July at O'Hare in 1959, the following instances were notable: In 1973 the high was 78 with a low of 71, a seven degree spread. In 1972 the high was 93 with a warm low of 79, a hot day and year of the record warmest low of 80 at Midway. In 1970 the low was a cold 51 degrees. 1970 was also the year of the record low of 51 at Midway. In 1966 the high was 81 with a cold low of 48, a 33 degree spread.

More recently, there was a large spread of 33 degrees in 1979 (86/53).

On the cool side, in 1996 (O'Hare's coolest July 21st) the high was the co-record coolest high of 67 with a low of 55. Four years earlier, in 1992, the high was the other co-record coolest high of 67 with a low of 56. The other cool high of 71 degrees occurred in 1981. The record low of 51 was set in 1970 at Midway. 25 of the last 33 lows (76%) were in the: 60s (14) and 70s (11). O'Hare recorded consecutive 60s & 70s lows as follows: eight from 1982 through 1989 and six from 2001 through 2006. Eight lows (24%) were in the cool 50s. The coldest low of the date was 48 degrees in 1966. The low was also a cold 51 degrees in 1970.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 5:03 and sunrise is at 5:36. Sunset is at 20:23 and twilight ends at 20:56. There's a total of 14 hours, 47 minutes of daylight today and 15 hours, 53 minutes between twilights. We lost two minutes of daylight from yesterday. Tomorrow twilight begins at 5:04 and sunrise is at 5:37. The moon is waxing crescent. Illumination was 5% at midnight, will be 7% at noon and 10% at midnight tonight. Moonrise is at 8:20 this morning. Moonset is at 21:30 tonight. Moonrise is at 9:27 tomorrow morning. Moonlight time is 13 hours, 10 minutes which is 35 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 90s and 100s occur infrequently. Parent's Day is tomorrow.