July 21: ORD

Today is July 21st. It's the 202nd 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.

According to the National Climatic Data Center (NCDC), the normal high is 84 and the normal low is 64. I've calculated the 32-year averages at 83 and 65.

On the hot side, in 2002 the high was 96 with a low of 78. Four years earlier, in 1998, the high was 94 with a low of 76. In 1983 the high was 96 with a low of 72. The record high of 103 was set way back in 1901. The record warmest low of 80 occurred in 1972. 30 of the past 32 highs (94%) were in the: 70s (7), 80s (15) and 90s (8). Two highs were in the cool 60s.

Yesterday's high of 99 at 16:17 and low of 73 at 5:43 produced the hottest July 20th of the last 33 years. The high of 99 fell two degrees short of the record high of 101, set in 1980. The low of 73 is now the second warmest. The mean of 86 in 2011 makes this the hottest. 1980, with it's record high of 101 and low of 69, is now in a tie for second place with 1991. In 1991, the high was 96 with our warmest low of 74. The high was also a hot 96 in 1987. Highs of 100 were recorded at the lakefront in Chicago, Midway Airport, Rockford and Kankakee. Joliet recorded a high of 99. It was 98 at the National Weather Service office in Romeoville and 97 in West Chicago at the DuPage County Airport.

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 cool 51 degrees. 1970 was also the year of the record low of 51 at Midway. In 1966 the high was 81 with a cool 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 the high was 67 with a low of 55. Four years earlier, in 1992, the high was 67 with a low of 56. These were the only 60s highs and they are the record coolest highs. Another cool high of 71 occurred in 1981. Our lowest temperature, 53 degrees, occurred in 1979. The record low of 51 was set in 1970 at Midway. 24 of the last 32 lows (75%) were in the: 60s (14) and 70s (10). Eight lows (25%) were in the cool 50s.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 5:02 and sunrise is at 5:35. Sunset is at 20:24 and twilight ends at 20:56. There's a total of 14 hours, 49 minutes of daylight today and 15 hours, 54 minutes between twilights. We lost a minute of daylight from yesterday. Tomorrow twilight begins at 5:03 and sunrise is at 5:36. The moon is waning gibbous. Illumination was 68% at midnight, will be 64% at noon and 59% at midnight tonight. Moonset is at 12:08 this afternoon. Moonrise is at 23:12 tonight. Moonset is at 13:08 tomorrow afternoon. Moonlight time is 12 hours, 56 minutes which is 34 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. Parent's Day is in three days.