July 20: ORD

Today is July 20th. It's the 202nd day of the year. 164 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 85 and 65.

On the hot side, last year, in 2011 (O'Hare's warmest July 20th), the high was 99 with a low of 73. In 1980 the high was the record high of 101 with a low of 69, a spread of 32 degrees. In 1991 the high was 96 with a low of 74. The other hot high was 96 in 1987. The record warmest low of 79 occurred in 1977 at Midway. 32 of the past 33 highs (97%) were in the: 70s (9), 80s (14) and 90s (9). One high, the record high of 101 in 1980, was in the 100s. O'Hare's second warmest July 20th was in 1977 as the high was 95 with a low of 76 (O'Hare's second warmest low of the date). O'Hare's warmest low of 77 degrees occurred in 1972. From 1979-2011, this is the average seventh hottest day of the year.

In looking back through the years to the first July at O'Hare in 1959, the following instances were notable: In 1977 the high was 95 with a warm low of 76, a hot day and year of the record warmest low of 79 at Midway. In 1972 the low was a warm 77 degrees. In 1970 the high was a cool 72 degrees. 1970 was the year of the record low of 53 degrees at Midway (O'Hare was slightly warmer at 55 degrees). In 1966 the low was a cold 51 degrees.

More recently, there were no other large spreads of at least 30 degrees or no small spreads of up to seven degrees.

On the cool side, in 1996 the high was 71 (O'Hare's coolest high of the date) with a low of 59. Three years ago, in 2009, the high was 76 with a low of 55.  The record low of 53 degrees was set in 1970 at Midway. O'Hare's second  coolest highs of the date were 72 degrees in 1970 and 1989. The record coolest high of 65 degrees occurred in 1950 at Midway. 19 of the last 33 lows (58%) were in the 60s. Seven lows (21%) were in the cold 50s. Seven lows (21%) were in the warm 70s. O'Hare's coolest July 20ths were in 1966 (high of 76 with O'Hare's coldest low of the date of 51) and 1970 (high of 72 with a low of 55). 

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 20:57. There's a total of 14 hours, 49 minutes of daylight today and 15 hours, 55 minutes between twilights. We lost two minutes of daylight from yesterday. Tomorrow twilight begins at 5:03 and sunrise is at 5:36. The moon is waxing crescent, having become new two nights ago at 23:24. Illumination was 1% at midnight, will be 3% at noon and 5% at midnight tonight. Moonrise was at 7:15 this morning. Moonset is at 21:00 tonight. Moonrise is at 8:20 tomorrow morning. Moonlight time is 13 hours, 45 minutes which is 33 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 in two days.