July 4: ORD

Today is July 4th. It's the 186th day of the year. 180 days remain. It's week 27 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 63. I've calculated the 33-year averages at 84 and 64.

On the hot side, in 1990 (with 1977, O'Hare's warmest July 4th) the high was 96 (O'Hare's hottest high of the date) with a low of 77. In 1999 (O'Hare's second warmest July 4th by just half a degree) the high was 93 with a low of 79 (O'Hare's warmest low of the date). The other warm lows were 75 in 1993 and 74 in 1994. The record high of 102 was set way back in 1911. The record warmest low of 80 occurred back in 1921. Both records occurred at a time when the official readings were taken adjacent to Lake Michigan. 17 of the past 33 highs (52%) were in the 80s. Seven highs (21%) were in the hot 90s. Nine highs (27%) were in the cool: 60s (2) and 70s (7). Along with 1990, 1977 is also O'Hare's warmest July 4th. In 1977 the high was 95 (O'Hare's second hottest high of the date) with a low of 78 (O'Hare's second 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 1977 the high was 95 with a warm low of 78, a hot day. In 1972 the high was 66 with a low of 49 (colder than the record low), a cool day. 1972 was also the year of the record low of 50 at Midway. In 1970 the high was a cool 72 degrees. In 1967 the high was 64 with a low of 50 (tied with the record low), a cool day. In 1964 the low was a cool 50 degrees (tied with the record low). In 1963 the high was 79 with a cold low of 46 (colder than the record low), a 33 degree spread and cool day. In 1961 the high was a cool 65 with a low of 60, a five degree spread and cool day.

More recently, there were large spreads of 33 degrees in 1988 (89/56) and 30 degrees in 1985 (91/61).

On the cool side, in 1997 the high was 66 with a low of 56. Three years ago, in 2009, the high was 69 with a low of 59. In the prior year of 2008, the high was 76 with a low of 52. In 1996 the high was 77 with a low of 51. In 1979 the high was 74 with a low of 54. The record low of 50 was set in 1972 at Midway. The record coolest high of 62 occurred back in 1920, at a time when the official readings were taken adjacent to Lake Michigan. 18 of the last 33 lows (55%) were in the 60s. Nine lows (28%) were in the cool 50s. Six lows (18%) were in the warm 70s. Our most recent 70s lows were 73 in 2003 and 70 in 2004. O'Hare's coolest July 4th was in 1967 as the high was 64 (O'Hare's coolest high of the date) with a low of 50 (tied with the record low). O'Hare's second coolest July 4th was in 1972 as the high was 66 with a low of 49 (O'Hare's second coldest low and colder than the record low). O'Hare's coldest low of the date was 46 degrees in 1963 (also colder than the record low). The low of 50 in 1964 is also tied with the record low.    

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 4:48 and sunrise is at 5:23. Sunset is at 20:33 and twilight ends at 21:07. There's a total of 15 hours, 10 minutes of daylight today and 16 hours, 19 minutes between twilights. We lost a minute of daylight from yesterday. Tomorrow twilight begins at 4:49 and sunrise is at 5:23. The moon is waning gibbous, having reached the full Buck moon yesterday at 13:52. Illumination was 100% at midnight, will be 99% at noon and 97% at midnight tonight. Moonset was at 6:27 this morning. Moonrise is at 21:08 tonight. Moonset is at 7:38 tomorrow morning. Moonlight time is 9 hours, 19 minutes which is 30 minutes longer 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. Independence Day is today.