May 11: ORD

Today is May 11th. It's the 132nd day of the year. 234 days remain. It's week 19 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 69 and the normal low is 47. I've calculated the 33-year averages at 71 and 48.

On the warm side, last year, in 2011 (O'Hare's warmest May 11th), the high was 87 with a low of 63 (O'Hare's warmest low of the date). In 1982 the high was the record high of 89 with a low of 58, a 31 degree spread. Three years later, in 1985, the high was 87 with a low of 60. In 1992 the high was 84 with a low of 61. Five years prior, in 1987, the high was 88 with a low of 53, a 35 degree spread. In 1991 the high was 85 with a low of 56. The other warm high was 82 in 2004. The record warmest low of 70 occurred way back in 1881, at a time when the official readings were taken adjacent to Lake Michigan. 27 of the past 33 highs (82%) were in the: 60s (8), 70s (11) and 80s (8). On the cool side, six highs were in the: 40s (1) and 50s (5).

In looking back through the years to the first May at O'Hare in 1959, the following instances were notable: In 1978 the high was 66 with a warm low of 59, a seven degree spread. In 1977 the high was 73 with a low of 39, a 34 degree spread. In 1972 the high was 73 with a low of 42, a 31 degree spread. In 1971 the high was 74 with a low of 44, a 30 degree spread. In 1969 the high was a cool 57 degrees. In 1967 the high was a cool 55 degrees. In 1966 the high was 45 with a low of 34, a cold day. In 1963 the high was a cool 53 degrees. In 1961 the high was 77 with a low of 44, a 33 degree spread. In 1960 the high was a cold 46 with a low of 37, a cold day.

More recently, we had additional large spreads of 33 degrees in 1983 (73/40), 30 degrees in 1986 (80/50) and 30 degrees in 1988 (75/45).
 
On the cool side, in 1981 (O'Hare's coldest May 11th) the high was 45 (along with 1966, O'Hare's coldest high) with the record low of 33. 15 years later, in 1996, the high was 52 with a low of 38. The other cool highs were 52 two years ago, in 2010 and 57 in 2002. The record coldest high of 41 occurred way back in 1907, at a time when the official readings were taken adjacent to Lake Michigan. 24 of the last 33 lows (73%) were in the: 40s (13) and 50s (11). O'Hare recorded nine consecutive 40s & 50s lows from 1997 through 2005. Six lows were in the cold 30s. Three lows were in the warm 60s. O'Hare's second coldest May 11th was in 1966 as the high was 45 (with 1981, O'Hare's coldest high of the date) and low of 34. The third coldest was in 1960 (high of 46, low of 37).

There was an instance of back-to-back years in which temperatures were just about as opposite as they can be. In 1981 the high was 45 with a low of 33,  overall 19 degrees BELOW normal. Then in 1982 the high was 89 with a low of 58, overall 15.5 degrees ABOVE normal.

Today is a major day in Chicago's snow history. In 1966, O'Hare recorded 1.6 inches of snow. Rockford had an inch.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 5:04 and sunrise is at 5:36. Sunset is at 20:04 and twilight ends at 20:36. There's a total of 14 hours, 28 minutes of daylight today and 15 hours, 32 minutes between twilights. We gained two minutes of daylight from yesterday. Tomorrow twilight begins at 5:03 and sunrise is at 5:35. The moon is waning gibbous, heading towards last quarter tomorrow at 16:47. Illumination was 67% at midnight, will be 62% at noon and 57% at midnight tonight. Moonrise was at 0:42 this morning. Moonset is at 11:10 this morning. Moonrise is at 1:17 tomorrow morning.  Moonlight time is 10 hours, 28 minutes which is 28 minutes longer than yesterday. 

Spring began on March 12th and runs for 84 days through June 3rd. Warm Spring began on April 13th and runs for 52 days through June 3rd. High temperatures in this period are usually in the 50s, 60s and 70s. Highs in the 80s and 90s occur infrequently. Mother's Day is in two days. Armed Forces Day is in eight days. National Maritime Day is in 11 days.