May 21: ORD

Today is May 21st. It's the 142nd day of the year. 224 days remain. It's week 21 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 72 and the normal low is 50. I've calculated the 33-year averages at 72 and 48.

On the warm side, in 1987 the high was 89 with a low of 59, a 30 degree spread. Three years ago, in 2009, the high was 84 with a low of 60. The other warm highs were 87 in 1994, 86 in 1992, 84 in 1991 and 84 three years ago in 2009. The record high of 92 was set in 1977 at Midway after first being set in 1934 at the University of Chicago. The record warmest low of 71 occurred in 1921 at a time when the official readings were taken adjacent to Lake Michigan. 13 of the past 33 highs (40%) were in the 70s. 12 highs (36%) were in the cool: 50s (5) and 60s (7). Our most recent 50s highs were 58 in 2002 and 57 in 2003. Eight highs (24%) were in the warm 80s. O'Hare's warmest May 21st was in 1977 as the high was 91 with a low of 67. Both the high and low in 1977 were O'Hare's warmest of the date. The second warmest was in 1970 with a high of 86 and low of 66. The third warmest was in 1959 with a high of 86 and low of 64.  

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 a cool 57 with a low of 41, a cool day. In 1977 the high was 91 with a low of 67, a hot day and year of the record high of 92 at Midway. In 1975 the high was 79 with a warm low of 66, a warm day. In 1974 the high was 84 with a low of 66, a warm day. In 1972 the high was a warm 84 degrees. In 1970 the high was 86 with a low of 66, a hot day. In 1969 the high was a cool 57 with a low of 42, a cool day. In 1965 the high was 83 with a low of 50, a 33 degree spread. In 1964 the high was 81 with a low of 39, a large 42 degree spread. In 1963 the high was a cool 58 degrees. In 1962 the high was a cool 56 degrees. In 1961 the high was 58 with a low of 36, a cool day. In 1959 the high was 86 with a low of 64, a hot day.

More recently, we had large spreads of 37 degrees in 1980 (82/45), 36 degrees in 1994 (87/51), 36 degrees in 2007 (80/44), 32 degrees in 1981 (79/47), 30 degrees in 1989 (80/50), 30 degrees in 1991 (84/54), 30 degrees in 1992 (86/56) and 30 degrees in 2005 (73/43).

On the cool side, in 2002 (O'Hare's coolest May 21st) the high was 58 with the record low of 31 (the former record lows were 36 in both 1883 and 1997). In 1997 (tied with 1961 as O'Hare's second coolest May 21st) the high was 58 with a low of 36. In 1979 the high was 58 with a low of 38. The other cool highs were 57 in 2003 and 58 in 1990. The record coolest high of 42 occurred way back in 1883, at a time when the official readings were taken adjacent to Lake Michigan. 29 of the last 33 lows (88%) were in the: 40s (15) and 50s (14). There were 17 consecutive 40s & 50s lows from 1980 through 1996. There were six consecutive 40s lows from 2003 through 2008. Three lows were in the cold 30s. One low was in the warm 60s (60 three years ago). O'Hare's second coolest May 21st (tied with 1997) was in 1961 as the high was 58 with a low of 36.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 4:54 and sunrise is at 5:26. Sunset is at 20:14 and twilight ends at 20:47. There's a total of 14 hours, 48 minutes of daylight today and 15 hours, 53 minutes between twilights. We gained two minutes of daylight from yesterday. Tomorrow twilight begins at 4:53 and sunrise is at 5:26. The moon is waxing crescent, having passed new last night at 18:47. Illumination was 0% at midnight, will be 1% at noon and 2% at midnight tonight. Moonrise was at 5:49 this morning. Moonset is at 21:05 tonight. Moonrise is at 6:36 tomorrow morning. Moonlight time is 15 hours, 16 minutes which is 12 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. National Maritime Day is tomorrow.