May 20: ORD

Today is May 20th. It's the 141st day of the year. 225 days remain. It's week 20 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 71 and the normal low is 49. I've calculated the 33-year averages at 72 and 47.

On the warm side, in 2004 the high was 84 with a low of 63. Three years ago, in 2009, the high was 84 with a low of 61. Another warm high was 84 in 1992. The record high of 95 and record warmest low of 70 were set on the same hot day in 1977 at Midway. 27 of the past 33 highs (82%) were in the: 60s (10) and 70s (17). Four highs were in the warm 80s. Two highs were in the cold: 40s (1) and 50s (1). O'Hare's warmest May 20th was in 1977 as the high was 93 (O'Hare's warmest high of the date) with a low of 64. O'Hare's second warmest May 20th was in 1975 as the high was 87 with a low of 67 (O'Hare's warmest low of the date).

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 78 with a low of 46, a 32 degree spread. In 1977 the high was 93 with a low of 64, a hot day and year of the record high and warmest low. In 1975 the high was 87 with a low of 67, another hot day. In 1972 the high was a hot 87 with a low of 54, a 33 degree spread. In 1970 the high was a warm 85 with a low of 52, a 33 degree spread. In 1965 the high was 74 with a low of 40, a 34 degree spread. In 1964 the high was a cool 58 degrees. In 1962 the high was a warm 85 with a low of 53, a 32 degree spread. In 1959 the high was 80 with a warm low of 64.

More recently, we had large spreads of 41 degrees in 1981 (76/35), 37 degrees in 1994 (78/41), 36 degrees in 2006 (74/38), 34 degrees in 1979 (78/44), 33 degrees in 1992 (84/51) and 31 degrees in 1980 (74/43).
 
On the cool side, in 2002 (O'Hare's coolest May 20th) the high was 48 (O'Hare's only 40s high of the date) with the record low of 33. The former record low was 35 in 1981. In 1986 the high was 56 with a low of 41. The other cold low was 35 in 1981. The record coolest high of 40 occurred way back in 1895, at a time when the official readings were taken adjacent to Lake Michigan. 27 of the last 33 lows (82%) were in the: 40s (17) and 50s (10).  Four lows were in the cold 30s. Two lows were in the warm 60s.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 4:54 and sunrise is at 5:27. Sunset is at 20:13 and twilight ends at 20:46. There's a total of 14 hours, 46 minutes of daylight today and 15 hours, 52 minutes between twilights. We gained two minutes of daylight from yesterday. Tomorrow twilight begins at 4:54 and sunrise is at 5:26. The moon reaches new tonight at 18:47. Illumination was 1% at midnight, will be 0% at noon and 0% at midnight tonight. Moonrise was at 5:08 this morning. Moonset is at 20:12 tonight. Moonrise is at 5:49 tomorrow morning. Moonlight time is 15 hours, 4 minutes which is 20 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 in two days.