April 21: ORD

Today is April 21st. It's the 112th day of the year. 254 days remain. It's week 16 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 61 and the normal low is 41. I've calculated the 33-year averages at 60 and 40.

On the warm side, in 1985 (O'Hare's warmest April 21st) the high was the record high of 88 with the record warmest low of 63. Until 1985, O'Hare's warmest low was 61 in 1974. This was also the former record warmest low at Midway. In 2001 the high was 82 with a low of 55. In 1980 the high was 79 with a low of 46, a 33 degree spread. The other warm highs were 77 in 2007 and 75 in 2008. 22 of the past 33 highs (67%) were in the: 50s (14) and 60s (8). O'Hare recorded 12 consecutive 50s and 60s highs from 1989 through 2000. On the warm side, seven highs (21%) were in the: 70s (5) and 80s (2). On the cold side, four highs (12%) were in the 40s. 

In looking back through the years to the first April at O'Hare in 1959, the following instances were notable: In 1975 the high was a cold 49 degrees. In 1974 the high was 70 with a warm low of 61, a warm day. In 1973 the high was 68 with a warm low of 59, a warm day. In 1972 the high was a cold 47 with a low of 41, a six degree spread. In 1968 the low was a warm 53 degrees.  In 1962 the high was a warm 80 with a low of 44, a warm day and 36 degree spread. In 1960 the high was 73 with a low of 55, a warm day. In 1959 the high was 47 with a low of 29, a cold day.

More recently, we had small spreads of three degrees in 2002 (42/39), six degrees in 1998 (53/47) and seven degrees three years ago in 2009 (44/37). The additional large spreads were: 34 degrees in 2007 (77/43), 33 degrees in 2008 (75/42) and 32 degrees in 1987 (72/40).
 
On the cold side, in 1986 (O'Hare's coldest April 21st) the high was 45 with the record low of 27. Two years later, in 1988, the high was 45 with a low of 29. The other cold lows were 29 in both 1981 and 1993 (our most recent 20s low). The other cold highs were 42 in 2002 (O'Hare's coldest high of April 21st) and 44 three years ago in 2009. The record coldest high of 35 was set back in 1936 at the University of Chicago. 27 of the last 33 lows (82%) were in the: 30s (9) and 40s (18).  Four lows were in the cold 20s. Prior to 1979, O'Hare's only sub-30 degree low was 29 in 1959. Two lows were in the warm: 50s (1) and 60s (1).

There was an instance of back-to-back years in which temperatures were just about as opposite as they can be. In 1985 the high was 88 with a low of 63, overall 24.5 degrees ABOVE normal. Then in 1986 the high was 45 with a low of 27, overall 15 degrees BELOW normal. 

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 5:33 and sunrise is at 6:02. Sunset is at 19:42 and twilight ends at 20:12. There's a total of 13 hours, 40 minutes of daylight today and 14 hours, 39 minutes between twilights. We gained three minutes of daylight from yesterday. Tomorrow twilight begins at 5:31 and sunrise is at 6:01. The moon becomes new today at 2:18. Illumination was 0% at midnight, will be 0% at noon and 1% at midnight tonight. Moonrise was at 5:58 this morning. Moonset was at 20:25 tonight. Moonrise is at 6:31 tomorrow morning. Moonlight time is 14 hours, 27 minutes which is 29 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. Earth Day is tomorrow. Arbor Day is in six days.