April 20: ORD

Today is April 20th. It's the 111th day of the year. 255 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 40. I've calculated the 33-year averages at 61 and 42.

On the warm side, in 1985 (O'Hare's warmest April 20th) the high was the record high of 87 with the near-record warmest low of 63. The record warmest low of 64 occurred in 1957 at Midway. Two years later, in 1987, the high was 86 with a low of 56, a 30 degree spread. The 80s highs of 1985 and 1987 were the only 80s highs for the date in O'Hare's history. In 2001 the high was 77 with a low of 51. Five years later, in 2006, the high was 75 with a low of 53. In 1992 the high was 71 with a low of 55. The other warm high was 76 in 1980. The other warm low was 54 in 1979. 19 of the past 33 highs (58%) were in the: 50s (9) and 60s (10). On the warm side, eight highs (24%) were in the: 70s (6) and 80s (2). O'Hare's third warmest April 20th was in 1977 (high of 78, low of 63). The fourth warmest was in 1973 (high of 77, low of 62). On the cold side, six highs (18%) 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 1978 the high was a cold 41 with a low of 34, a seven degree spread and cold day. In 1977 the high was 78 with a near-record warmest low of 63, a warm day. In 1975 the high was a cold 47 degrees. In 1974 the high was a warm 79 with a low of 46, a 33 degree spread and warm day. In 1973 the high was 77 with a low of 62, a warm day. In 1971 the high was a warm 76 degrees. In 1969 the high was 66 with a low of 33, a 33 degree spread. In 1968 the high was a warm 75 with a low of 50, a warm day. In 1960 the high was a warm 74 degrees. In 1959 the high was a cold 42 with a low of 32, a cold day.

More recently, we had additional small spreads of six degrees in 1991 (44/38), six degrees in 1995 (47/41) and seven degrees in 2002 (48/41). We had additional large spreads of 31 degrees in 1989 (69/38) and 30 degrees in 2007 (68/38).

On the cold side, in 1981 (O'Hare's coldest April 20th) the high was 41 with a low of 30. Last year, in 2011, the high was 43 with a low of 32. In 1993 the high was 41 with a low of 35. The other cold low was 27 in 1983 (the only 20s low for the date in O'Hare's history). The other cold highs were 44 in 1991, 47 in 1995 and 48 in 2002. The record low of 24 was set way back in 1897. The record coldest high of 32 occurred way back in 1904. Both records were set at a time when the official readings were taken adjacent to Lake Michigan. 24 of the last 33 lows (73%) were in the: 30s (15) and 40s (9). O'Hare recorded eight consecutive 30s & 40s lows from 1993 through 2000. One low (3%) was in the cold 20s. Eight lows (24%) were in the warm: 50s (7) and 60s (1). O'Hare's warmest lows of the date were 63 degrees, in 1977 and 1985. 

There was an instance of back-to-back years in which temperatures were just about as opposite as they can be. In 1992 the high was 71 with a low of 55, overall 12 degrees ABOVE normal. Then in 1993 the high was 41 with a low of 35, overall 13 degrees BELOW normal. 

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 5:34 and sunrise is at 6:04. Sunset is at 19:41 and twilight ends at 20:11. There's a total of 13 hours, 37 minutes of daylight today and 14 hours, 37 minutes between twilights. We gained three minutes of daylight from yesterday. Tomorrow twilight begins at 5:33 and sunrise is at 6:02. The moon is waning crescent, heading towards new tomorrow at 2:18. Illumination was 1% at midnight, will be 0% at noon and 0% at midnight tonight. Moonrise was at 5:28 this morning. Moonset was at 19:26 tonight. Moonrise is at 5:58 tomorrow morning. Moonlight time is 13 hours, 58 minutes which is 32 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 in two days. Arbor Day is in seven days.