April 4: ORD

Today is April 4th. It's the 95th day of the year. 271 days remain. It's week 14 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. 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 54 and the normal low is 35. I've calculated the 33-year averages at 53 and 34.

On the warm side, in 1997 (O'Hare's warmest April 4th) the high was 75 with a low of 51 (O'Hare's only 50s low of the date). In 1988 the high was 68 with a low of 47. Last year, in 2010, the high was 76 (O'Hare's warmest high of the date) with a low of 39, a 37 degree spread. The other warm highs were 68  (1981, 2005, 2011 (last year)) and 67 in 1999. The other warm low was 48 in 1991. The record high of 79 and record warmest low of 60 were set on the same warm day back in 1929, at a time when the official readings were taken adjacent to Lake Michigan. The record high of 79 first occurred in 1921. 26 of the past 33 highs (79%) were in the: 40s (9), 50s (9) and 60s (8). Our most recent 40s high was 45 in 2004. On the warm side, two highs (6%) were in the 70s. On the cold side, five highs (15%) were in the 30s. 

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 36 with a low of 13, a cold day and year of the record low of 16 at Midway. In 1973 the high was a cold 39 with a low of 34, a five degree spread. In 1972 the high was 40 with a low of 22, a cold day. In 1965 the high was a cold 42 degrees. In 1964 the high was a cold 38 with a low of 25, a cold day.

More recently, we had a small spreads of seven degrees in 1984 (44/37) and 2003 (39/32). There were large spreads of: 31 degrees in 1987 (50/19) and 32 degrees in 1981 (68/36) and 2005 (68/36).
 
On the cold side, in 1982 the high was 39 with a low of 20. Two decades later, in 2002, the high was 37 with a low of 25. In 1993 the high was 41 with a low of 21. In 2007 the high was 35 (O'Hare's coldest high of the date) with a low of 28, a seven degree spread. In 1995 the high was 44 with a low of 21. A year later, in 1996, the high was 37 with a low of 20. The other cold low was 19 in 1987. The other cold high was 39 in 2003. The record low of 16 was set in 1975 at Midway. The record coldest high of 28 occurred way back in 1891, at a time when the official readings were taken adjacent to Lake Michigan. 17 of the last 33 lows (52%) were in the 30s. Nine lows (27%) were in the cold: 10s (a.k.a. teens) (1) and 20s (8). O'Hare's coldest April 4th was in 1975 when the high was 36 with a low of 13 (O'Hare's coldest low of the date). There were only two 10s lows (a.k.a. teens lows) in O'Hare's history on April 4th: 13 in 1975 and 19 in 1987. Six lows (21%) were in the warm: 40s (6) and 50s (1).

There was an instance of back-to-back years in which temperatures were just about as opposite as they can be. In 1996 the high was 37 with a low of 30, overall 11.5 degrees BELOW normal. Then in 1997 the high was 75 with a low of 51, overall 18 degrees ABOVE normal. 

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 6:02 and sunrise is at 6:30. Sunset is at 19:23 and twilight ends at 19:52. There's a total of 12 hours, 53 minutes of daylight today and 13 hours, 50 minutes between twilights. We gained three minutes of daylight from yesterday. Tomorrow twilight begins at 6:00 and sunrise is at 6:28. The moon is waxing gibbous and will reach full in two days as the full Pink (a.k.a. Sprouting Grass/Egg/Fish) moon at 14:19. Illumination was 91% at midnight, will be 94% at noon and 96% at midnight tonight. Moonset was at 4:54 this morning. Moonrise is at 17:14 this afternoon. Moonset is at 5:25 tomorrow morning. Moonlight time is 11 hours, 40 minutes which is 41 minutes shorter than yesterday.

Spring began on March 12th and runs for 84 days through June 3rd. Cool Spring began on March 12th and runs for 32 days through April 12th. High temperatures in this period are usually in the 30s, 40s and 50s. Highs in the 60s and 70s usually occur infrequently. Good Friday is in two days. Easter is in four days.