April 17: ORD

Today is April 17th. It's the 108th day of the year. 258 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 60 and the normal low is 39. I've calculated the 33-year averages at 60 and 39.

On the warm side, in 2002 (O'Hare's second warmest April 17th) the high was 85 with a low of 65. In 1981 the high was 84 with a low of 59. In 2004 the high was 77 with a low of 51. The year later, in 2005, the high was 76 with a low of 47. The other warm highs were 76 in 1985 and 74 in 1988. The record high of 88 and record warmest low of 68 were set on the same hot day in 1976 at Midway. 30 of the past 33 highs (91%) were in the: 40s (8), 50s (9), 60s (6) and 70s (7). On the warm side, two highs were in the 80s. O'Hare's warmest April 17th was in 1976 as the high was 85 with a low of 67 (O'Hare's warmest low of the date). O'Hare's warmest temperature for the date was 87 in 1977. O'Hare also had another 80s high of 81 in 1964. On the cold side, one high was in the 30s. 

In looking back through the years to the first April at O'Hare in 1959, the following instances were notable: In 1977 the high was 87 with a low of 51, a 36 degree spread and hot day. In 1976 the high was 85 with a low of 67, a hot day. 1976 was the year of the record high of 88 and record warmest low of 68 at Midway. In 1975 the high was a warm 73 with a low of 42, a 31 degree spread. In 1972 the high was 71 with a low of 37, a 34 degree spread. In 1966 the high was 66 with a low of 32, a 34 degree spread. In 1964 the high was 81 with a low of 56, a warm day. In 1963 the high was 75 with a low of 56, a warm day. In 1961 the high was a cold 40 with a low of 33, a seven degree spread and cold day. In 1959 the high was 75 with a low of 57, a warm day.

More recently, we had a small spreads of three degrees in 2000 (42/39), five degrees in 1992 (44/39) and five degrees in 2003 (44/39). There were large spreads of 38 degrees in 1985 (76/38), 38 degrees in 1996 (65/27), 38 degrees in 2009 (70/32), 34 degrees in 2007 (70/36), 32 degrees in 1980 (60/28) and 30 degrees in 1988 (74/44).

On the cold side, in 1983 (O'Hare's coldest April 17th) the high was 36 (O'Hare's only 30s high of the date) with a low of 26 (O'Hare's coldest low of the date). In 2001 the high was 46 with a low of 29 (our most recent 20s low). In 1990 the high was 46 with a low of 31. The other cold low was 27 in 1996. The fourth 20s low of the date was 28 in 1980. The other cold highs were 42 in 2000, 44 in 1984, 44 in 1992 and 44 in 2003 (our most recent 40s high). The record low of 17 and record coldest high of 26 were set on the same cold day way back in 1875, at a time when the official readings were taken adjacent to Lake Michigan. 18 of the last 33 lows (55%) were in the 30s. Four lows (12%) were in the cold 20s. O'Hare's second coldest April 17th was in 1961 as the high was 40 with a low of 33. 11 lows (33%) were in the warm: 40s (8), 50s (2) 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 2001 the high was 46 with a low of 29, overall 12.5 degrees BELOW normal. Then in 2002 the high was 85 with a low of 65, overall 25 degrees ABOVE normal. 

Today is a major day in Chicago's snow history. 1961 was the year. Midway had 0.3 inches, the third of three days of 6.8 inches of snow.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 5:39 and sunrise is at 6:09. Sunset is at 19:38 and twilight ends at 20:07. There's a total of 13 hours, 29 minutes of daylight today and 14 hours, 28 minutes between twilights. We gained three minutes of daylight from yesterday. Tomorrow twilight begins at 5:38 and sunrise is at 6:07. The moon is waning crescent. Illumination was 15% at midnight, will be 12% at noon and 9% at midnight tonight. Moonrise was at 4:10 this morning. Moonset was at 16:28 this afternoon. Moonrise is at 4:35 tomorrow morning. Moonlight time is 12 hours, 18 minutes which is 35 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 five days. Arbor Day is in ten days.