April 18: ORD

Today is April 18th. It's the 109th day of the year. 257 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 40. I've calculated the 33-year averages at 63 and 40.

On the warm side, in 2002 (O'Hare's warmest April 18th) the high was the record high of 89 with a low of 63. The former record high of 88 occurred at Midway in 1977. In 1985 (tied with 1977 as O'Hare's second warmest April 18th) the high was 83 with a low of 62. Those lows of 63 and 62 were the only 60s lows in O'Hare's history for the date. In 2004 the high was 86 with a low of 52, a 34 degree spread. A decade earlier, in 1994, the high was 84 with a low of 50, a 34 degree spread. In 2005 the high was 83 (our most recent 80s high) with a low of 45, a 38 degree spread. In 1996 the high was 76 with a low of 51. The year prior, 1995, the high was 76 with a low of 47. The other warm highs were 77 in 1987, 75 in 2008 and 73 three years ago, in 2009. The record warmest low of 66 occurred way back in 1896, at a time when the official readings were taken adjacent to Lake Michigan. 22 of the past 33 highs (67%) were in the: 50s (9), 60s (7) and 70s (6). On the warm side, five highs (15%) were in the 80s. Tied with 1985 as O'Hare's second warmest April 18th is 1977 with a high of 86 and low of 59. O'Hare reached a high of 80 in the prior year of 1976. On the cold side, six highs (18%) were in the: 30s (1) and 40s (5). 

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 86 with a low of 59, a warm day. This was also the year of the former record high (88 at Midway). In 1976 the high was 80 with a low of 57, a warm day. In 1972 the high was a warm 77 with a low of 45, a 32 degree spread. In 1969 the high was a cold 46 with a low of 40, a six degree spread. In 1967 the high was a cold 46 degrees. In 1966 the high was a warm 76 with a low of 50, a warm day.

More recently, we had a small spreads of seven degrees in 1999 (45/38) and seven degrees in 2003 (46/39). There were additional large spreads of 37 degrees 1986 (69/32), 33 degrees three years ago (in 2009) (73/40), 32 degrees in 1980 (70/38), 32 degrees in 1987 (77/45) and 30 degrees in 2008 (75/45).

On the cold side, in 1983 (O'Hare's coldest April 18th) the high was 37 (O'Hare's only 30s high of the date) with the record-tying low of 23. The record low of 23 first occurred way back in 1875. Last year, in 2011, the high was 41 with a low of 32. The other cold lows were 26 in 1997, 26 in 2001 (our most recent 20s low) and 28 in 1990. The other cold highs were 45 in 1989, 45 in 1999, 46 in 2003 and 47 in 1984. The record coldest high of 35 occurred back in 1926 at the University of Chicago. 24 of the last 33 lows (73%) were in the: 30s (16) and 40s (8). Four lows (12%) were in the cold 20s. Five lows (15%) were in the warm: 50s (3) and 60s (2).

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 5:38 and sunrise is at 6:07. Sunset is at 19:39 and twilight ends at 20:08. There's a total of 13 hours, 32 minutes of daylight today and 14 hours, 30 minutes between twilights. We gained three minutes of daylight from yesterday. Tomorrow twilight begins at 5:36 and sunrise is at 6:06. The moon is waning crescent. Illumination was 9% at midnight, will be 6% at noon and 4% at midnight tonight. Moonrise was at 4:35 this morning. Moonset was at 17:28 this afternoon. Moonrise is at 5:01 tomorrow morning. Moonlight time is 12 hours, 53 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 four days. Arbor Day is in nine days.