Nearly six weeks have elapsed since the mercury cracked 90-degree levels in Chicago, but that is about to change. The persistent pattern that brought record July coolness to parts of the Midwest and all-time record heat to South Texas and the Southwest is finally showing signs of breaking, allowing hot weather to return to the Midwest. The heat will tease the area Monday as a burst of southwest winds pushes the mercury into the upper 80s along with a noticeable increase in humidity. That could trigger a few thunderstorms along the Wisconsin border Monday morning and some additional storms by evening.
The midweek period will feature more of the comfortable weather that dominated July, but the latest computer models show an increased probability of temperatures rising into the 90s here by next weekend. If that occurs, four of the next seven days will feature above-normal temperatures — the same number that the city mustered during the entire month of July. Chicago has logged only three days in the 90s this year: three 94s on June 23-25.
Why haven’t there been any hurricanes yet this year?
The tropical Atlantic has been very quiet so far this season. Not only haven’t there been any named storms, but forecasters have had only a smattering of disturbances to investigate. The developing El Niño may be part of the reason. During an El Niño episode, hurricane frequency tends to decrease in the Atlantic Basin. The reason is an increase in the winds aloft which creates more wind shear, a condition that tears storms apart and disrupts their formation. With no named storms through Aug. 3, this season becomes the latest starting Atlantic season since 1992. However, that season more than made up for its late start. The first named storm which finally formed on Aug. 16 developed into Category 5 Andrew, an historic storm that devastated far south Florida.
July 2009 was not only cool and cloudy, but also dry. The month’s high temperature was just 86 degrees, logged on July 6. This marks the first time since weather records began here in 1871 that July had so low a maximum temperature. Sub-par rainfall totaled just 1.53 inches –ending a five month string of above-normal precipitation and making July the driest since 1991. Sunshine was deficient, not only in July but in June as well. Since June 1, the amount of possible sunshine here has averaged just 52 percent, the least since the 49 percent recorded in 1992.
Heat may be on the way
August’s opening week will feature seasonally warm weather in the lower 80s, though a surge of warmth could send readings into the upper 80s Monday. Real heat may make inroads into the city by Saturday for the first time since June 25 when Chicago recorded its third consecutive official high of 94 degrees at O’Hare International Airport.
Sometimes dew forms all night. Can it be measured? If so, how?
The National Weather Service’s weather observation program does not include measurements of dew. However, the amount of water contributed by dew can be determined, and a variety of techniques exist to do the job, though with varying degrees of accuracy. Dew can be measured most easily and accurately by weighing it. A piece of sod (or other material representative of the soil surface in the area for which dew measurements are desired) is placed on a scale that records the weight increase as dew accumulates. Unfortunately, accurate measurements of dew are not possible in densely vegetated areas. For example, little or no dew forms under a dense forest canopy, but it does accumulate on leaves at the top of the canopy.
July closed with eye-catching temperature deficits over a wide swath of the northern U.S. overnight. Chicago’s 69.4-degree average July temperature at O’Hare International Airport was the coolest of the past 17 years. But at Midway Airport, the month’s 71.0-degree average temperature was the site’s coolest in 42 years. Estimates based on the month’s temperatures suggest the need for air conditioning was 30 percent below the long-term average.
Cool as it’s been in Chicago, in a number of Midwest cities July has never been cooler. Records were established in Rockford; Madison, Wis.; South Bend and Ft. Wayne, Ind.; and Benton Harbor, Saginaw and Flint, Mich. The month’s temperature in those cities finished 4.5 to more than 7 degrees below normal.
The month’s lack of rainfall was impressive — and a huge change from the wet spring that kept farmers out of their fields. Only 1.53 inches of rain was measured here in July, less than half the 3.51 inches considered normal. The dry weather has led to browning lawns.