How hot has July 2019 really been?

By Stuart A. Foster, State Climatologist for Kentucky

As residents welcome the advance of cool, dry air across the region, how hot has it been?

At the Bowling Green-Warren County Regional Airport, the official source of temperatures collected by the National Weather Service for the city, the average temperature through the first 21 days of July has been 81.1 degrees, or 2.4 degrees above normal. Taking a closer look, however, tells a different story. The average daily high temperature has been 90.1, only 0.8 degrees above normal. Meanwhile, the average low temperature of 72.1 is 4.2 degrees above normal.

Normal temperatures are based on observed temperatures over the period 1981-2010. New normals will be calculated after 2020 by the National Centers for Environmental Information to reflect the period 1991-2020.

The warm overnight low temperatures are consistent with the very humid air that has dominated over the first three weeks of the month. As a greenhouse gas, water vapor in the atmosphere absorbs long-wave radiation from the heating of the earth’s surface, acting like a blanket and effectively limiting overnight cooling.

The high humidity that we have been experiencing is partially the result of the extremely wet weather over the first half of the year, and even going back to last year. With the abundance of moisture in the environment, that moisture can be evaporated and transpired by plants into the atmosphere.

While that moisture, coupled with summertime temperatures, has led to sticky conditions and elevated heat stress, it has actually helped to limit the daily high temperatures, as a dry environment, such as a desert, heats more quickly than a moist one under the summer sun. Not surprisingly, high temperature records are typically set during droughts, when the lack of moisture allows temperatures to soar.

With cooler, drier air expected over the next few days, the average daily high and low temperatures for the month are not likely to increase, and may drop slightly as July comes to a close.

A review of historical temperature data for July going back to 1894 for Bowling Green tells an interesting story. Looking at the average daily high temperature, the list of 10 warmest Julys is dominated by years from the first half of the 1900s. The exception is 2012, when severe drought conditions developed during the spring and were followed by hot, humid conditions during the summer. Through the 21st, July 2019 is on track to rank 59th, but may drop a few notches by the end of the month.

However, the list of average daily low temperatures tells a different story. Five of the 10 warmest Julys on the list occurred after 2000. If included on the list, Bowling Green’s current average daily low temperature for July 2019 would rank as the third warmest on record. Even with some cooler evenings ahead, it may very well remain in the top 10.

 

Top 10 Warmest Julys in Bowling Green, Kentucky (1894-2018)

Average Daily High Temperature

Average Daily Low Temperature

Rank

Year

Temperature

Rank

Year

Temperature

1

1901

99.1

1

2012

73.7

2

1930

98.1

2

2011

72.5

3

1919

97.0

3

2016

71.6

3

1921

97.0

4

2015

71.5

5

1931

96.1

5

1942

71.1

6

1936

95.6

6

1998

71.0

7

2012

95.3

6

1980

71.0

8

1902

95.1

8

2010

70.9

9

1907

95.0

8

1999

70.9

9

1952

95.0

10

1943

70.6

9

1934

95.0

10

1936

70.6

 

The occurrence of warmer daily low temperatures may reflect multiple factors. Examination of data from a selection of other locations across the region show broadly similar patterns, with recent years dominating their lists of warmest average daily low July temperatures. At the same time, the continued urban development of Bowling Green, specifically the area around the Bowling Green-Warren County Regional Airport, may be contributing to an enhanced urban heat island effect, wherein the built environment of the city leads to a warmer environment. Observations dating back to 2007 from the Kentucky Mesonet station located at the WKU Farm about five miles from the airport show daily low temperatures for July that average about 1.7 degrees cooler over the same period.

So, how hot has it been? Well, perhaps the story is not so much about the heat, but about the humidity instead. In any case, perhaps it is time for a change in the weather.

Contact: Stuart Foster, (270) 745-5983

Posted with permission from WKU News

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