The Farmer Logo

Revisiting excessively high dew points in July

Climate Observations: Dangerous heat index values have occurred in six Julys since ’95

Mark Seeley

June 14, 2024

3 Min Read
closeup of temperature gauge reading 100 degrees
HOW HIGH? Relative abundance of rainfall this July may be more of a factor in determining whether we will record any 80-degree dew points and high heat index values.JJ Gouin/Getty Images

Dangerous heat index values have occurred in six Julys since ’95

Excessively high dew point temperatures indicate large water vapor content in the atmosphere. These values are used in combination with air temperature in many countries besides the United States to calculate so-called comfort indices or heat indices.

Dew points of 70 degrees F are considered high for Minnesota, but occur with some frequency nearly each summer, often causing the National Weather Service to issue excessive heat advisories or excessive heat warnings, as they inflate heat index values to over 100 degrees, levels that produce stress for humans, as well as for poultry, hogs and cows.

Dew points of 80 degrees or greater are usually record-setting in the Minnesota climate. Such readings are far more common in countries of Central America as well as the Persian Gulf. Dew points of 80 degrees invariably produce high heat index values and have at times caused large-scale mortality in poultry flocks (in 1995 and 1999, for example).

From statehood in 1858 until the summer of 1983 there was no evidence for an 80-degree dew point temperature in the state of Minnesota climate network. But in the summer of 1983, there were two dates that produced dew points of 80 degrees in Minnesota causing the heat index to soar to 105 to 110 degrees. These two days with the enormously large water vapor in the atmosphere were precursors to strong thunderstorms in July that year.

Twelve years later in July 1995, four separate days brought dew points of 80 degrees or greater, creating heat index values of 106 degrees to 115 degrees. There was high mortality inflicted on turkey farms by this excessive heat, and heavy thunderstorms were also associated with these extreme dew points.

Only four years later in 1999, July brought two more days of 80-degree dew points and heat index values of 105 degrees to 123 degrees. Again, there were reports of high mortality losses on turkey farms and again, some associated heavy thunderstorms (over 2-inch rains).

Dew trend

Since the new millennium began 24 years ago, there have been four more Julys that have brought 80-degree dew points to Minnesota. These occurred in 2002, 2005, 2011 and 2019. The heat wave of July 17-19, 2011, was perhaps the worst with heat index values that ranged from 110 degrees to 124 degrees. At 5 p.m. on July 19, 2011, Moorhead reported a 95-degree temperature with a dew point of 88 degrees (highest value ever measured in Minnesota) that produced a heat index of 133 degrees, the highest ever measured in our region of the country.

What are some commonalities of these seven Julys that have brought 80- degree dew points to Minnesota? In all cases, except 2005, the weather had brought a wetter-than-normal July. In addition, in all these cases field crops were healthy and at full canopy in July with adequate stored soil moisture.

Winds were from the south, bringing in warm, moist air to Minnesota as well.

Crops were near the maximum daily water usage, taking moisture out of the soil and transpiring it through the leaves to the atmosphere, evaporation from area lakes was also extremely high. It is certainly understandable that all these factors could have contributed to the excessively high water vapor content of the atmosphere in those years.

How about prospects for 80-degree dew points in July this year? Climate trends show that eight of the most recent 10 Julys have been warmer than normal, while only five of the most recent 10 have been wetter than normal. So, the relative abundance of rainfall this July may be more of a factor in determining whether we will record any 80-degree dew points and high heat index values.

More on Minnesota’s weather history is available in my book, Minnesota Weather Almanac: Second Edition, available at most bookstores or through the Minnesota Historical Society Press.

Seeley is an Extension professor emeritus of meteorology and climatology at the University of Minnesota.

About the Author(s)

Mark Seeley

Mark Seeley is an Extension professor emeritus of meteorology and climatology at the University of Minnesota.

Subscribe to receive top agriculture news
Be informed daily with these free e-newsletters

You May Also Like