Where corn sweat exacerbated heat, and where sultry conditions will spread

Sultry air over the central United States — fueled by a heat dome, corn sweat and tropical winds from warmer than average Atlantic Ocean temperatures — will spread eastward in the days ahead, bringing record temperatures to parts of the U.S.

Over the past several days, dew points, a direct measure of humidity, were just a few degrees short of national records in the Corn Belt. The combination of heat and humidity exacerbated by corn fields caused heat index values to surge to 128 degrees in southern Missouri, 117 degrees in western Kentucky and 116 degrees in central Iowa this week. Now, that heat will spread into other states.

On Thursday, the most oppressively hot conditions were found in the Midwest and Great Lakes before surging today into the Northeast and Mid-Atlantic, where temperatures will be near 100 degrees in some places. Dangerous and long-duration heat will settle in the Southeast and redevelop in the Corn Belt into next week — with heat indexes of up to 120 degrees possible.

Over the next week, more than 180 temperature records will be challenged or broken, especially at night, as 100 million people are covered by heat alerts and a more humid than normal summer continues.

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Maximum humidity in the Corn Belt

Meteorologists use dew point thresholds to describe and communicate humidity; values above 75 degrees qualify as rainforest-like — or in this case, cornfield-like.

The dew point is the temperature at which air becomes fully saturated with moisture, causing water vapor to condense into dew.

Here are the highest dew point and heat index values observed in the Corn Belt from Monday to Wednesday, according to weather stations operated by the National Weather Service:

Daryl Herzmann, who works in the department of agronomy at Iowa State University, cautioned that some weather stations could be “buried on all sides in corn,” reporting humidity levels that are representative of the corn field itself.

Corn can release up to 4,000 gallons of water per acre into the lower atmosphere each day, a process known as evapotranspiration, contributing to extreme humidity and flooding rainfall.

The 88-degree dew point observed in southern Missouri — where a heat index of 128 degrees occurred around midday Wednesday — was measured at an airport, where Herzmann noted there are stricter rules around clearing vegetation near weather instrumentation. In more populated places in Missouri, such as St. Louis and Kansas City, heat indexes reached 107 degrees.

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Where heat will go next

Extreme heat and humidity aren’t going away for at least another week as a heat dome continues to blow in tropical air and corn keeps sweating.

Here’s an overview of where dangerous heat will be into next week.

Today

High heat will move into the Northeast and Mid-Atlantic, where widespread temperatures in the mid- to upper 90s are expected — though afternoon thunderstorms will temper heat across the interior.

A few locations could reach 100 degrees, such as in central New Jersey and around Philadelphia, Baltimore and D.C., though late-day downpours will probably keep most places just below the century mark.

Saturday

Though not quite as hot as Friday, it will be another sultry day in the Mid-Atlantic.

The hottest weather will be found in Virginia and the Carolinas, where 100-degree temperatures are likely, including areas south of Richmond and near Charlotte. Heat index values may reach 110 degrees.

Sunday

Dangerous heat will expand and intensify in the Southeast, with triple-digit high temperatures and heat index values of up to 115 degrees stretching from Virginia to Georgia — leading to an extreme risk (Level 4 out of 4) for heat-related impacts, according to NOAA.

Extreme humidity will return to the Corn Belt, sending heat index values toward 115 to 120 degrees in some places.

Monday

Exceptional humidity combined with high heat may cause widespread and dangerous heat index values of 110 to 120 degrees in the Southeast, Florida and the Corn Belt.

Meteorologist Brad Panovich wrote that an unusual multiday stretch of 100-degree temperatures is likely in Charlotte — perhaps the longest on record there.

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When will the heat wane?

There could be some relief next month. There are signs it may turn notably cooler in central and eastern states around the start of August.

Below-average temperatures could reach as far south as the Southeast during the first week of next month, as an air mass from northern Greenland dives southward.

This surge of cooler air shouldn’t be mistaken for the end of summer, as above-average heat and humidity will probably return during mid-August.