Ag Weather Forum

Research published this year finds that drought can lengthen heat waves if these two features occur at the same time. This connection is familiar to anyone who has labored through a blistering hot day during a dry spell; however, the quantifying of this drought-heat wave relationship is noteworthy, nonetheless.

The study was funded by NOAA's drought information division, the National Integrated Drought Information System (NIDIS). Research results were recently published in the Journal of Applied Meteorology and Climatology. A NOAA/NIDIS news release on this project stated that "researchers examined the influence of drought on heat wave intensity, duration, and human exposure to extreme heat using air temperature, soil moisture, and soil temperature observations from across the country."

Research team members found that drought increased the average duration of heat waves by 13 to 48 hours at most of the weather stations included in the study. Nationally, heat waves during drought had an average of about nine more hours of extreme heat (hours where temperatures are in the top 90th percentile of historical conditions) compared to heat waves that occur in non-drought conditions.

The NOAA/NIDIS summary of the drought-heat wave research study highlights how these two features can complicate safety and commerce.

"Drought and extreme temperatures can dry out vegetation, leading to billions of dollars in agricultural losses and building up fuels that can ignite in wildfires. These hazards can also reduce water supplies and lead to low river levels, impacting freight transportation on major waterways like the Mississippi River," the summary noted.

In addition, this drought-heat wave feature is something that is not confined to the drier regions of the western United States. It also shows up across the lower 48 states. "Heat waves more often co-occur with drought in the western United States, particularly the Southwest from Texas to California -- areas that have seen periods of long-term drought over the last 25 years. Even in the eastern United States, though, heat waves co-occurring with drought resulted in higher daily maximum temperatures compared to heat waves without drought," the study summary said.

The study also found a key interaction of dry soils with heat retention and then release of that heat into the atmosphere. This attribute acts as a booster to the air temperature. "It is understood that dry soils reduce local evapotranspiration (ET) rates, but this study also found that the soils retained more heat. The combination of reduced ET and warmer soils likely allowed heat to escape into the atmosphere, driving warmer air temperatures," the NOAA/NIDIS summary said. The effect of soils releasing stored-up heat into the atmosphere is especially prominent in the early evening, which drives higher evening temperatures.

The study notes that heat waves occurring at the same time as drought bring increased risk of late-afternoon or early-evening heat exposure, both from the higher temperatures and a longer length of exposure to heat.

The NOAA/NIDIS study summary stated that "Measuring soil moisture and soil temperature conditions therefore could improve assessments of how crops, economies, wildfire risk, infrastructure, and human health are impacted by extreme heat."

More details on the drought-heat wave research are available here: https://www.drought.gov/…

Bryce Anderson can be reached at bryce.anderson@dtn.com

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