Research Tracks Extreme Midwest Dew Points

Research into the weather patterns in place during periods of extreme dew point temperatures in the Midwestern U.S. was recently published in the Royal Meteorological Society’s International Journal of Climatology. The research was conducted by Drs. Mace Bentley (NIU meteorology professor) and Tony Stallins. This is the first study to examine and composite multiple extreme dew point events and chronicle their evolution. Heat stress, especially in large metropolitan Midwest cities, is a serious hazard during the dog days of summer. Anticipating weather conditions leading to the development of periods of high heat and humidity are important in warning and mitigating these events. For a PDF copy of the article, please contact Dr. Bentley.


A summary of major findings follows.

1) In nearly all cases, the evolution of the synoptic environment associated with extreme dew point events includes the development and movement of low pressure from the High Plains through the upper Great Lakes. The low pressure increases and backs the surface winds that transport low-level moisture from eastern Kansas, Iowa, and Missouri eastward into Illinois and Indiana. The surface wind field acts to advect and also focus the low-level moisture already trapped within the boundary layer in the Midwest, likely enhancing the apparent temperatures throughout the region. The progression of the low pressure also acts to modulate the length of the event as thunderstorms often propagate through the Midwest as the system moves through the Great Lakes. In some cases, this low pressure development and evolution took place in several days, while in other events it took over one week. In a few events, it took the development and propagation of several low pressure centers before the ridge was suppressed southward far enough to end the extreme dew point event in the Midwest.

2) Examination of soil moisture over the region for extreme dew point events illustrates that evapotranspiration from crops over eastern Kansas, Iowa, and Missouri provides a rich source of boundary layer moisture. The surface analyses indicate that low-level flow transports moisture from this region into the Midwest.

Crops, such as corn (left) and soybeans (right), are major contributors to Midwestern extreme dew point events.

3) The vertical thermal profile of the atmosphere during extreme dew point events along with analysis of turbulent kinetic energy within the boundary layer further highlight the importance of restricted low-level mixing as instrumental in allowing near-surface moisture to increase.

The major mechanisms listed above act in concert to create dangerous combinations of low-level warm air and moisture. If one of these ingredients is missing, it is likely that extreme amounts of low-level moisture will not develop and focus over the region. Meteorologists and climatologists should monitor soil moisture and vegetative health in the region as well as ridge amplification, the development of shallow mixing layers, and increasing winds on the backside of high pressure (in response to developing lee-side low pressure) as indicators of the initiation of an extreme dew point event.

Dr. Bentley’s manuscript is another example of the exciting research ongoing at NIU – research that bridges meteorology, climatology, and hazards.