Path of Thunderstorms - Map by CoCoRaHS; discussion by Donald Collins
Earlier on this final week of the semester for Warren Wilson College (Tuesday, May 4, 2022) we had a fairly strong thunder storm that created some minor flooding near the campus, but was strongly localized. No rain appeared in Asheville that afternoon after the storm passed WWC campus. A map of rain gauge readings contributed by citizen science participants for the same date showed the rain pattern in definite bands ranging from the southeast towards the northeast USA. The darker and more intense colors indicate a larger amount of rainfall.
The image at right shows a spring-time thunderstorm over the Great Craggy Mountains near Warren Wilson College photographed about six years ago (May 12, 2016) and posted last spring on PPOW for April 30, 2021. (I was busy on an errand in Asheville during this week's storm and was unable to photograph this week's storm after it had passed.)
Click on the 2016 thunderstorm photo at right and it takes your browser to the volunteers' rain-gauge data map for May 12, 2016. The archive map shows that the spring rain covered much of the central US on that date, with some hints of the bands of thunderstorms, but the linear features are not nearly as prominent as this week's events.
Why do the rain storms show the linear pattern? My guess is that warm, moist air typically moves northeast from the Gulf of Mexico, especially during the spring months as warm weather fronts. Cold fronts flowing toward the southeast from the northwest trigger the thunderstorms along the cold fronts. The cold air forces the warm air to rise above it. As the warm moist air rises, it expands due to lower pressure at higher altitudes and condenses the rain. The heat released causes the warm air to rise higher and eventually trigger thunder storms. (Convective storms). As a major storm forms at the cold front, it it continues its path to the northeast (bordering the cold front) and contributes to the rainfall in the gauges of volunteers.
The citizen science organization that collects the rain data from volunteers is called: the COmmunity COllaborative Rain, Hail and Snow Network (CoCoRaHS network). It was founded in 1998 by Professor Nolan Doesken of Colorado State University. Anyone can join CoCoRaHS (just search for the acronym in any browser), purchase a relatively inexpensive precision plastic rain gauge, and contribute daily rainfall data. My contributions in those two events are two of the many dots on these maps. I remember suggesting the data for a recent WWC student research project where they needed a record of rainfall for the growing season.
Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department. These photos feature interesting phenomena in the world around us. Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation. Atmospheric phenomena are especially welcome. Please send any photos to dcollins@warren-wilson.edu.
All photos and discussions are copyright by Donald Collins or by the person credited for the photo and/or discussion. These photos and discussions may be used for private individual use or educational use. Any commercial use without written permission of the photoprovider is forbidden.
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