Unraveling Weird Weather Mysteries of the Monsoon

Recent thunderstorm activity in the North Valley. In&Out Publications photo

Monsoon whether is weird. Weather forecasting can be a little strange, too. Based on some reader questions, we reached out to Andrew Deemer, a meteorologist at the National Weather Service office in Phoenix, to explore some finer points of monsoon meteorology.

IO Does a 20 percent chance of rain mean there’s a 20 percent chance it will rain somewhere in the forecast area, or a 20 percent chance it will rain everywhere in the forecast area?

AD “This is a tough one and I am not sure anyone can really give a solid answer,” Deemer said. The official NWS definition says it’s “the chance of precipitation occurring at any point” in the forecast area. But Deemer thinks of it this way: If there’s a 20 percent chance of rain, he’d like to see 20 percent of the forecast area getting at least a trace of rain.


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Is a 20 percent chance of rain really significant?

“As someone who grew up here, I have attached much more significance to hearing 30 percent (or greater) chance for rain, and I tend to dismiss anything less. But that’s just a personal bias.”

Why do temperatures drop during the monsoon?

“It takes much more energy to raise the temperature of a liquid than dry air,” Deemer explained. Just think of how quickly the air warms up on a sunny day in May, while a pool takes several days or weeks to warm up. When the air is dry in spring and early summer, temperatures soar. When monsoon moisture moves in, temperatures simply can’t climb as high.

Why does it get downright cool when a thunderstorm rolls through?

The amount of moisture in the air relative to the temperature is called relative humidity (think of it as relative stickiness). In a thunderstorm, some of the rain evaporates as it falls. Like an evaporative cooler, this cools the air, causing the relative humidity to rise.

Are rain gauges accurate in heavy, sideways downpours?

“Not really. Sideways rain is a problem. Very heavy rain is a problem—both often report under what probably fell.”

All About the Monsoon Learn where the name came from, what causes a haboob, and why forecasting monsoon storms is so difficult.

lightning storm
Desert thunderstorms like this one in the North Valley offer vibrant colors and shocking shows. Photo by Tracee Struchen
monsoon thunderstorm
Lightning crackled just north of Anthem July 28, 2016, but no rain fell in town during this storm. Photo By Marius Britt
ominous monsoon clouds
A monsoon storm looking all ominous from New River, a the edge of North Phoenix. Photo by Tracee Struchen
monsoon storm lightning
Rain and lightning seen from Anthem during a monsoon storm in 2016. Photo: Marius Britt
Robert Roy Britt
NoPho resident Robert Roy Britt has written for In&Out publications since its inception in 2005. Britt began his journalism career in New Jersey newspapers in the early 1990s. He later became a science writer and was editor-in-chief of the online media sites Space.com and Live Science. He has written four novels. Email the author.
Robert Roy Britt on Email

Robert Roy Britt

NoPho resident Robert Roy Britt has written for In&Out publications since its inception in 2005. Britt began his journalism career in New Jersey newspapers in the early 1990s. He later became a science writer and was editor-in-chief of the online media sites Space.com and Live Science. He has written four novels. Email the author.

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