1. Weather

What's the science behind our daily downpours?

Almost like clockwork, a late-afternoon storm, packed with thunder and rain, rolls across St. Petersburg’s skyline last week.
Published Jun. 29, 2015

It can be like clockwork during the summer. The dark sky. The low pressure. The lightning, often too close for comfort. The rain, sheets of it, pouring from the thunderheads faster than windshield wipers can slash it away.

Storms are a ubiquitous part of life in the Tampa Bay area, one of the stormiest regions in the country. Tampa, in particular, regularly racks up more than 80 days of storms in a year, most of which hit on summer afternoons.

So why do we see so many squalls — and why during rush hour?

Blame the heat and humidity, said WTSP 10Weather meteorologist Bobby Deskins. That, and our proximity to so many bodies of water.

Especially in the summer, the Atlantic Ocean and Gulf of Mexico produce breezes. So do Tampa Bay, smaller lakes and the remnants of previous thunderstorms. All of those winds can form stormy weather on their own, but they can also hurtle across the state on a storm-producing collision course, fed by moisture in the hot air.

"You really just have a real soupy, prime atmosphere for storm development," Deskins said.

Every storm has three essential "ingredients," said National Weather Service meteorologist Dan Noah: moisture, lift and air instability. Moisture becomes rain, lift starts the air moving, and instability — pockets of hot air rising and giving off energy — allows thunderclouds to grow.

"Think of moisture as the gasoline," Noah said. "The lift is the match, and unstable air is like blowing oxygen on a fire to get it really hot."

When it gets hot, air near the surface of the Earth starts to rise, carrying water vapor with it, Noah said. But when that warm air meets the cold air several thousand feet up, it starts to cool down again — and the water condenses, forming clouds. The tops of those clouds are positively charged, and the bottom of the cloud and the ground are negatively charged.

When the condensed water gets too heavy, it falls. Rain.

If the electrical charge on the ground changes, a kind of path forms between earth and sky, and electrons stream from clouds to meet another stream from the ground. Lightning.

When lightning strikes, the air around it expands, which creates a loud shock wave. Thunder.

And during Tampa Bay summers, all of this is supercharged. It's hotter, so air rises easily, and it's more humid, so there's more moisture. Sometimes, those factors can produce storms on their own if the air is unstable enough, Deskins said.

But the driving force of most storms is sea breezes — that extra lift. That's what makes Florida unique.

"We get sea breezes on both our coasts," Noah said. "Texas also gets a sea breeze, but they just have one. We get two."

How those breezes interact varies — and so do the time and place of storms. But here's how it usually works: as it gets hotter throughout the day, the air heats up and rises. Cooler air from over the Gulf of Mexico rushes in as a sea breeze, straight toward another wind.

It's like swiping your arm through the water in a swimming pool, Noah said. The sea breeze lifts the air and the moisture in it, the same way your arm creates a wave of swirling, fast-moving water. If there are two breezes moving toward each other — if you swing your arms together — "that water goes straight up," Noah said.

Then, it comes straight back down, right over your evening commute.

Contact Emily McConville at or (813) 226-3374. Follow @emmcconville.


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