What is a heat dome and why is it so dangerous?

• A dangerous "heat dome" is expected to impact millions of Americans this week and into the Fourth of July holiday weekend.
• Heat advisories have been issued June 29 for 41 Florida counties.
• The heat index could reach as high as 110 to 112 in some Florida counties June 29.

A dangerous "heat dome" is expected to impact millions of Americans this week and into the Fourth of July holiday weekend.

"The extreme heat will impact everything from everyday life to beach vacations and outdoor cookouts to watching the fireworks," according to AccuWeather.

"Dangerous heat" is expected for most of Florida June 29. Heat advisories have been issued June 29 for 41 Florida counties. The National Weather Service offices in Jacksonville, Melbourne, Tampa Bay and Miami all warned the feels-like temperatures could reach as high as 110.

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The National Weather Service in Tallahassee warned residents the heat index could reach as high as 112 across the Panhandle and Big Bend.

Here's what a heat dome is and how you can keep yourself and your family safe.

Most of Florida under heat advisory

How hot will it get in Florida?

Temperatures expected June 29:

The 'feels-like' temperatures for June 29:

What is a heat dome? Heat dome map

Simply put, a heat dome is when hot air is trapped over a certain area. It can stay for days, or even weeks, according to nationalgeographic.com.

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"When hot air attempts to rise it's blocked by the high-pressure ridge. As it sinks, it compresses and grows steadily hotter, producing conditions of extreme heat near the surface over areas hundreds of miles in size."

The result is extreme heat under the area under the "dome," according to climaterealityproject.org. "It's like a lid on a pot, trapping the hot air and preventing it from escaping.

"Heat domes can cause dangerously high temperatures that linger for days or even weeks, posing significant health risks."

"Weather inside these domes will typically be clear with nary much moisture available for cloud development, allowing the sun to cook the ground while free from obstructions," according to Fox Weather.

"Combine the sun’s heating with the sinking air, and you get a double-whammy of warming that can many times bring near-record or record heat." 

Here's what happens:

• "A high-pressure system stalls: A strong area of high pressure in the upper levels of the atmosphere gets stuck over a particular region.
• "This traps hot air in place: Warm air gets trapped under the stalled high-pressure system and can’t get out. This “dome” of hot air can't rise up and escape, so it sits over a region.
• "Sunshine further intensifies the heat: With little to no clouds because of the high-pressure system being in place, the sun's radiation directly hits the ground, further increasing the temperature within the heat dome."

How is a heat dome different from a heat wave?

"Unlike a heat wave, in which various factors might cause hot weather, it takes specific weather conditions to create a heat dome," according to nationalgeographic.com.  

"A heat wave is a prolonged period of abnormally warm weather, typically lasting for days or sometimes even weeks. It's defined in relation to a specific location's historical temperature averages," according to climaterealityproject.org.

"If a particular place experiences above-average heat for a period of time, that place is enduring a heat wave."

"A heat dome is a specific weather phenomenon. It involves a high-pressure system lingering over a set region and trapping hot air. This can, in some cases, cause more intense and longer-lasting heat compared to regular heat waves."

How long can a heat dome stay in place?

A heat dome can stay in place, making life miserable for those under them, for days or even weeks.

"Unfortunately for those experiencing them, heat domes tend to be stagnant or move slowly, squatting over large areas," according to nationalgeographic.com.

"Their course and duration are generally steered by patterns of atmospheric circulation, and particularly, the jet stream." 

 “When the jet stream shifts or weakens the dome may expand in size or move into new areas," according to AccuWeather Meteorologist Brandon Buckingham.

"Relief finally arrives when the ridge of high-pressure breaks down, causing the dome to dissipate quickly.” 

How can you stay cool in extreme Florida heat?

Heat is one of the leading causes of weather-related deaths in the United States, according to the Centers for Disease Control, so it's important to heed warnings when temps and the heat index climb. Here are a few ideas to help you stay cool:

• Drink plenty of water.
• Take a shower, jump in a pool, pour a bottle of water over your head, or use a mister. Water helps your body cool off.
  • Tempting though it may be, don't take a cold shower. Your core temperature will actually increase.
• Use a wet cooling cloth or even a bandana, especially on your wrists, ankles and the back of your neck.
• Wear loose clothing in breathable fabric. Make sure they're light-colored.
• Sleep on the ground floor or the basement if you're lucky enough in Florida to have one.
• Try to spend part of your day in a public place that's cool, such as a mall, library or movie theater.

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Cheryl McCloud is a journalist for the USA TODAY Network-Florida's service journalism Connect team. You can get all of Florida’s best content directly in your inbox each weekday day by signing up for the free newsletter, Florida TODAY, at https://tcpalm.com/newsletters.