What’s a heat index? From warnings to watches, here are the weather terms you need to know

From heat waves and floods to tornadoes and severe storms, the Philadelphia region has been on the receiving end of some extreme weather — and extreme weather alerts — lately.

There was the great 3 a.m. tornado alert that awoke the region and generated countless memes, tornadoes that have touched down in parts of Pennsylvania and New Jersey and heat advisories that have blanketed the area during summer months.

But what do they all mean, exactly? Here are the severe weather terms you need to know.

Not exactly. A heat wave refers to a specific length of time: It’s typically defined as three or more consecutive days with temperatures of 90 degrees or above. Excessive heat, however, is when there’s a combination of both high temperatures and high humidity.

For Philadelphia and its immediate suburbs, the weather service issues an excessive heat warning when the heat index is expected to reach or surpass 105 degrees for at least two hours a day, typically over multiple days. Heat advisories are issued when the heat index is forecast to be above 100, usually for a single day.

The heat index is important because it can determine just how dangerous the hot weather can be to your health. The National Oceanic and Atmospheric Administration defines it as “apparent temperature” or “an accurate measure of how hot it really feels" by combining relative humidity and temperature. Your body cools itself off by sweating, but isn’t able to do so as well when it’s humid and the perspiration can’t evaporate, the National Weather Service explains. It’s why meteorologists might say it’s 95 degrees out, but feels like 100.

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A flash flood is classified as “flooding that begins within 6 hours, and often within 3 hours, of the heavy rainfall (or other cause)," according to the weather service. Most often caused by extremely heavy rainfall from thunderstorms, flash floods can take just minutes to hours to develop.

Flash floods are especially prone to occur in urban areas where concrete surfaces to do not absorb rain, and the water runs off to the low spots very quickly, according to the weather service.

The floods are particularly dangerous because they can occur so quickly that the rapidly rising water often catches people off-guard, the weather service says.

A flash flood warning means a flash flood is imminent. If you are in a flood-prone area and receive a flash flood warning, the weather service advises you move immediately to higher ground. It is even possible to experience a flash flood in an area not immediately receiving rain, according to the National Weather Service.

A flood warning signals a longer term event, when hazardous weather is imminent or already happening, according to the weather service.

» READ MORE: Drenching storms blamed for 3 deaths in Southeast Pa., tornado in South Jersey

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The weather service only issues alerts for a “severe thunderstorm," said Valerie Meola, a meteorologist with the service in Mount Holly.

Meola said there’s certain criteria that needs to be met for a storm to be deemed “severe,” including winds 58 mph or greater or hail with a one-inch diameter or greater — think the size of a quarter.

A regular thunderstorm? Those are just the typical rain and lightning you see throughout the summer.

“Lighting does not make a storm severe," Meola said. "It is a part of every thunderstorm.”

» READ MORE: The region is getting wetter. But there’s a plan to help towns cope.

The National Weather Service defines a tornado as a “violently rotating column of air extending from the base of a thunderstorm down to the ground.” Tornadoes typically last for less than 15 minutes but can cause a lot of destruction with winds that can reach more than 300 mph, according to the weather service.

“Fifteen minutes is a long time for a tornado to be on the ground," Meola said. “Most of our tornadoes are not on the ground for nearly that long.”

It’s those high wind speeds and ability to cause debris to go flying that make tornadoes so dangerous, according to the weather service. They’re called “nature’s most violent storms." Though, mountainous terrain, like in some parts of Pennsylvania, could help a storm dissipate faster than it would across the Plains.

Pennsylvania averages about eight confirmed twisters a year.

“We do get tornadoes here, they’re not as common, but we’ve seen our fair share this season so far," Meola said.

» READ MORE: Tornadoes touch down in South Jersey: ‘It just happened so fast’

The truth is that we don’t know for sure, but we can say that severe tornadoes are caused by supercells — or “rotating thunderstorms with a well-defined radar circulation called mesocyclone,” according to the weather service.

Tornadoes are often recognized by their funnels, and there’s often a calm felt before the storm — with the air turning very still before a tornado makes its way through the region. Hail is not uncommon, either.

The two terms are used interchangeably.

“They’re very similar," Meola said. "Tornado is more, I guess, the official term that we use, but twister is a term a lot of people know as well.”

Hearing about a tornado watch or warning in your area can be scary, but there are differences between the two. A watch, as declared by the Storm Prediction Center, means that weather conditions capable of producing tornadoes are possible, according to the weather service. A warning, issued by your local forecast office, means a tornado has been spotted and “there is imminent danger to life and property.” Think of a watch as a cautionary message while a warning is when you should seek shelter.

Of course, watches vs. warnings aren’t applied to just tornadoes. For the Philadelphia area, you probably associate them with severe thunderstorm watches and warnings.

“A watch can be described as it could happen," Meola said. “It means that the conditions of the atmosphere say we could see severe storms or there’s potential for tornadoes. A warning means imminent. Means it’s occurring, on its way, take shelter, it’s more of an immediate thing.”

» READ MORE: Why is it raining so much? Twisters, downpours, iPhone alerts: the harvest of a wild spring.