If you’ve ever noticed the skies suddenly darkening just as you’re heading to the pool, firing up the grill or settling in for an afternoon walk, you’re seeing the science of summer in action.

Those classic pop-up thunderstorms have a favorite time to strike: the hottest part of the day.

"The ingredients for afternoon pop-up thunderstorms are moisture, unstable rising air and a lifting mechanism to give it that nudge," meteorologist Caitlin Kaiser explained.

It’s all about timing

Those ingredients tend to come together in the afternoon, which is why these types of storms have become synonymous with the hours before dinnertime. As the sun beats down throughout the day, it heats the ground, which in turn warms the air just above the surface.

(MORE: Why heat lightning is actually a myth)

"That warm air rises because it's less dense than cooler air," Kaiser said. "That's when clouds start to form."

Get More of Our News on Google

Set The Weather Company as a 'Preferred Source' to get quicker access to the news you value.

Those small, fluffy cumulus clouds can quickly grow into towering cumulonimbus clouds capable of producing heavy rain.

"Heating continues to increase throughout the duration of the day," Kaiser said. "Normally the hottest time of day is around 3 to 6 p.m., so this is when that heat is peaking as well as the instability, and that is when we see these afternoon pop-up thunderstorms."

That’s why so many summer storms seem to arrive just as outdoor plans are getting started.

(MORE: Optimize your outdoor activities with the weather conditions)

Why during the summer?

Summer provides another key ingredient: moisture.

Warm air can hold much more water vapor than cold air, creating a sort of thunderstorm fuel.

"In the summer there is so much moisture in the air," Kaiser said. "When that warm air rises and conflicts with that colder air, we get that clash that creates the instability the storms need."

Small storms can still pack a punch

"These storms are typically not severe, but that doesn't mean they're harmless," Kaiser warned. "We can see downbursts, which are really strong winds, heavy rain, lightning and even small hail."

(MORE: Why weather moves slower in the summer)

The good news? They usually don’t last long.

"Once the sun goes down, we start to see a lot of cooling, and heat-fueled storms will dissipate," Kaiser said.

So if the forecast calls for scattered afternoon thunderstorms, don't be surprised if your sunny day takes a brief detour. It's simply the atmosphere doing what it does best after hours of summer sunshine.