It seems to happen every year. After a long, hot summer, the calendar flips to September, and one day you wake up and it's ... cool? Crisp, even.

"It's fall!" you think to yourself as you pull on a cozy sweater, grab the fall decor out of the closet, order a pumpkin spice latte and make weekend plans to go apple-picking.

(MORE: Leaves turning red already? It’s a distress signal, not early autumn)

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But a couple of days later, it's hot again. You reluctantly take the sweater back off and wonder where you went wrong.

What is 'false' fall?

It's a cold front. An unfortunately timed cold front that comes right when you want the dog days of summer to be over.

But after a couple of days, up to a week, summer returns, and it somehow feels hotter than it was before the seasonal teaser of "false" fall. The sun is still high, daylight (though shrinking) is still long, and the atmosphere hasn't fully shifted to autumn yet.

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(MORE: First day of fall: why there are two answers)

How the jet stream dictates when fall starts

So, how can you know if the dip in temperature is fall you can believe in? It's all about the jet stream.

The jet stream’s location is dictated by the temperature contrast between Canada and the U.S. When it’s still too far north, that’s a recipe for “false" fall fakeouts. But as real fall settles in and cooler temperatures take over in the Northern Hemisphere, the average location of the jet stream moves farther south.

The easiest way to tell is by checking your forecast on The Weather Channel app. Are temperatures going to shoot back up in a few days? It's "false" fall. Are consistently cooler temperatures expected for the next 10 days? It's the real thing.

(MORE: September outlook: How your first month of fall will feel)