Jet stream visualization on Sept. 29. (Ventusky.com)

Mother Nature has flipped the calendar to fall. The days are getting shorter, the nights are feeling crisper, and at any moment now Walmart will throw up all its Christmas displays. We all know the seasons are changing, but what exactly does that mean? It all comes down to one thing: the jet stream.

The jet stream is a river of air snaking along about 6 to 7 miles above the ground. It weaves a serpentine path, acting as the border between warm, tropical air to the south and cooler, drier polar air banked up north. It’s along this boundary that the atmosphere unleashes its fury in vicious battles, and it comes as no surprise that the jet stream is commonly referred to as the “storm track.”

Even though the jet stream is one of the biggest factors in our day-to-day weather, it’s actually the change of seasons and the amount of heating at different parts of the Earth that give rise to the jet stream. The equator stays warm and is heated by the sun year round but the poles see little in the way of sunlight, except in the summer. This creates a pool of cold air centered at the North Pole, which sinks toward the ground and leaves a void in the upper atmosphere.

Warmer air from the equatorial regions is drawn northward to fill this void. In a sense, it’s just a consequence of gravity; since warmer air expands and cool air shrinks, there’s a natural downward slope in the pressure surfaces of the atmosphere. This generates a “density ramp” that masses of air can slide down on their way to the north pole. The steepness of the slope is determined by how much the temperature changes over horizontal distances at the Earth’s surface. The greater this temperature gradient, the steeper this atmospheric slope, and the quicker that air flows downhill to the pole.

The slope is what gives rise to the jet stream. This swiftly-moving river of air forms along the zone of greatest temperature difference, since that’s where the density slope will be the greatest, and where the air will rush downhill fastest. That’s why the jet stream moves so incredibly quickly.

But wait a second — the jet stream doesn’t go from the equator to the pole; it goes in a latitude circle around the earth. What gives?

Remember the earth is spinning. Air can’t flow in a straight south-north line, because it has rotational momentum. Think of the twirling skater who spins faster when she pulls in her arms. Her radius shrinks, and to conserve angular momentum, she rotates at a greater rate. The same is true in the atmosphere. Since the earth is a sphere, the radius of the earth dwindles as one heads from the equator to the north pole. Imagine taking horizontal slices through the earth at the middle, and then at the top — the top slice would be tiny. That’s why the air is forced to speed up as the earth curves inward, and the air travels so fast that it can’t make it all the way to the pole. Instead, it snakes around the poles at the mid-latitudes, and thus the jet stream is born.

The jet stream is inherently wavy. Waves that ripple through the jet stream’s flow can help spin up weather systems. Cyclones or storms form downwind of the troughs of these waves, whereas ridges of high pressure and sunshine from downwind of their crests.

The jet stream shifts north and south seasonally; because there is less cold air, it retreats northward into Canada during the northern hemisphere’s summer. That’s why we see relatively mild and tranquil conditions without any nor’easters or large-scale storm systems in the warmer months. But as the chilly air mass builds heading into fall and winter, that jet stream is surged southward — and parks the battleground of the seasons over us here in the Lower 48. It’s this contrast that can give rise to some of the powerhouse winter storms and blizzards that buffet us from December through March.

Here in the mid-latitudes, we’ve noticed a change in the weather lately — we’re seeing fewer 80-degree days, and the humidity has begun to abate. You may have also detected that the winds are flowing more from the northwest, ushering in that cooler, continental airmass from Canada. That’s exactly what happens when the jet stream begins to make its annual descent.

In the meantime, the sun will start to retreat as well. We’re barely a week past the autumn equinox, when the sun’s most direct rays shine into the southern hemisphere. The sun will soar lower in the sky, the birds (and everyone’s grandparents) will migrate south for the winter, and we all know winter’s just around the corner.