This week's question comes from Morning Brief reader Jim, who asks, “Where does wind come from?”

Meteorologist Jonathan Erdman: The reason there are winds at all is because there are differences in air pressure.

You can think of air pressure as a reflection of how heavy the air is above you. If it’s heavier, it’s high pressure. If lighter, it’s low pressure.

You’ve seen these pressure differences before when looking at a weather map. This is easiest to visualize with water, so follow along with our pool setup above.

The red Ls and blue Hs correspond to centers of low and high pressure, respectively. In the atmosphere, high pressure systems are quite literally hills of air. In our hypothetical pressure pool, there's just more water under a high pressure system.

The atmosphere, and any fluid, really, doesn’t like differences in pressure.

So, air flows from areas of higher pressure toward lower pressure. That air flow is wind.

On a weather map, or in the pool above, you might notice some lines between the high and the low. Meteorologists call them isobars, not because they’re thinking ahead to a night on the town, but these lines of equal pressure can tell you if it’s windy or not.

If there are large differences in pressure, say from a strong winter storm system and a cold area of high pressure behind it, you may notice many isobars tightly packed, meaning it’s windy.

It turns out that the atmosphere can’t quite eliminate these differences in pressure, though.

That’s because there are other factors in play influencing the wind, including friction with the earth’s surface and the apparent deflection of wind direction due to the planet’s rotation, known as the Coriolis effect.