Air is heavy. The stack of air exerts pressure on the earth and on the air around it.
The air also moves. Air flows from high pressure to low pressure areas.
Because of various forces the air can not flow straight from the center of a high pressure region to the center of a low pressure region. These forces are basically a result of the inertia that air also has, and the fact that the earth is "larger all the way around" at the equator compared to up near the poles.
So the moving air, as it trys to go from high to low, gets deflected by its own momentum and it swirls around.
The bigger the pressure difference between the highs and the lows, the more air moves, and the faster it moves, and the more it swirls.
In normal weather the highs and lows they speak of are relatively mild and quite large. That is they are large in area and small in pressure difference.
In the case of a "tropical storm" or a "hurricane" a low pressure area starts doing its normal swirl thing, and that creates a barrier of sorts. (The same sort of barrier forms when you see a whirlpool over the drain when you empty a large tub or whatever). But inside the barrier there is the warm ocean water. Warm water evaporates. Warm wet air rises faster than other kinds of air. This rising warm wet air makes the pressure even lower and so the air swirls even faster and the barrier becomes more stable and coherent.
So basically the heat of the water and the cold of the air and the rising air and the barrier all turn into an engine. The walls conduits of the engine (there are literally "pipes" of different kinds of air if you ever look at a cut-away diagram of these sorts of storms) are all made out of swirling air. And the fact that the different kinds of air are moving at different speeds, and are different temperatures, and have different amounts of water in them, "produces" the storm.
One of the main "bad things" about such a storm, worse in many ways than the wind or rain itself, is that the low pressure in the center, combined with the walls of the storm, basically form a straw to suck up the water.
Yes, like a drinking straw.
The storm causes the water to bulge upward right under the storm just like when you suck on a straw just a little and the drink in the straw becomes somewhat higher than the rest of the drink.
When this "high mound of ocean water", called the "storm surge", hits land it does so like a tidal wave, sort of. In the case of Katrina the bulge was about eight feet, though normal is more like five feet. So it's like the storm raises the tide by five feet above whatever tide is normal.
https://www.reddit.com/r/explainlik...li5_what_does_pressure_andor_900mb_mean_when/