[Physics] How to the atmospheric pressure be different in distinct points at the same altitude

Question

From an hydrostatic point of view, the pressure in a fluid should be the same at the same depth/altitude.

Obviously, in our atmosphere that does not happen. I am guessing that the main reason is the fact that the atmosphere cannot be regarded as hydrostatic.

Is this the reason? How exactly can we explain these pressure differences?

I understand that a higher pressure region must have a higher density, and therefore it would take time for reducing such density gradient. But how fast is this? In the order of the speed of sound? Or it has nothing to do with it?

Answer 1

You asked a number of questions in this question.

From a hydrostatic point of view, the pressure in a fluid should be the same at the same depth/altitude.

That "should be" assumes hydrostatic equilibrium. That is a simplifying assumption. It's a reasonable starting point, but it's not a hard and fast rule. The Earth's atmosphere, it's oceans, and even its interior are approximately in hydrostatic equilibrium.

I am guessing that the main reason is the fact that the atmosphere cannot be regarded as hydrostatic. Is this the reason?

Significant deviations from hydrostatic equilibrium do occur. This is an effect, not a cause.

How exactly can we explain these pressure differences?

Ultimately, it's because the Earth

• Is round,
• Is lit by the Sun,
• Rotates about its axis once per day,
• Has distinct rotational and orbital axes, separated by about 23 degrees,
• Has a fairly clear atmosphere, and
• Is covered by lots of liquid water.

These result in climate and weather, which in turn result in the Earth's atmosphere being only approximately in hydrostatic equilibrium.

Equatorial regions receive a lot more sunlight than do polar regions. The resulting temperature gradient is one of the key drivers of the climate. On Venus, which rotates slowly, this energy transfer occurs in a pair of Hadley cells that reach from the equator almost to the poles. On Titan, which rotates in about 16 days, the Hadley cells breaks up at about 60 degrees latitude. Jupiter and Saturn are so large and rotate so quickly that they have bands instead of Hadley-type cells.

On the Earth, which rotates once per day, the Hadley cells extend to only 30 degrees. Polar cells form around the poles, and the Ferrel cells act as intermediaries between the Hadley and Polar cells.

http://www.metoffice.gov.uk/media/image/f/s/Figure-4-Global-cells(edit)2.jpg

But how fast is this? In the order of the speed of sound? Or it has nothing to do with it?

The speed of sound has nothing to do with it. Winds do, and winds generally move much slower than the speed of sound. The fastest winds recorded are inside tornados, and even there things only move at about 40% of the speed of sound.