-- When there is cold weather as in winter, does the atmospheric pressure 'shifts' to a higher range than in hot summer for the same clear sky ? It would make sense, as there is denser atmosphere when cold.

 

-- When the weather reports say as example 'northwest wind' ... do they mean coming from northwest or heading towards northwest? Is there a standard meaning ?

-- When the weather reports say as example 'northwest wind' ... do they mean coming from northwest or heading towards northwest? Is there a standard meaning ?

 

From.

Atmospheric pressure does not depend on "density" alone.

At the descending air current condition, the temperature of moving down air is low. And, at that state, the atmospheric pressure is high.

-- When there is cold weather as in winter, does the atmospheric pressure 'shifts' to a higher range than in hot summer for the same clear sky ? It would make sense, as there is denser atmosphere when cold.

 

-- When the weather reports say as example 'northwest wind' ... do they mean coming from northwest or heading towards northwest? Is there a standard meaning ?

 

But for ocean currents, westerly means to the west; instead of from the west, as with winds.

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Hmmm. Cold high pressure might be higher than warm high pressure; it would make sense. But to look at weather, it is the relative pressures that matter; rather than the difference in absolute pressures of an air mass between summer and winter.

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It seems easiest to think of air masses as either expanding (being heated) or contracting due to heat loss (relative to the surrounding air).

For the Northern Hemisphere, as an air mass expands, the Coriolis Effect causes clockwise rotation; and conversely a contracting airmass will move counterclockwise. Cold air that gets dragged out behind (or south) of the polar jet stream (or "dips" in the jet stream) will begin warming, expanding to rotate clockwise, forming a high pressure system.

And moist tropical air, which develops shade-producing clouds as it rises, will not heat as much as the surrounding, cloud-free air; so it will contract (lowering the pressure), and rotate counterclockwise. Similarly, a warm air mass that gets entrained north of the polar jet stream (or is "run over" by a dip in the jet stream) will also begin contracting to become a low pressure system.

The same is more or less true for the subtropical jet steam--and the air masses on either side of it. Expanding air masses (high pressure) rotate clockwise, and contracting (low pressure) air masses rotate counterclockwise.

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