so would it be true that above or below1013 surface pressure would indicate a rising (l) or falling(h) pressur

Question

appreciate previous answers, but i have read that 1013 is the divide at sea level.You do not see1016 LOW or conversely 1012 HIGH. It must be a critical point at which air is thought to be rising or falling? I know i m sad on a fri night but its bugged me for years! even sadder

Answer 1

101312 mb or 29.92 Inches of Mercury is the average air pressure around the Globe.
Highs or Lows are not measured relative to any value, they are determined by the rotation of the air around the centre of the weather system.
So a High Pressure System has an anticlockwise flow of air around the centre, and a Low Pressure System has a clockwise flow around the centre. This is true for the Northern Hemisphere, and is reversed in the Southern Hemisphere.

Answer 2

The pressure is only a measurement of air pressure. Below 1012 it is low and above 1013 it is high but that does not indicate rising of falling air pressure. Pressure measurements taken over a period of time show the trend. It is this trend that indicates rising or falling pressure. So pressure of 1016 this hour and it was 1018 last hour is falling pressure but it is still high pressure.

Answer 3

1013.5mb is the sea level pressure defined in the Standard Atmosphere, which is used by aviation. That doesn't mean that the sea level pressure everywhere IS 1013.5mb , it is just a value used to ensure that everyone is measuring from the same starting point.

Let's try a vacuum cleaner analogy 1
Imagine a brand new vacuum cleaner. There is no dust in the cleaner (= low/zero pressure of dust) but the carpet is dusty (= high pressure of dust).

Switch the cleaner on and air rises up the nozzle, and is dumped inside the cleaner. The dust is also lifted in the air and if we clean as much of the carpet as we can see and switch off, we now have a higher pressure of dust in the cleaner than when we started and a lower pressure of dust on the carpet.

But imagine that we hadn't pulled out the sofa. There would still be dust on the carpet there. But there would be less (pressure of) dust under the sofa than in the cleaner (we cleaned a lot of carpet!) now, so comparing the cleaner to under the sofa gives a HIGH(er) pressure of dust in the cleaner and a LOW(er) pressure of dust under the sofa.

But if we compare the dust amount under the sofa to the dust amount in the middle of the carpet where we cleaned, we get a HIGH pressure of dust under the sofa, compared to a LOW(er) pressure of dust in the middle carpet. So, comparisons are important.

Let's get back to air. Wind at the ground is slowed by things like friction with trees, buildings, mountains, etc. So winds higher up usually move faster. So if we lift air up from the surface by heating it or forcing it over mountains, then the lifted air starts moving away faster than it comes in at ground level.

Obviously, if this carried on winds at the ground would need to speed up a lot to compensate, but fortunately, these strong high winds usually keep right on moving along.

Analogy 2
[If you keep your cleaner over one spot, you sometimes hear the engine whine louder - like it's trying to speed up - as it starts sucking the carpet up, as well as the dust! We avoid that by moving the cleaner around and allowing the "dust to settle" - here, read "air" for "dust"]

The bands of strong winds high up that suck up air from below and move it somewhere else before dumping it are called jet streams. If they are sucking up and moving then (air/dust) pressure will be falling and if they are slowing and dumping (e.g. switching off) pressure will be rising.

How much rising and falling is going on depends on how long the jet (vacuum nozzle) stays in one place, and how powerful the jet (vacuum cleaner engine) is.

So
1) Falling surface pressure is caused by air lifting and moving away and rising pressure is caused by extra air from above being dumped.
2) A single value of the pressure CAN'T indicate if surface pressure will be rising or falling.

(If [high] dust pressure under the sofa starts falling, then that's probably because the nozzle is near and beginning to suck up the dust. So comparing 2 or more values at one spot will, over time, allow you to guess if pressure is going to rise or fall).