lows and highs?

Question

we all know n.hemis low air rotates anti-clock trev.pressure intensity is directly correspondant to mB measurement. No one has answered why a system measured 1012 is not considered to be a high pressuire.?

Answer 1

The single pressure reading at a point is not what determines it to be high or low pressure. You will need to also compare this pressure reading with other pressure readings found in the surrounding areas. For example: If you measured 1012 at your house, but your pressure reading was the lowest in your neighborhood, your house location would be defined as an area of low pressure.

However if your 1012 reading was the highest pressure reading in the neighborhood, then you house would be the center of the high pressure cell.

So you can see, the single value or number is not important. Instead, it is how this number is related to pressure readings found the area around you.

After reading some of your previous questions on nearly the same topic, let me re-write my answer in hopes that it will help you understand why we can't just say 1012 is high pressure. The pressure reading of 1012 only tells me the weight of the air above the location of where this reading was taken. In order to know whether this is considered part of a high pressure system or a low pressure system, you must know what the surrounding pressure readings are and also know how the pressure pattern looks like. To get a pressure pattern at a given height level, we must get other pressure readings and plot them on a map. When we have enough readings to plot a chart of lines connecting points of equal pressure, we will then have a better picture as to what that single reading really was either high or low pressure.


For more information, see the following links...

http://ww2010.atmos.uiuc.edu/(Gh)/wwhlpr/anticyclone.rxml?hret=/guides/crclm/act/prs.rxml

http://ww2010.atmos.uiuc.edu/(Gh)/wwhlpr/low_pressure_center.rxml?hret=/guides/crclm/act/prs.rxml

http://www.srh.weather.gov/jetstream/synoptic/wind.htm

http://ww2010.atmos.uiuc.edu/(Gh)/guides/crclm/act/prs.rxml

http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fw/prs/isb.rxml

Answer 2

Kirsty, a High Pressure system CAN measure less than 1013,
just as a Low Pressure system can measure more than 1013mb

If you google "synoptic charts" now (Sat 16 June) - and select Met Office Surface Charts, you will see a High of 1012mb North of Turkey.

I think your interest may be in what happens to a High Pressure system as it fades away and its pressure drops - at what point does it change from a High to a Low and what changes take place. Unfortunately it never actually changes, they just fade away or get swamped by other Highs or mixed with Lows, creating Ridges (High Pressure), Troughs (Low) or Cols (Areas between 2 Highs and 2 Lows).

Hope that helps - nothing to do with 1013.2 mb

Answer 3

A high pressure system is an area surrounded by lower pressure. So you can easily get Highs of less that 1012 if the pressure in the surrounding area is lower still.

Answer 4

It's all relative to adjacent systems -
1012 is the low if it is adjacent to an e.g. 1020 mb then 1012 is the low.
If 1012 is the high if it is adjacent to a 999mb system.

Answer 5

I know this is not an answer to your questions this evening, but have a look at the pressure over the Black Sea on tonight's chart...

Answer 6

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).

Answer 7

Because that is average sea level air pressure. Neither high nor low. Same are 29.92" is average air pressure in inches of mercury.

Answer 8

UALog is talking absolute cr*p!
Trevor H has got the right answer, i can't add much to what he said.

Answer 9

don't we all.

Answer 10

ahhh what???