Who decides what is "High Pressure" ?

[Achilles]

On the weather forecast they are always talking about high and low pressure areas. These have various values. The “Low” maybe from about 1110 millibars(???) downwards and the “High” from about 1115 upwards.

My question is : Where is the cut-off point?

Is it relative (to the surrounding pressure systems), or is it absolute - i.e. over a certain value is “High”, under a certain value is “low” ?

Anyone know ?

[Achilles]

Bump !

[Q.E.D]

It’s not an absolute. High pressure refers to an area that is higher in pressure than the surrounding air. Low pressure means just the opposite.

Some additional information on atmospheric pressure here.

[Achilles]

Thanks QED.

[chillihead]

Atmospheric pressure is defined as 1013.25 millibars.

High is above this, etc

[Q.E.D]

*Originally posted by chillihead *
**Atmospheric pressure is defined as 1013.25 millibars.

High is above this, etc **

No, this is not correct. Regardless of the definition of what atmopsheric pressure is high-pressure region has a higher pressure than the surrunding air – say it with me – even if the high-pressure region is lower than standard atmospheric pressure.

Your actual quoted figure is correct, but your definition of high- and low-pressure regions is not.

[MonkeyMensch]

QED can you give a cite or some elaboration to back up your side? I understand a fair amount re meteorology and the idea of below/above average makes sense. Of course your counter-argument makes sense too.

[Popup]

*Originally posted by MonkeyMensch *
**QED can you give a cite or some elaboration to back up your side? I understand a fair amount re meteorology and the idea of below/above average makes sense. Of course your counter-argument makes sense too. **

While USA Today is maybe not the gold standard of meteorology, the link provided by QED says:

As the name says, a “high” is an area where they air’s pressure is higher than the pressure of the surrounding air. A "low’ is where it’s lower. Meteorologists don’t have any particular number that divides high from low pressure; it’s the relative differences that count.

In order to be completely useless, I can also provide a quote from the Swedish meteorological organization. They claim here pretty much the same thing:

Högtryck - Område som har högre lufttryck än omgivningen, kallas också anticyklon.

[Richard_Pearse]

*Originally posted by chillihead *
**Atmospheric pressure is defined as 1013.25 millibars.

High is above this, etc **

1013.25 is an international standard and is not necessarily an “average” it is simply a convenient standard pressure. It is the setting that aircraft altimeters use at flight levels (generally above 11000’).

As stated above a high is relative. Due to the coriolis affect, airflow around a high is in a particular direction (anti-clockwise in the southerm hemisphere) and airflow around a low is in the opposite direction, if highs could live next to each other then they would not have a consistent direction of airflow, some would be cyclones, and the others anticyclones. This is not the case.

[Richard_Pearse]

Högtryck - Område som har högre lufttryck än omgivningen, kallas också anticyklon.

That doesn’t seem quite right to me, do you have anything else to back it up?

oh, ah, I guess I should add this: