What is the definition of High and Low weather pressure?

Question

Understanding High and Low weather pressure

Answer 1

Think in terms of air molecules...if the number of air molecules above a surface increases, there will be more molecules to exert a force on that surface and the pressure increases. The opposite is also true, where a reduction in the number of air molecules above a surface will result in a decrease in pressure. Atmospheric pressure is measured with an instrument called a barometer, which is why atmospheric pressure is also referred to as barometric pressure.

In aviation and television weather reports, pressure is referred to in inches of mercury ("Hg), while meteorologists use millibars (mb), the unit of pressure found on weather maps.

Atmospheric Pressure
force exerted by the weight of the air
Atmospheric pressure is defined as the force per unit area exerted against a surface by the weight of the air molecules above that surface. In the diagram below, the pressure at point "X" increases as the weight of the air above it increases. The same can be said about decreasing pressure, where the pressure at point "X" decreases if the weight of the air above it also decreases.

As an example, let us consider a "unit area" to be 1 square inch. At sea level, on average, the weight of the air above this unit area would weigh 14.7 pounds! The pressure applied by this air on the unit area would be 14.7 pounds per square inch. Meteorologists use a metric unit for pressure called a millibar. Average pressure at sea level is 1013.25 millibars.

Answer 2

Just wanting to clear up a few things about meteorologists in the United States.

(1) We use millibars. We also use hectopascals. Since they are the same number (500mb = 500hPa) it isn't a big deal what unit you stick on the end, you can always change it for a conversion.
(2) We report in inches of mercury so that the public will understand the air pressure. We do not use it outside of reporting it to the public. We use millibars/hectopascals.
(3) We only use Fahrenheit for surface temperatures, again, so that the public will understand. All upper air obs are in Celsius, all calculations use either Celsius or Kelvin.

A low pressure is an area where the pressure is lower relative to the surrounding air. This is usually caused by upward motion, mass is being removed from the area. Since the pressure lowers, the air converges (comes together) in the low, resulting in more upward motion. Sometimes the convergence comes first.
A high pressure is an area where the pressure is higher relative to the surrounding air. This is usually cause by downward motion, mass is being added to the area. Since the pressure rises, the air diverges (moves away from) in the high, resulting in more downward motion. Sometimes the divergence comes first.

Answer 3

Meteorologists do not use millibars and have not done so for some years. Outside the USA, the rest of the world - meterologists and the general public - use hectoPascals (hPa) to measure atmospheric pressure. Meteorologists and aviation in the USA use the same unit. The Pascal is part of the standard metric system based on the metre and kilogram while the bar is a unit in the old centimetre-gram system. The figure in hPa is the same as that in millibars so 1013hPa equals 1013mb.

It is only the USA that still reports pressure in inches of mercury and is possibly the only country that still uses Fahrenheit for temperatures. Don't worry, you'll catch up with the world one of these days.

In a low pressure system, air moves in towards the centre at the surface, rises and moves out high in the troposphere. In a high pressure system, air moves in high in the troposphere, sinks to the surface and then blows out from the centre of the high.

Answer 4

Pey's description of pressure in general is right.

But if you're asking what is "high pressure" and what is "low pressure"??

There is no actual definition for it. It's more of a relative term. Like pey said 1013.25 is standard pressure, but for example, on a day where the pressure in a large area is 1016, an smaller area within, with 1013.25 would be considered "Low pressure"
But then on a day where the pressure in a large area is 1011, then the same area of 1013.25 would be considered "High pressure"

I hope that makes sense...