My grandfather has a big classy barometer on his wall, and occasionally checks the pressure.
And they keep talking about the pressure for hurricane Katrina.

Whats the signifigance of pressure?

low pressure is bad, high pressure is good mostly
changing from hi-lo or lo-hi rapidly is also bad

Why is low pressure bad? Whats causes low pressure, or what is caused by low pressure?
I figured out low is bad and high is good from the barometer. The low side had thunderclouds, the high side a sun.
I'm curious to the inner workings of pressure.

Understanding air pressure


The air's pressure is caused by the weight of the air pressing down on the Earth, the ocean and on the air below. Earth's gravity, of course, causes the downward force that we know as "weight." Since the pressure depends on the amount of air above the point where you're measuring the pressure, the pressure falls as you go higher.

The air's pressure is related to its density, its which is related to the air's temperature and height above the Earth's surface.

The air's pressure changes with the weather. Air pressure, in fact, is one of the important that determines what the weather is like. (Related: How high and low pressure affect the weather). You can do some basic weather forecasting by using the wind and barometric pressure.

Air pressure is also called barometric pressure because instruments called barometers are used to measure it.

The U.S. National Weather Service reports air pressure at the surface in inches of mercury while air pressure aloft is reported in millibars, also known as hectopascals (hPa). Scientists, however, generally use pressures in hectopascals.

In the rest of the world, measurements are usually given in hectopascals although you will sometimes see them in centimeters of mercury, especially on older barometers.

The term "hectopascals" is replacing the term "millibars." The hectopascal is a direct measure of pressure, like pounds per square inch, but in the metric system. Since the measurement is in the metric system, 1,000 millibars equal one bar. A bar is a force of 100,000 Newtons acting on a square meter, which is too large a unit to be a convenient measure of Earth's air pressure.

Inches of mercury and centimeters of mercury measure how high the pressure pushes the mercury in a barometer.

The use of direct pressure measurements goes back to the late 19th century when the great Norwegian meteorologist Vilhelm Bjerknes, a leader in making meteorology a mathematical science, urged weather services to use direct pressure measurements because they can be used in the formulas that describe the weather, unlike measures of the height of the mercury in a barometer.

A sidelight: In the International System (SI) of measurements, the unit of pressure is the Pascal, named after Blaise Pascal, the 17th century scientist who made important discoveries about air pressure. The standard atmospheric pressure at the Earth's surface of 1013.25 millibars is equal to 101,325 Pascals. To avoid large numbers, air pressure is reported in hectoPascals, which are the same as millibars. In many nations, you are now likely to hear reports such as, "air pressure, 1020.0 hectoPascals." This is the same as 1020.0 millibars.



Air pressure and your body

Changes in air pressure, especially rather quick changes, can affect your body. The most obvious of these are the discomfort or even pain you feel in your ears when your gain or lose altitude rather quickly, such as in an aircraft, or even a fast elevator that goes up or down several stories.


When you read a barometer the reading directly from it is the "station pressure."

Two things affect the barometer's reading, the high or low air pressure caused by weather systems, and the air pressure caused by the station's elevation, or how high it is above sea level.

No matter what weather systems are doing, the air's pressure decreases with height. If you're trying to draw a weather map of air pressure patterns, you need a way to remove the effects of the station's elevation. That is, you want to see what the pressure would be at the station if it were at sea level. Otherwise, all high-elevation locations would be mapped as having low pressure.

You need to calculate, sea-level pressure, which is defined as: "A pressure value obtained by the theoretical reduction of barometric pressure to sea level. Where the Earth's surface is above sea level, it is assumed that the atmosphere extends to sea level below the station and that the properties of that hypothetical atmosphere are related to conditions observed at the station."

To do this, you have to take into account the barometric reading at the station, the elevation above sea level, and the temperature.

Another kind of barometric reading is the altimeter setting, which aircraft use. It's defined as: "The pressure value to which an aircraft altimeter scale is set so that it will indicate the altitude above mean sea level of an aircraft on the ground at the location for which the value was determined." For it, all you need is the station pressure and the elevation, you can ignore the temperature. (Related: Calculate altimeter setting).

How pressure decreases with altitude

As you go higher in the air, the atmospheric pressure decreases.

The exact pressure at a particular altitude depends of weather conditions, but a couple of rules of thumb (approximations) and a formula give you a general idea of how pressure decreases with altitude.

A rule of thumb for the altimeter correction is that the pressure drops about 1 inch of mercury for each 1,000 foot altitude gain. If you're using millibars, the correction is 1 millibar for each 8 meters of altitude gain. These rules of thumb work pretty well for elevations or altitudes of less than a two or three thousand feet.

The standard atmosphere is a table giving values of air pressure, temperature, and air density for various altitudes from the ground up. You can think of these values as averages for the entire Earth over the course of a year.

Pressure is the depth or thickness of the atmosphere.  This atmosphere is stratified, naturally at the stratosphere.  There, the air is MUCH cooler and is where cumulus clouds are blocked, else they turn into ice.  

A thinner atmosphere means the moisture has less distance to go to condense and form rain.  Violent rain, actually starts out as ice which melts as it falls, making the drops large and causing intense downdrafts.


What causes the atmosphere to thin?  Jetstream currents dip and this bend means velocity must change.  A change in velocity means the pressure must change.  And when the pressure drops, it means the atmosphere is sinking, bringing cold stratosphere closer to the surface.

The simple explanation.... for those who need the simple explanation.  This is how it was explained to me when I was a kid.


Clouds are lighter than air.  They create "Less" pressure.

So... When there's a lot of clouds in an area, it's a "low pressure" area.

Low pressure means more clouds, and all the groovy things more clouds bring to an area.

If you don't like the explanation, bummer!    It was good enough for me when I was about 8.  It may not be correct, but it correlates well.

The rest the weatherman extrapolates from all the other information.  

Quoted: The simple explanation.... for those who need the simple explanation. Clouds are lighter than air.  They create "Less" pressure. So... When there's a lot of clouds in an area, it's a "low pressure" area. Low pressure means more clouds, and all the groovy things more clouds bring to an area. If you don't like the explanation, bummer!    It was good enough for me when I was in my 8th grade science class.  It may not be correct, but it correlates well. The rest the weatherman extrapolates from all the other information.

Thanks for the simple explaination...that's all I have patience for anyway

Quoted:

You're pretty groovy.  Goodnight again sweetums.

Quoted: Quoted: You're pretty groovy.  Goodnight again sweetums.

Thanks Stealth

Goodnight

Simple but less than correct.  Clouds rise until their density matches the surrounding air.

No, there is something else here and it is velocity/position of the jetstream which causes the stratosphere to dip into the lower atmosphere.

Air flows from high to low press.

In a hurricaine the lowest pressure is in the eye, caused by the swirling air around it. The bigger the storm/the faster the "wind speed", the lower that press. drops. Same with a tornado. But those are not a "normal" situations.

I understand some of the phenomena that create higher pressure areas. Do not have a good handle on what creates low press. areas. I do know that moving air creates a lower press. area around it. For example, in a norm. aspirated internal combustion engine there is a press. drop in the intake manifold. Look up venturi effect.

Temp and humidity effect all this too. Monitoring barometric press. can give insight into what is going on in the atmosphere, if you understand what can/is causing the conditions. As I said, I do not understand all that much.

In general, falling press. means deteriorating conditions and rising press. means improving conditions. At least in terms of conditions we like to see.

In a nutshell . . .

Low presure is caused by warm air.  Warm air is less dense and rises, creating lower pressure.

High pressure is caused by cold air.  Cold air is more dense and falls, creaing higher pressure.

A cold front will increase barmetric pressure, whereas a warm front will decrease it.  

Swings in barometric pressure indicate that a boundary between a warm and cold air mass are passing.  Weather (rain, winds, snow, ect) occurs on these boundaries.

Disconnector