Barometric Pressure question

n57flyguy

n57flyguy

Well-Known Member
Hello again.
Another question. I know that bad weather (clouds, rain, snow, ect) is asociated with a low pressure system, and good weather with a high pressure system. I am really confused here though. What is barometric pressure? I know there is something related when you set the altimetar in the cockpit, something like set it to a certain pressure, I really dont know. I post alot more questions about this, I am so frustrated trying to find this stuff I can think clearly now. Thanks again.
 
Barometric pressure is the air pressure that meteorologists read off a barometer (the instrument used to measure air pressure) to give you the altimeter setting. A barometer is basically a glass tube filled with mercury that is marked off in inches that sits inside a bowl full of mercury. You can think of it as like a McDonald's cup full of Coke with a straw in it. When the air pressure is high, air pushes down on the Coke in the cup. Since the coke can't bust through the bottom of the cup, it goes up into the straw. If the straw were marked off in inches (like a barometer), you could measure the air pressure by seeing how far the coke is being pushed up into the straw.

Hope that helps... if not, let me know.
 
Think of high pressure as pushing all the bad weather down and away from the location of the pressure zone. In the low pressure zone, all the bad weather comes in, and there is greater vertical development (thunderstorms, hail, etc.) because of the lower pressure.
 
Your altimeter is basically an instrument that acts like what jawright described. But instead of using a glass tube and bowl it uses a capsule inside the instrument and static vent. Let's say your local barometric station pressure is 30.25" hg and you set that into your altimeter so now your capsule of the altimeter instrument inside(which is the bowl) will reflect this setting. You fly up to altitude where the pressure is 29.25" hg. Your static vent(which is the glass tube) detects this lowered atmospheric pressure but your capsule(bowl) will still indicate 30.25" hg. So your altitude is the difference between what your static vent detects and the setting of local barometric setting. This local barometric setting is your altimeter setting which is 30.25" hg like stated before. The difference is one inch so your altimeter would indicate 1,000ft. Practically speaking, barometric pressure is the altimeter setting. You use the altimeter setting for proper altitude reading.
 
Lets clarify here just to be sure:

When setting the altimeter, you are setting your Kolsman window to the barometric pressure as felt at sea level (for ex. 29.92"). If you're at a field that is 5000' msl the barometer would real approximately 5" less than sea level (e.g. 24.92"). But because the altimeter is a pressure sensing device, we want to know the difference between the pressure we are feeling outside relative to the sea level pressure.

-ColM
 
Isobars are lines on some weather charts that show what areas have equal barometric pressure. "Iso-" means "equal" and "bar" refers to pressure. Especially remember the "iso-" prefix; it will come up again in other terminology.

Besides just knowing where areas of high and low pressure lie, isobars are valuable to you as a pilot because winds want to flow from areas of high pressure to areas of low pressure. Knowing this, you can use the isobars on weather charts to see which way the winds are going to blow (but you also have to factor in surface friction on low level winds and coriolis force on upper level winds). The closer together the isobars (lines of equal pressure) are, the stronger the winds will be.

In the upper level charts, this stuff also helps you predict where weather systems will go because the upper level winds tend to "steer" weather systems around.

Here are two Advisory Circulars that should be able to answer all your questions:

AC 00-6A "Aviation Weather"
which you can download here in pdf format:
http://www.airweb.faa.gov/Regulator...C2DF8D9D7471617786256A020078083A?OpenDocument

and

AC 00-45E "Aviation Weather Services"
also available in pdf format here:
http://www.airweb.faa.gov/Regulator...702EF0BFE021AB3986256BB2005C1458?OpenDocument

You should definitely check those out. Hope that helps.
 
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