altimeter setting and pressure alt ???

[KBC]

Can anyone explain this:

As alt increases outside pressure falls. So if pressure at sea level is 30.02, then at 100 feet the pressure should be less than that, something like 30.00. Since the pressure falls 1 inch for every thousand feet, then at 1000 feet it should be approximately 29.02.

However, to compute pressure altitude the conversion table tells to do exactly the opposite. The pressure altitude (which is the altitude where pressure is equal to 29.92) in the above example will be equal -75 feet, which is exactly the opposite of what logically should be happening.

Where I am wrong? Thanks.

 

[flyguy]

Pressure altitude is not the altitude where the pressure is 29.92, it is the altitude above where the pressure is 29.92. On a standard day, the pressure altitude at sea level is 0. If the pressure at sea level is higher than 29.92 (30.02 for example), that means the altitude where 29.92 exists is above sea level, so the pressure altitude at sea level is a negetive number.

 

[Alchemy]

maybe it would help you to see how a mercury barometer works....

http://www.usatoday.com/weather/wbaromtr.htm

On a high pressure day you have more air molecules stacked on top or your head, which has the same effect of being lower than sea level on a standard 29.92" Hg day.

On a low pressure day you have fewer air molecules stacked on top of your head, which has the same effect of being at a higher altitude on a standard 29.92" Hg day.

 

[JHines]

Pressure altitude is actually the altitude indicated when the altimeter is set to 29.92.

Conceptually, think of what would happen if you had the altimeter set to local station pressure and then reset to 29.92. If you're at 30.02" at sea level, your local pressure is 0.1" Hg higher than the reference plane of 29.92, and the altimeter shows increasing pressure by a lower altitude indication.

In your example, the pressure altitude ought to be -100 feet. The pressure gradient is 1" Hg per 1000' (or 1000' per 1" Hg) like you said, so I don't know why the chart would give you -75.

 

[AirmetTango]

Keep in mind that 1" Hg for every 1000' altitude change is only an approximation. If you look at a density altitude conversion chart, you'll see that the actual conversion factor varies. That's why the conversion chart shows -75' instead of -100' for 30.02.