Altimeter readings "mystery".
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Altimeter readings "mystery".
Hello
I am an Air Traffic Controller, currently being trained for a position in the Guadalajara Airport Radar for Terminal Control.
My question is regarding Meteorology, I hope someone will help me out on solving this "mystery".
We all know that air density, and thus atmospheric pressure, decreases at higher altitudes, given a certain temperature.
Here most of us don't think about this often, we just read our altimeters and transmit the reading to pilots, and that's it. Maybe sometimes we also think about worsening weather conditions based on our knowledge of pressure changes.
One member of our team insists that whenever you see a decrease in the altimeter reading, what is showing is an increase in pressure. He says so because we hear about altimeter readings on the aiports near or on the coast, always much lower readings than miles inland like in Guadalajara, and at higher elevations. So, he concludes, a lower pressure (in Puerto Vallarta for example, one of the nearest airports on the coast, fellow controllers tell us that they read 29.98 inches Hg) indicates a higher atmospheric pressure, whilst in Guadalajara, on a higer elevation and at the same time of day, teh altimeter reads, say, 30.22"Hg.
To me, that is contrary to intuition because as far as I know, the reading is based on the height of the mercury column inside a tube, so the higher the atmospheric pressure, the higher the column.
My guess is that temperature has a very clear influence on altimeter readings on the coast, usually a much higher temperature there than locally on Guadalajara, and that causes that discrepancy.
Any help? Thank you!
I am an Air Traffic Controller, currently being trained for a position in the Guadalajara Airport Radar for Terminal Control.
My question is regarding Meteorology, I hope someone will help me out on solving this "mystery".
We all know that air density, and thus atmospheric pressure, decreases at higher altitudes, given a certain temperature.
Here most of us don't think about this often, we just read our altimeters and transmit the reading to pilots, and that's it. Maybe sometimes we also think about worsening weather conditions based on our knowledge of pressure changes.
One member of our team insists that whenever you see a decrease in the altimeter reading, what is showing is an increase in pressure. He says so because we hear about altimeter readings on the aiports near or on the coast, always much lower readings than miles inland like in Guadalajara, and at higher elevations. So, he concludes, a lower pressure (in Puerto Vallarta for example, one of the nearest airports on the coast, fellow controllers tell us that they read 29.98 inches Hg) indicates a higher atmospheric pressure, whilst in Guadalajara, on a higer elevation and at the same time of day, teh altimeter reads, say, 30.22"Hg.
To me, that is contrary to intuition because as far as I know, the reading is based on the height of the mercury column inside a tube, so the higher the atmospheric pressure, the higher the column.
My guess is that temperature has a very clear influence on altimeter readings on the coast, usually a much higher temperature there than locally on Guadalajara, and that causes that discrepancy.
Any help? Thank you!
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The instrument you seem to be referring to isn't an altimeter but a barometer, and altimeter reads height/altitude based on a reference pressure which is set by the user, it doesn't tell you the pressure although it can be roughly shown by using the altimeter in reverse as such and adjusting the reference pressure until the altimeter shows 0.
Aside from that (and if I understand your question correctly), the higher Hg then the higher the pressure as you said yourself, so your colleague who would think that 29.98 is higher pressure than 30.22 is wrong. However, if you genuinely did mean an altimeter and not a barometer, then if the reference pressure is not changed and the altimeter decreases (for example if you adjusted the reference so the altimeter reads 500' and over the course of an hour it decreases to 400') then that does mean that the pressure is increasing.
Aside from that (and if I understand your question correctly), the higher Hg then the higher the pressure as you said yourself, so your colleague who would think that 29.98 is higher pressure than 30.22 is wrong. However, if you genuinely did mean an altimeter and not a barometer, then if the reference pressure is not changed and the altimeter decreases (for example if you adjusted the reference so the altimeter reads 500' and over the course of an hour it decreases to 400') then that does mean that the pressure is increasing.
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remember too that your instrument for giving ALTIMETER SETTINGS is compensated for your local elevation. IF you brought your instrument to the top of the mountain without compensating, there would be a problem.
NOW I don't mean giving out QFE. QNH (also known as altitude above mean sea level)
There is confusion in my mind whether your friend means the altimeter changes or the ALTIMETER SETTING CHANGES.
I do ask this. Some places just use an altimeter to get the SETTING. They just keep turning the knob until the elevation is read in the altimeter and then they look at the KOLLSMAN window for the setting.
hope this doesn 't confuse you too much.
NOW I don't mean giving out QFE. QNH (also known as altitude above mean sea level)
There is confusion in my mind whether your friend means the altimeter changes or the ALTIMETER SETTING CHANGES.
I do ask this. Some places just use an altimeter to get the SETTING. They just keep turning the knob until the elevation is read in the altimeter and then they look at the KOLLSMAN window for the setting.
hope this doesn 't confuse you too much.
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Nacho, you are so close... Let's get the height of the column of mercury sorted out first. Can you remember what is above the mercury? A (Torricelli) vacuum. The atmosphere pushes down on the mercury opposite to the vacuum. The greater the pressure, the more the column of mercury is pushed into the vacuum, the higher the reading. As for altimeters, the lower the pressure, the higher the reading. Given a day with no pressure gradient, if you took an altimeter that read zero at sea level from sea level and took it to your airport, it should read your airport's elevation. No change in the sub-scale should be required. But here's a question. Do you use an altimeter to measure pressure? I've seen some places that do. You bang on the table, reset the sub-scale until you see the field elevation and pass the value. If that is the case, the point your friend makes is correct.
As for temperature affecting pressure, it's a yes and no. In hot countries with large diurnal temperature variations and big land masses, the pressure changes (and bounces) because of diurnal heating. As the day warms up and the air in the centre of the land mass expands, causing pressure ripples to spread outwards (I'm thinking of Australia here. The QNH is forecast based on this phenomena). What is certain is the the air density us reduced because of in read in temperature, the pressure may remain constant.
Clear as mud?
As for temperature affecting pressure, it's a yes and no. In hot countries with large diurnal temperature variations and big land masses, the pressure changes (and bounces) because of diurnal heating. As the day warms up and the air in the centre of the land mass expands, causing pressure ripples to spread outwards (I'm thinking of Australia here. The QNH is forecast based on this phenomena). What is certain is the the air density us reduced because of in read in temperature, the pressure may remain constant.
Clear as mud?
