Weather Question

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giaviv

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giaviv
hey guys. i've decided to expand my weather knowledge (i'm a ppl and working on an instrument rating) and i have a question for you weather experts.

when the temperature is higher, by definition your indicated altitude at a standard altimeter setting will be HIGHER than your true altitude. in my ppl i learned that this is because when its warm the entire air mass "raises" and the 29.92 line is above sea level (there is higher pressure at sea level).

but one thing that came to mind is that at higher temperatures, density and pressure are both lower. the two explanations are contradicting in terms of the pressure; does that simply mean that higher (and lower of course) temperature has a bigger effect with respect to raising the "atmosphere" and therefore creating lower pressure at sea level than the higher temperature's effect on density and pressure?

thanks!
 
giaviv said:
when the temperature is higher, by definition your indicated altitude at a standard altimeter setting will be HIGHER than your true altitude. in my ppl i learned that this is because when its warm the entire air mass "raises" and the 29.92 line is above sea level (there is higher pressure at sea level).
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This statement is backwards. True altitude is greater than indicated altitude at warmer than standard temperatures. The reverse is true in colder than standard temps. This is often discussed when conducting instrument approaches in very cold weather, especially in mountainous terrain and with the use of Baro-VNAV equipment which has temperature specific limitations due to the altimetry error at extreme temperatures.

Think, "low to high, look to the sky" because the altimeter is indicating lower than it should and you are really above the indicated altitude. The phrase works both with changes in atmospheric pressure and/or temperature.
 
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HPNPilot1200 said:
This statement is backwards. True altitude is greater than indicated altitude at warmer than standard temperatures. The reverse is true in colder than standard temps. This is often discussed when conducting instrument approaches in very cold weather, especially in mountainous terrain and with the use of Baro-VNAV equipment which has temperature specific limitations due to the altimetry error at extreme temperatures.

Think, "low to high, look to the sky" because the altimeter is indicating lower than it should and you are really above the indicated altitude. The phrase works both with changes in atmospheric pressure and/or temperature.
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Or "High to Low, look out below" because the altimeter will read higher than it should and you are really below the indicated altitude. The IFR flight rules give you a buffer around the stuff that doesn't move with temp/pressure gradients (rocks!), so unless the temperature or pressure is WAY off from standard, you shouldn't hit anything.

But, combine an "acceptable" altimeter error, and "just within standards" piloting technique, AND significantly nonstandard temperature or pressure, and you might find yourself with a very small margin before you hit something.
 
Hot --> H --> High
If you're hot, you're high.
 
Also worth noting is that the effect of temperature on altimeter error is proportional to the relative height of the airplane and the ground station used to generate the altimeter setting. If you're at the same elevation as the ground station there's no temperature based error regardless of the temp or altitude.
 
gismo said:
Also worth noting is that the effect of temperature on altimeter error is proportional to the relative height of the airplane and the ground station used to generate the altimeter setting. If you're at the same elevation as the ground station there's no temperature based error regardless of the temp or altitude.
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Good point. Here is a table which describes cold temperature effects on altimetry based on temperature and height above the station.
 

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  • ICAO Cold Temp Error Table.pdf
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Oops, I wrote the statement wrong and I think that made you guys miss my actual question.

Yes, indicated alt will be lower at high temperatures. I just mistyped it.

Let me try to explain my question better.

When it's warmer than standard, the pressure at sea level would be higher than expected (because the air column ascends). But, higher temperature also has an inverse effect on pressure. So my question is - is the "rising column" effect greater than the inverse relationship between temperature and pressure? Well it including is, but why?
 
EdFred said:
PV = nRT
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What he said. I recently studied up on air density as a function of temperature, pressure, and humidity. These should get you started:

http://wahiduddin.net/calc/density_altitude.htm
http://psas.pdx.edu/RocketScience/PressureAltitude_Derived.pdf
http://en.wikipedia.org/wiki/Density_altitude
http://en.wikipedia.org/wiki/True_airspeed
http://en.wikipedia.org/wiki/Lapse_rate

You can derive most of the common rules of thumb from first principles using that info.
 
scottd said:
Not necessarily true. Warmer temps at the surface don't imply higher pressure. Pressure at the surface depends largely on the amount of mass (molecules) in the column of air above that point. Changing the temperature doesn't necessarily add or subtract mass from the column. You need to watch the preview I posted. It'll probably answer some of your questions.
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Bob
 
SCAM alert

This "Bob Albertson" fellow mentioned "regional pilot stability" and "disposable income" all in one sentence. Obviously he is a fake and imposter. :D:D:D
 
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giaviv said:
When it's warmer than standard, the pressure at sea level would be higher than expected (because the air column ascends). But, higher temperature also has an inverse effect on pressure. So my question is - is the "rising column" effect greater than the inverse relationship between temperature and pressure? Well it including is, but why?
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In a sealed container, increasing temperature would increase pressure.

The atmosphere is not a sealed container. Increase temperature, and the volume increases. That air mass then ascends due to buoyancy. Momentum carries it past equilibrium, until it's cooled enough to contract again. The same happens along the edges, but not as much. This pulls more air out of the warm column than surface warming alone.

Now, other air masses can come in, compress things, expand things, change the temperature, etc. which is why the pressure is measured rather than calculated.

But all things considered, higher temperature means lower pressure and higher indicated altitude.
 
xaminmo said:
The atmosphere is not a sealed container. Increase temperature, and the volume increases. That air mass then ascends due to buoyancy. Momentum carries it past equilibrium, until it's cooled enough to contract again.
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So far so good, but how does that affect pressure? If the column of air expands, so that at a given altitude you now have a greater mass of air above you than before, does the pressure there increase or decrease?

The way I think about it is that when the column expands, all the isobars move up with the column. Pressure decreases with altitude. So at a given real altitude, I'm at a lower pressure altitude (altimeter indicates lower) if the temperature is higher.
 
Gah. There I go confusing things again. Air density got mixed into this in my head, which is where I went astray. Also, I think somewhere in there I was thinking the expansion was more than vertical, ignoring that an updraft will pull in, not push away air.

Double check me here:
* The pressure altitude is based on the weight of the column of air above.
* Temperature changes expand or contract the column vertically
* Higher temperature decreases density to maintain the same pressure.
 
scottd said:
1. Assume you are hovering over the earth at 10,000 feet AGL with the air pressure at 700 mb and standard temperature in the column.

2. Let's say you increase the temperature of the entire column above and below your hovercraft.

3. Pressure at the surface does NOT change since the mass (the weight of the air) in this column of air has not changed.

4. The increase in temperature of the column will cause the 700 mb level to expand upward as the density of air is decreased in the column.

5. The pressure outside of your aircraft is now higher (say 650 mb) since the 700 mb level is now higher than your aircraft (it expanded upward).

6. Your altimeter now sees higher pressure since it didn't move vertically.

7. Higher pressure at your current fixed altitude above the earth (10,000 feet) means lower indicated altitude.

8. Why does it show a lower indicated altitude? Your altimeter is just a barometer. When you descend, it senses the higher pressure below you and the altimeter is designed to show a lower altitude.

Make sense?
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I was following along just fine, up to #5.

The 650mb level was already above me, and is lower pressure than 700mb. If the entire column expands, then the 650mb level went higher than it already was, just like every other level. I agree the pressure outside the aircraft would now be higher than it was, but I'd expect it to be something greater than the original 700mb.
 
TangoWhiskey said:
I was following along just fine, up to #5.

The 650mb level was already above me, and is lower pressure than 700mb. If the entire column expands, then the 650mb level went higher than it already was, just like every other level. I agree the pressure outside the aircraft would now be higher than it was, but I'd expect it to be something greater than the original 700mb.
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I was having the same issue. I'm guessing he meant to type 750mb as his example level.
 
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