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Altimeter Errors and Meteorology

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Flightinfo's sexiest user
Dec 21, 2001
Hey guys,
I hope everyone is having a good Christmas so far. Pilots do get Christmas off, right?

I have another question, but this relates to aircraft instruments, particularly the altimeter.

My understanding is that the altimeter functions as a barometer. It displays an altitude for a given pressure, calibrated so that a 1 inch of mercury decrease in pressure equates to 1000ft gain in altitude. Pressure decreases as you climb, and it increases as you descend.

My first question has to do with the rate of pressure decrease. I read that in the lower levels of the atmosphere (below 5000ft), pressure decreases 1 inch of mercury for every 1000ft gain in altitude, but only .3 inches for every 1000ft in the higher levels (all under ISA). Why does the rate of pressure decrease vary? Does it have to do with the density of the air? I figured that since the density of the air decreases, it must be exerting a lesser force (i.e. less weight, less force). Something tells me I am wrong though.

I read elsewhere that warm air has higher pressure than colder air, also, cold air has a higher density than warm air. If cold air is more dense, and it sinks, wouldn't this translate to higher pressure? Maybe the cold air produces (or is a factor in producing) the higher pressure but has lower pressure in itself. Is this right? How is density related to pressure, and how does it relate to varying pressures?

Also, I am confused with temperature errors and altimetry. I do not understand how pressure changes in different temperatures, and what types of errors are related (i.e. why does the altimeter overread in colder than ISA conditions?) I read that pressure decreases more rapidly in colder air than in warmer air, but again I do not understand why (does it have to do with density?). This must be a reason for the altimeter temperature errors, and I hope someone can please clarify this up.

Maybe I'm going over my head with this stuff, but I just want to make sure I understand this stuff. I know how important meteorology is, and I don't just want to memorize it- I want to understand it. Basically, my two questions are about the rate of pressure decrease, and temperature errors. These have been causing me great difficulties, and I want to get this stuff down cold.

I really appreciate you guys taking the time to reply. I hope you all have a great Christmas!

Almost all the earths atmosphere exists within 160,000 feet of the surface 90% ofits mass is contained within 50,000 feet of the suface
the density of air decreases with altitude because the force of gravity attracts the air molecules toward the surface of the earth,causing them to squeeze together closer at the surface
the density of air means its mass per unit volume cold air is dense becuse the molecules that compose it are moving relatively slowly and are packed closely together , warm air is less dense because the molecules which compose it are moving rapidly hence they take up ore space and consequently there are fewer molecules in a given volume , since cold air is denser it is therefore heavier and it sinks to the bottom of the atmosphere

remember air is a gas there fore it is compressible ,l so the top part of the atmosphere is basically squeezing the bottom part

continious fluctuations of temperature and pressure in our restless atmosphere create some problems for the engineers and the meerologists who require a fixed standard reference , to arrive at this standard they averaged conditions troughout the atmosphere for all latitudes,seasons and altitudes the result is a " standard atmosphere " witha specified sea level pressure and temp , and specific rates of change of temperature and pressure with height ---------------- it is the standard for calibrating the pressure altimeter .
hope this helps

Altimetry is very simple if you keep it that way. An altimeter is nothing more than a set of gears and a pointer attached to a little pressure chamber. The chamber is sealed, and expands or contracts in relation to the pressure of the air around it. It knows nothing of temperature. You can adjust the reading on the altimeter by adjusting the tension on the chamber, internally. This is done with the adjustment knob, and you can set it precicely using the "kollsman window" which displays inches of mercury or millibars. You can also simply set the altimeter to field elevation.

High altitude aircraft require their altimeters to be compensated. The linear expansion that takes place internally at lower altitudes doesn't happen at higher altitudes. Air does not decrease in pressure at the same rate we expect down low. If you get a chance to ride in a high performance airplane or a light jet, you'll often see that one altimeter (the Captains, or left seat altimeter) is compensated by a correction module, while the other altimeter is not. At higher altitudes, one altimeter will read correctly, while the other will usually be off by 500-1,000 feet or more. This is because the pressure lapse rate by which the unit determines altitude has differed that it can no longer properly compare ambient air pressure accurately with the reference air pressure in the sealed chamber in the unit (the "aneroid").

Another cause of this error is static errors which occur as airflow changes at higher speeds. Some airplanes will produce static correction charts to show the correction to altimeter readings for a given speed.

Air pressure at eighteen thousand feet is roughly one half of sea level pressure. But only a few thousand feet higher, it drops by half again. Above that, it begins to drop rapidly. From a practical point of view, the time of useful consciousness (TUC at eighteen thousand feet can be up to 30 minutes or more, while at 25,000 feet it's measured in minutes. By the time 40,000 is reached, it's measured in seconds.

Air has pressure at sea level because it has mass, and under the influence of gravity, weight. Air at the surface of the earth has a greater mass pushing down from above, than air at say, eighteen thousand feet. The weight pushing down tends to increase the pressure. With less and less atmosphere "above" a given column of air, there is less pressure. There really isn't a column of air, as air is a fluid, but you get the picture.

You asked about altimeter errors with respect to temperature. The saying is that when flying from hot to cold, or high to low, look out below. In other words, you're going to be lower in actuality, than indicated on your altimeter. The amount of error depends on the ambient conditions, but I've had an altimeter show a 1,000' error during a short cross country flight of only about 75 miles. It involved crossing a front that involved both a strong pressure change and a temperature change, and the altimeter was off 1,000 feet on arrival. It can make a big difference.

An easy way to think of it is this. Imagine a toy airplane sitting on a sealed balloon. glue it on top of the balloon so it doesn't fall off. Put the balloon in the freezer. As the air in the balloon cools, it contracts, and soon the airplane won't be sitting so high off the ground. Pressure has necessarily changed as the temperature has, and as a result, the level of the airplane in reality has changed, while the indication in the altimeter has not. Remember, the altimeter knows nothing of temperature, only of pressure. The pilot of this airplane needs a new altimeter setting.

The saying is, when flying from hot to cold, or high to low, look out below. The airplane doesn't actually fly or follow an altitude, but a pressure line, or a line of constant pressure. If that line dips due to temperature, the airplane dips with it. You may think of reduced pressure at the surface as someone letting the air out of the inside of the balloon. The altimeter should be reset to the new pressure in the balloon, because now the pressure has dropped. However, if the altimeter remains set the same, it will be lower than it's indicating. The pressure has dropped, and so has the airplane, but unless the altimeter setting is manually changed, it will indicate higher than it really is.

Therefore, in the case of the temperature change or pressure change, a change in the altimeter setting is needed to accurately reflect the true altitude of the airplane. The balloon represents the air mass in which the airplane is flying, or over which it's flying; it gives a graphic representation of the "column" of air beneath the airplane. Increase the temperature of that air mass, and it expands. The pressure gradient set in the altimeter will be forced higher, and if the pilot follows that gradient to keep the same indication on the altimeter, the aircraft will assume a higher true altitude. Again, a new altimeter setting is in order.

Someone here will have a better explaination for you, but that's the basics.

As far as the *rate* of pressure decrease (lapse rate), decreasing with altitude; you are correct, it is a result of the decreasing density of the air. Because gasses are compressible, they are much more dense at the bottom of the atmosphere than at the top. It might help to think of water. Water is essentially incompressible, so a cubic foot of water at the top of the ocean will weigh exactly as much as a cubic foot of water at the bottom of the Mariannas Trench. OK, there will be a slight difference, due to temperature and salinity differences, but it will only be a few percent. So, as you go deeper in the ocean, the rate of pressure change with depth remains constant, because the water is all the same density. On the other hand, if you go up in the atmosphere, the density of the air becomes a lot less, so the rate of pressure change with altitude becomes a lot less.

Does that make your first question clearer?

Second question, you wrote:

>>>>I read elsewhere that warm air has higher pressure than colder air.

Be careful here, I think that is referring to air in a closed container. If you heat a gas in a closed container, it will try to expand and exert more pressure, that much is true, but the atmosphere is not a closed container. Colder air IS more dense, and in a free atmosphere it WILL exert more pressure than warmer air. The atmosphere is complex, though, and pressure is a function of many factors, not just temperature. Generally though, colder air has higher barometric pressure than warmer air.

On the altimetry errors due to temperature:

>>>>I read that pressure decreases more rapidly in colder air than in warmer air, but again I do not understand why (does it have to do with density?)

Yes, it has to do with density. The colder air will have a greater pressure lapse rate than warmer air. (The pressure will decease more with a given increase in altitude) Think if you were using the altimeter under water. The density of water is much greater than air of course, so much more that a difference in pressure of 1" of mercury, equates to a change in water depth (change in altitude) of about 13 inches. So, if you're under water and you go up about a foot, your altimeter will show an increase in altitude of about 1000 feet. Now let's go the other way, let's suppose the atmosphere were filled with Helium (it's mostly Nitrogen) Of course, we'd all be talking like Donald Duck, but I digress. Helium is about 14 % as dense as nitrogen, or nitrogen is about 7 times as dense as helium. In an atmosphere of helium, a diffence in pressure of 1 inch of mercury would be a difference of 7000 feet of altitude. in other words, to get your altimeter to read 1000 ft higher, you'd have to go up 7000 feet in the helium atmosphere. OK so if you can see that 1"hg is 13 inches of "altitude" in water, and 7000 feet of altitude in helium, you should also see that 1" of mercury is not the same altitude difference in cold and warm air. On a standard day, you already know that 1"hg equals about 1000 feet. Now let's say it's much colder than standard. Today, because the air is more dense, the actual lapse rate is greater than standard. just picking a number out of the air, let's say the lapse rate is 1" hg per 920 feet. OK, let's go flying. We hop in the plane, dial up the ATIS and the altimeter setting is 30.12 (remember, it's a cold day, so the altimeter will be higher than standard.) we set that, and the altimeter reads 0 feet (for simplicity, our airport is at sea level) OK, now the altimeter setting only adjusts for pressure at field elevation, we can't do anything about the non-standard lapse rate. Now, we take off and climb over the airport until out altimeter reads 5000 feet. This means that our altimeter senses that the pressure is about 5"hg less than it was at sea level. But, our lapse rate is only 1"hg per 920 feet (or 1.09" hg per 1000 feet) so if the pressure at our altitude is 5"hg less than it was at sea level, our altitude is 5 x 920 feet, or 4600 feel above sea level. The opposite would be true on a day which was warmer than standard, if we set the altimeter on the ground, then climed till it read 5000 ft. we'd actually be more than 5000 ft above sea level.

Hope this helps you visualize better how this all works.

If all else fails, Like Avbug says, from high to low, look out below. It works for both temp and pressure. ie: if you're going from higher pressure to lower pressure, or higher temp to lower temp, you're actual altitude will be lower (look out below)

Please tell us you were joking about pilots having Xmas off...doh! Not a chance if you're junior (although if you're super junior, you do get Xmas off...Happy Furlough!). Gotta love getting the holidays off for once:)
Thank you so much guys! It is much more clearer now. The altimeter cannot tell if the temperature changes, and thus, cannot adjust for the lapse rate, since it is calibrated to a standard atmosphere. Hence, there will be errors, unless the atmosphere is standard or the plane is parked on the ground. Also, dense air will have a steeper lapse rate, as there are more air molecules pushing down and more pressure, compared to a mass of air that is less dense.

... and yes, I was joking about getting Christmas off! :) I have three weeks off- how about you ;)

Thanks a lot guys, and happy flying and happy holidays!!
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