So I was wondering...

 

We measure altitude by measuring the static airpressure which decreases as altitude increases.

Nowadays all aircraft are fitted with barometric altimeters.

 

Now temperature also decreases as altitude increases so I suppose that it would be possible to measure altitude by looking at a thermometer, right?

So what I was wondering is; has this ever been done?

And if it has, why did the barometric altimeter 'win'?

Hi.

 

It's not really possible to measure the depth of air by its temperature because the change with altitude is neither steady nor constant.

 

Consider weather fronts... First a warm front where a body of significantly warmer air lies over a cooler mass. Climbing through that interface would give a sudden increase in temperature resulting in an apparent loss of altitude. Second, consider a cold front where a body of cooler air is nudging a warmer mass aside. Flying through it would cause your apparent altitude to increase quite drastically; not what you'd want around mountains or near the ground.

 

Imagine also the effect had by climbing (or descending) through an inversion where the OAT increases suddenly as altitude increases. Your altimeter would show a sudden loss of altitude as you climbed.

 

The temperature gardient is also affected by the air's moisture content: the drop in temperature with increasing altitude is reduced when the air is wetter...

 

Best to stick with pressure. Or radar. Or GPS...

 

Cheers,

D

Because the temperature does not always decrease with altitude. It can rise as well. There are also regions of the upper atmosphere where it also stays constant with altitude or rises. So it won't work to merely measure altitude by measuring the decrease in temperature.

 

 

It's not really possible to measure the depth of air by its temperature because the change with altitude is neither steady nor constant.

 

The change of pressure with altitude is also neither  steady of constant.

 

The reason why temperature isn't used is to measure altitude is because it doesn't change between 11000 m (36089 ft) and 20000 m (65600 ft) in the ICAO Standard Atmosphere.

Pressure does decrease as you go up and increase as you go down though it will happen at different rates under different temperature conditions. You don't encounter significant pressure gradients as you encounter temperature gradients in horizontal layers of the atmosphere. The differences are small because winds act quickly to balance the forces.

The change of pressure with altitude is also neither  steady of constant.

 

The reason why temperature isn't used is to measure altitude is because it doesn't change between 11000 m (36089 ft) and 20000 m (65600 ft) in the ICAO Standard Atmosphere.

 

Hmm... are there such things as pressure inversions then?

 

 

Hmm... are there such things as pressure inversions then?

 

Read again what I posted.

 

 

 

The change of pressure with altitude is also neither steady of constant.

 

I read it again. I still don't understand what 'steady of constant' means.

It's a typo - should be "steady nor constant".

The change of pressure with altitude is also neither  steady of constant.

 

The reason why temperature isn't used is to measure altitude is because it doesn't change between 11000 m (36089 ft) and 20000 m (65600 ft) in the ICAO Standard Atmosphere.

 

And the opposite becomes true as you go even higher...

 

 

And the opposite becomes true as you go even higher...

 

true but it doesn't affect the point at issue - why temperature can't be used to determine altitude.

 

Read again what I posted.

 

 

You don't mention positive or negative. Not steady & not constant can still have a gradient entirely in the positive range.

 

So... can you get pressure inversions?

 

Or are you just hoping for an argument?

true but it doesn't affect the point at issue - why temperature can't be used to determine altitude.

 

Well, it further substantiates the fact that temperature and pressure don't increase (or decrease) consistently with altitude variations.

You don't mention positive or negative. Not steady & not constant can still have a gradient entirely in the positive range.

 

So... can you get pressure inversions?

 

Or are you just hoping for an argument?

 

What difference would it make?  You don't set pressure based on altitude, anyway.

 

My answer would be yes, it should be possible to get a pressure inversion.  Things like Temperature Inversions, Low and High Pressure Fronts, Land and See Breezes, etc. all forcefully displace air masses.

It's not really possible to measure the depth of air by its temperature because the change with altitude is neither steady nor constant.

 

 

And the change in  pressure with altitude also is nether steady nor constant. What's your point?

Remember one of the basic properties of gases in that they will expand to full whatever space they are in. Pressure inversions don't form in horizontal layers because the column of air will expand or contract. Winds form and mix things up.

 

Temp inversions occur for several reasons. One reason is due to warmer air that is rising up cooling off below the temperature of a layer of less dense warmer air above. This can be a stable environment.

 

Another is when the ground quickly cools of at night and cools the layer of air right above it the air increases in density and remains below the warmer air.