How Temperature & Pressure Effect Atmosphere & Our Altimeter

Ajouté : 2026-05-28

[00:00:00]In this video, we're going to talk about how pressure and temperature changes have an effect on the atmosphere we fly in and what is indicated on our altimeter. There are two pneummonic devices pilots are typically taught to remember when dealing with altimeter errors from temperature and pressure.

[00:00:17]They are from high to low, look out below, and from low to high, clear the sky. From high to low, lookout below memory aid works for both pressure and temperature. It tells us that when we fly from an area of high pressure or temperature to an area of low pressure or temperature that our altimeter is going to sense a high altitude.

[00:00:38]Therefore, we need to look out below because we think we are higher than we actually are and we might be in danger of hitting terrain or obstacles below us that we think we have clearance from, but we really don't. Low to high, clear the sky also works for both pressure and temperature.

[00:00:57]This tells us that when we fly from an area of low pressure or temperature to an area of high pressure or temperature that our altimeter is going to sense a lower altitude, the opposite of our previous pneummonic situation, high to low, lookout below. Therefore, we need to clear the sky above us because we think we are lower than we really are and may be in danger of running into traffic above us or busting an airspace above us that we think we are safe from.

[00:01:25]For example, say we are flying at 3,500 ft and we fly from an area of low pressure to high pressure without changing our altimeter setting and our altimeter tells us that we are now at 3,000 ft. So, we decide to climb 500 ft so that our altimeter now says 3,500 ft, which is what we want. But because of the change in pressure, our actual true altitude is 4,000 ft now. So any traffic at 4,000 ft or air sport or airspace we don't want to be in will be in danger from us unknowingly flying at that altitude. Now one of the most common questions we get about this subject and the reason this subject can be so confusing to learn and honestly difficult to explain is I thought increasing temperature decreases air density and less air density means less pressure. This is true for the atmosphere in general senses. So don't forget it. But when it comes to our altimeter, it does not help us and actually makes things more confusing. So we have to decouple in our brains the two concepts even though they seem related. So why is this? Let's show it visually and break it down for you.

[00:02:41]First, let's visualize the example we just talked about. Here we have an area of low pressure in red and an area of high pressure on the left in blue. Our aircraft is at a true altitude of 7,000 ft MSL mean above mean sea level and initially our altimeter setting is set correctly and our indicated altitude is indicating the correct true altitude of 7,000 ft. In other words, there are no errors from pressure or temperature on our altimeter. But let's say we continue to fly without changing anything except the fact that we are flying forward on the same heading from the low pressure area in red to the high pressure area in blue. We don't change our altimeter setting and we don't climb or descend. When we get to the area of high pressure in blue, we are still at a true altitude of 7,000 ft because remember again, we haven't made any actual altitude changes. However, our altimeter now indicates slightly lower than 7,000 ft at 6,835 ft. So, if we say, "Oh, shoot. We need to climb to get back to 7,000 ft because our altimeter sends us something lower.

[00:03:51]We must have descended a bit." And we climb to get to an indicated altitude of 7,000 ft. we'll actually now be at a true altitude of more than 7,000 ft at something like 7,165 feet as we show here in this visualization. So our altimeter says 7,000 ft because we've climbed our altimeter was sensing lower at 6,800 ft.

[00:04:13]So we climbed to make that say 7,000 but now our actual true altitude is 7,1665 ft because our altimeter is having airs.

[00:04:24]We could have done the same example with temperature. All of it would have been the same. We would have traveled from low temperature to high temperature and again our altimeter would have sensed a lower uh altitude and then we would again you know need to climb to make that say 7,000 ft but then our climb would be put us above 7,000 ft just like we had in the same example. It's the same exact thing for temperature as it would be for pressure. So the important thing to remember is that when it comes to our altimeter, we need to think differently. When temperature or pressure drops, our altimeter senses both of these situations as an increase in altitude. And when temperature or pressure rises, our altimeter senses both of these situ situations as a decrease in altitude. And these follow our memory aids of high to low, look out below, and low to high clear the sky. So when it comes to the altimeter, student pilots must first and foremost remember these manonics pneummonic devices to be true. So now let's explain why our altimeter acts this way and why an increase in temperature outside our aircraft doesn't have our altimeter sensing lower pressure and higher altitudes. To understand, we need to remember how alimeters work. In an alttimeter, an anoid wafer, think of like a sealed balloon inside your altimeter, is filled with sea level pressure air and it expands or contracts with changes in outside air pressure. We cover this with detailed views of the inside of an altimeter in our lesson on how alimeters work.

[00:05:58]Gears connected to the wafer turn with the expansion or contraction and translate the balloon's movement into the movement of the needle on the face of our altimeter and thus a gain or drop in the indicated altitude. The altimeter needle, the gears connected to it and the wafer are all specifically calibrated for a specific atmosphere that we call standard atmosphere.

[00:06:23]Standard atmosphere tells us an average or standard lapse rate for temperature and pressure with altitude. In other words, we can use the standard atmosphere to say what the temperature and pressure would be at any altitude when conditions are exactly standard.

[00:06:38]The standard atmosphere atmospheric lap lapse rates are for every 1,000 ft of altitude gain, the standard pressure drops 1 in of mercury and the standard temperature drops about 2° per C. So here we show our aircraft at 5,000 ft and we show the pressure dropping incrementally for each 1,000 ft we go up in standard atmosphere. So on a standard day when our altimeter senses a 5 in drop in pressure from sea level like in our aircraft here, the anoid wafer expands into the lower pressure surrounding it and the gears are calibrated to show a 5,000 ft altitude increase for that specific amount of expansion of the wafer. So, let's consider some situations. First, let's start with our standard conditions with an altitude of 4,000 ft MSL. We have standard conditions and have the standard pressure set in our pressure setting window. So, our altimeter is giving us an accurate reading of 4,000 ft. Now, let's consider a new situation and compare it to our standard one. Our aircraft is still at the same true altitude of 4,000 ft, but we are now in this red column of air.

[00:07:49]Let me get a little pointer here. This red column of air here. We're still at 4,000 ft. These are lined up, but we're in a different column of air. This situation is going to represent when our temperature is higher than standard.

[00:08:01]When temperature is higher than standard, the column of air we are in expands. So it's it's much taller column as you can see such that each 1-in drop in pressure represented by these black lines here is now more than a th00and ft of altitude gain. We have sea level pressure here at the bottom at 29.92 in of mercury. But now we can see because of the expansion of the column each of these 1 in pressure blocks in the column are now taller. As you can see this line is a little bit higher than this line.

[00:08:35]This line's higher than this line in standard, right? So 28.92 1 in drop here in our hotter than standard column of air. That usually represents 1 in drop in pressure, right?

[00:08:49]Usually represents 1,000 ft of altitude gain, but it's now taller than this 1,00 ft line here on standard because of the expansion of air and so on as we go up for each 1-in drop in pressure. But our altimeter wasn't told of this change and can't be recalibrated to accommodate the change. Our altimeter still thinks that a 1-in drop in pressure is 1,000 ft as we've labeled here. So our this is what our altimeter is thinking even though we're in this different hotter and expanded column of air. So it sees 28.92 and it thinks 1,000 ft. It sees 27.92 it and it sees 2,000 ft. So these altitudes listed here again our what our altimeter is thinking as it senses the pressure.

[00:09:37]So again our aircraft has not moved.

[00:09:40]It's still here at 4,000 ft. Except now our indicated altitude on our altimeter here in this hotter than standard column of air reads 3,100 ft. Much lower than 4,000 ft. This is because of that expansion in the column of air from high temperature that our aircraft is only sensing about 3 in of pressure drop instead of 4 in like before as we can see by the 26.92 in of mercury line right here. So here in standard we got one 2 3 4 blocks of pressure drop. So 4 in of pressure drop here because it's expanded. We only got one two three and a little bit more than that. So our altimeter is sensing, you know, just a just over a 3 in drop in pressure. And so it's going to tell us that we're at 3,100 ft. Now, let's consider this blue column of air here. That's going to represent colder than standard temperatures.

[00:10:44]lower than standard temperature contracts the column of air such that each 1- in drop in pressure is now less than 1,000 ft of altitude gain. So it's the opposite. You can see these again these are our 1-in drop in pressure blocks here represented by the black lines and they're now squished. They're contracted because of the cold air. Here we can see the column is contracted or squished and the blocks for each 1-in drop of pressure are shorter than standard level. So we have 29.92 inches of merker here. Then 1in drop is right here. We're in standard.

[00:11:19]It's up here, right? It's a little bit taller in standard. It's been everything's been squished. So our altimeter again doesn't know of this change and still thinks a 1 in drop in pressure is 1,000 ft of altitude gain. So, our altimeter will show 1,000 ft lower than the actual true 1,000 ft. Again, our aircraft hasn't moved.

[00:11:42]We're still here at a true altitude of 4,000 ft. But now, our altimeter indicates almost 5,000 ft, right? As you can see down here in the blue, because it is sensing almost 5 in. So, if we count the 1 in pressure drops, we get 1 2 3 4 five blocks. And that's what our altimeter is sensing because of this contraction of air and it associates those pressure drops with each with 1,000 ft of altitude because it doesn't know any better. Let's summarize again.

[00:12:14]In each of these situations, the aircraft's altitude is the same at 4,000 ft. We can see this if we replace these altitude levels from what our altimeter thinks they are to what they actually are. So if we cross all these out and we replace them with the actual true altitude, we can see that in each situation, the true 4,000 ft true altitude is right here where aircraft is. And then what we can also see is the new relationship of 1 in drop in pressure and the actual altitude for each of these situations. Ideally, we would want to have an altimeter that we could automatically calibrate on the go and tell it the new relationship between pressure and altitude. For example, when it gets colder, our altimeter adjusts its calibration to associate the contracted column of air and the fact that a 1 in drop in pressure is now equal to less than 1,000 ft of altitude gain. As we can see here, 1 in drop in pressure, everything's contracted. So, it's it's not 1,000 ft anymore. It's associated with 800 ft. So, we wish we could tell our altimeter on the go, hey, 1 in drop in pressure is no longer 1,000 ft. It's now 800 ft. Or when it gets warmer, all our altimeter would adjust its calibration on the go and associate this expanded column of air where now a drop in pressure is something like 1,200 ft or more than 1,000 ft. This contraction and expansion of air can also help us explain the concept of density altitude. Here again we have three three aircraft all at the true altitude of 4,000 ft right here along this line of our true altitude. One is at standard this one here in the middle. Uh one is above standard temperature again the red one and on the right and then one is below here in the blue column of air.

[00:14:09]Density is the amount of mass per volume. So in terms of the air we fly in, this is essentially the amount of air molecules in a set volume of air. So we see a specific amount of air molecules in our standard column of air surrounding our aircraft. And let's say that this box is the set volume of air in which we will use to measure the density. Here we see about 20 to 25, you know, 1 2 3 4 5 20 to 25 air molecules in this box that we've made. That's going to be the volume of air we measure density by. When temperature decreases here in the blue, the column of air contracts, right? It's shorter. It's more contracted and packs all these air molecules more tightly together. So, if we use that same box, that volume of air box, we now see that there are more molecules in that same box. where there was about 20 to 25 here in our standard uh column of air in this volume box. The same size volume box here in the cold column of air has about maybe 30 to 35 molecules. So therefore the density is increased because we have more molecules in the same size volume box. And finally, when the temperature increases, the column of air expands and it does the opposite of when it was cold, right? It expands. This allows the molecules to have more space and now they're less tightly packed. So again, you guessed it, if we use that same size volume box in the hot column of air, we now see that they're only about maybe six to eight air molecules, you know, 1 2 3 4 5 6 7. Uh, and in other words, the amount of air molecules per this volume of box has decreased. So our density has decreased. Less air molecules per volume surrounding our aircraft like this, like in this hot air column means less performance of our wings, engine, and more. And this is why we care about density altitude. When conditions are standard, density altitude equals pressure altitude as we see here in our standard column of air. So if we use this level as the pressure altitude right here in the standard column, the column matches the height of the column matches our pressure altitude here in standard air. When the temperature is colder than standard, density altitude is now less than pressure altitude as evident by our cold column of air being shorter. It's contracted. So and the height of these columns can perfectly represent the density of altitude. So up here, this is where our pressure altitude was. And now this one you can see is contracted. So this height represent the height of the column represents our density density altitude.

[00:16:54]You can see that it's now less than the pressure altitude which is up here. And then of course for our hotter than standard air it is even higher. So our density altitude is higher than our pressure altitude when the air is hotter than standard. How do we use density altitude?

[00:17:12]Well, the truth is we don't actually use the number directly. But this won't stop the FAA from asking us to compute it using figure 8 in the airman knowledge testing supplement on the written exam like you see here. Here in the graph on the left, we can match a temperature with a pressure altitude. So down here we have temperature and then we go up and we match it with one of these lines for pressure altitude and then we see what the density altitude here is on the y ais. We won't calculate this here in this video, but we will in our lesson specifically on density altitude, and we have a video for that specifically. The true application of density altitude comes when we need to calculate our aircraft's performance.

[00:17:54]Here we see a climb performance and similar to those used in aircraft like Cherokee Warriors. The goal of this chart is to tell us the performance of our climb in terms of the time it will take, the fuel it will take and the distance it will take to climb to our desired altitude. There is no output on this chart for density altitude.

[00:18:14]However, if you notice the first step in using this chart is very similar to figure 8 over here. We ma we take we find a temperature and we go up and we match it with a pressure altitude line.

[00:18:26]This is the same thing as how we found density altitude. So these performance charts are taking in the information of our temperature and matching with it with our pressure altitude to understand the effect the density will have on our performance at that altitude and temperature. But we just don't see the final numbers of what density altitude actual are because again the goal of this is to tell us the fuel time and distance it will take for us to climb.

[00:18:52]For an aircraft like a Sesna, the climb performance chart is in tabular form.

[00:18:56]But again, look at the first two columns here. We have temperature and pressure altitude. And then we also have this note right here, this note number three that tells us to increase or decrease by 10% for each 10° C above standard temperature. And so again, what this is doing, it's taking into account the density of the air by way of matching our pressure, altitude with a temperature to tell us that our performance. So it's taking into account the density altitude and the density of the air for the performance of our aircraft. We just don't see it as an output. We can also calculate the density altitude on our E6B flight computer using the pressure, altitude, and temperature window right here. So we match a pressure altitude with a temperature here on the outer scale and then we read off density altitude here in the density altitude window. So this suggests that it can be useful information for pilots to know the actual density altitude and it certainly is. Knowing that our density altitude is much higher than our actual altitude can warn us that our aircraft will not perform like we want it to. Again, we will show how to calculate density altitude in our density altitude lesson specifically.