What is Density Altitude - For Student Pilots

[@kypaik]

This is the first video that makes me fully understand what density altitude actually means. Thank you.

[@PartTimePilot]

Anytime! Thanks for watching

[@andrelafreniere8604]

This is the explanation that worked for me, now I get it :) Thanks

[@PartTimePilot]

Yay! Glad to have helped and thanks for watching!

[@yamidgaray9074]

The explanation in this video is amazing. Thank you.

[@PartTimePilot]

Thanks for watching!

[@ianjeakins1438]

Thank you so much for this video series. It has been invaluable in helping me understand basic aerodynamics. I am studying to get my engineering licence in the UK and your videos have really helped.

[@PartTimePilot]

Oh awesome! No problem. Thank you for watching

[@MAXLOPEZ-oj8gc]

thank you
you just mad it more eazy

[@nancicoia3116]

Excellent explanations!

[@sundaysareforreading]

How did you calculate the takeoff ground roll distance?

[@PartTimePilot]

We have a video that shows how: ua-cam.com/video/NOAaWvkVjhk/v-deo.htmlsi=fwsCYkDKhIRuvaLc

[@dr3ww._]

I understand you said air is less dense at higher altitude but I’m trying to make sense of this but I just can’t somehow. If temperature decreases with altitude, wouldn’t that make the air more dense at higher altitudes though? since those air molecules are more close to each other? I’m overthinking this and I’m trying to figure out, but I just can’t I thought I understood it at first but now it’s like I’m going back and just thinking about it.

[@PartTimePilot]

It’s true that to describe a lot of phenomena in weather we use the fact that cold temperatures and high density go hand and hand. But when we increase in altitude this goes out the window. It goes out the window because the volume of the air changes with altitude and the air thins. The amount of molecules per volume decreases because the volume expands so much. With less molecules per volume (exact definition of density) you also get less collisions of molecules and less heat release (exact definition of temperature).
So whenever you say high density equals low temperature you are essentially assuming a constant volume of a gas or liquid. You are also assuming a constant pressure. If volume or pressure changes, these relationships change. That’s what makes weather so confusing. It’s not a closed system.

[@pearceblaney]

Great video! I had a question. So higher temperature is less dense than cold air. However, higher pressure is also more dense and I thought when temperature rises so does pressure, but then that wouldn’t make a lot of sense. Hopefully you can help. Thank you!

[@PartTimePilot]

The thing that makes it confusing is that the relationship temperature goes up with pressure is from the ideal gas law which says Pressure = Density*Temperature*GasConstant. So as you can see if temperature goes up, pressure has to go up and vise versa IF we assume the density remains the same. Which is the case if we have a gas contains in an adiabatic container. Adiabatic means nothing can go in our out essentially. So the density inside the container is all the same.
But our atmosphere is not an adiabatic container and we have various effects which can change the temperature (heat from the sun, radiation from the surface), density (humidity, temperature, pressure) and pressure (terrain, temperature, humidity). So in our atmosphere it is not as clear cut and you can't explain it with the ideal gas law.
However, I still teach my students that when they are thinking of their altimeter to remember temperature goes with pressure. Meaning that when you travel from a place of low/high temperature to a place that is the opposite your altimeter will react the same as it would with changes in pressure. This is explained by how altimeters measure pressure. Pressure is just a measure of the column of air above the device doing the measuring and its a measure of the force of that column on the device. When you have a drop in temperature it condenses the column such that less column is above you and your altimeter feels less pressure. When there is a rise in temperature is expands and there is more column above you so your altimeter feels more. Hopefully this helps

[@Mike-dt6um]

Hopefully I can ask this question properly.
The pressure altitude numbers in the POH for performance matched with the temperatures . Is that the density altitude not corrected yet? 😬 if I asked that right?
The pressure altitude at 4000ft at 20c above standard
.
It’s not corrected yet
And this is what I’m climbing too? Hopefully I asked correctly

[@PartTimePilot]

The lines on your performance charts for altitude are for pressure altitude. The reason these charts first make us match it with a temperature is because the chart is converting the pressure altitude to density altitude for us!
Because density altitude is what our aircraft really feels and what we want to calculate performance at.
Take a look at figure 7 I believe in the airmen testing supplement for
Private Pilot. This is a chart just for converting pressure altitude to density altitude. Look familiar? It’s the same first step on all our performance charts. The only difference is it actually gives us a value for density altitude. On our performance charts figuring out density altitude isn’t our goal… our goal is climb performance or
True airspeed or engine power. So we don’t need to know the number for density altitude but we do need to make sure we correct for it.
Hopefully this makes sense. Not 100% sure that was what you were asking but should clear things up