# Temperature variation at constant altitude?

Just curious if anyone has observed if there is ever much, or any, temperature change in cruise at a constant altitude?

The reason I ask is because I was trying to get maximum ground speed in solo mode. So I moved the wind sliders so that I had the highest possible tail wind. But I also increased the temperature slider to the highest position.

It made a good boost to ground speed.

But then when I moved the temp slider a good bit left and went back to check airspeed, it shot me way over maximum speed.

I just wondered in live mode, how much temp change there is to affect airspeed.

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I donâ€™t know if this is the answer you are looking for but, the temp. Is based on the live weather mode so it can change accordingly to the wind example as the wind changes from warm to cool the farther north it gets, but it also wind does tend to be greater when in jet streams because the constant circulation between the polar winds and the warm wind that gets heated up and pushed back down, like a circle but Iâ€™m assuming you probably know half of this.

Edit: Iâ€™m not for sure IF used real weather physics either like warm air rises cold air sinks and such but, it is very much possible they do since itâ€™s live weather

Edit 2: @Ksisky said it best

This has to do with density of the air.

When air gets cold it gets dense, total pressure with increase and youâ€™ll overspeed at a lower ground speed. If I had to guess, if either just uses altitude forecast data or uses some type of equation to estimate the temperature drop at altitude based on the ground temperature.

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I was surprised when I saw how dramatically much I shot over max speed when I pushed the temperature slider back to a lower number. It caused me to wondered if temperature change could ever happen fast enough in live mode to make someone violate max speed if they are flying too close to redline.

In the past itâ€™s been mentioned that wind gusts could cause a violation if flying too close to max speed. Which is sort of common sense. But I never thought about temperature gradient, how fast the temperature changes.

When I caused this speed overshoot, the auto throttles of course went immediately to zero. But it took a long time for the speed to come back down to the autothrottle setting.

Thank you for the diagram and equations which Iâ€™m still thinking about.

As you indicated I was going for lower air density so that I could fly with the highest possible ground speed. I was surveying a route so I wanted to use x5 speed which is available of course only in solo mode. But x5 is not available for the fighter jets so I took a 737 and adjusted the weather sliders to give me as much ground speed boost as possible. You can get nearly 100kts of tailwind and then another good boost from making the air hot.

Basically, the winds and temperature in InfiniteFlight are all in sync with weather in real life. Thatâ€™s the best short short answer I can give and simple one.

So my question would be, temperature aloft is forecasted by weather services and used directly in IF as published?

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Can anyone answer why the HUD temperature is different from the Weather slider setting? I must be missing something obvious(?).

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So for the first one

This is possible for sure if someone is flying close enough ti redline and auto throttle at 0 the wind would still push because the aircraft is neither descending nor ascending and stays the same so there is really no drag in some sense you could say. But also I donâ€™t think it would happen it very much could but only because most people fly realistic and not close enough and you would have to have an insane tailwind to overshoot redline.

This is because well the aircraft has little drag, well wind drag obviously but like no forced pressure drag causing the aircraft to slow down. Weight will slow is down and auto throttle too but it would take a minute due to how fast the air achy is flying and how much it weighs.

For example take a 777, itâ€™s heavy and you are at cruise but say you are flying 360-370 knots it would take a minute or two for the plane to reach 320-330 because (assuming you had a tailwind or no wind since this is solo) because well there is no extreme opposing force applying pressure to the aircraft it is just flying straight with a tailwind or no wind.

Hope this helps

Also yes, IF uses real time weather to create realistic conditions, kind of like when a hurricane hits, you can find the very windy patches and you can also find the eye all while flying in Infinite flight

In short, yes it does change

I have to admit Iâ€™ve never paid attention to whether temp changed by much when Iâ€™m settled at cruise, and by how much and how fast. Itâ€™s clear the auto throttles have to adjust for that change.

But again, I couldnâ€™t find any info on how much it changes.

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It appears that the temp gradient data may be that which is available here, which can be selected by FL: AWC - Winds/Temps Data

Though the Pitot Tube measures IAS, the V in this equation is TAS?

Correct. V as in local velocity, meaning the true airspeed. TAS can be constant but if density changes so will IAS.

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Itâ€™s interesting that to get IAS all the pressure transducer has to work with is the ability to sense and report on the dynamic pressure: comparing total pressure less static pressure. And the term that represents that comparative stress from the relative wind is:

which is simply kinetic energy, that is (1/2)mv^2, but where rho is mass density rather than simply total mass.

So this says the IAS is a measure of the airâ€™s available energy for pilots in making flight decisions.

The density term rho weighs in the fact that the energy content of the air depends not only on the true relative velocity of the mass in motion, but also on the amount of mass available in each parcel of passing air.

This is why youâ€™ll see never exceed speeds in KIAS. The actual velocity of the stream does not matter, itâ€™s the pressure that counts. That dynamic pressure will always be constant for a given KIAS.

You guys are too smart for me

I donâ€™t believe this at all. I think the brain follows curiosity. The more curious you are about something, the more intensely youâ€™re compelled to dig into something. You obviously are very curious about things.

and in the process of digging we make mistakes. Example?:

I thought later â€śwhat was I thinking?â€ť You need the single standard temp setting at a standard altitude and the OAT on the HUD is what the translates that into at your experience at your flying altitude.

What I meant here was that redline is given in IAS but we have this inertia of our mass moving forward that wants to keep moving. That inertia is best considered in this case to be represented by our ground speed: you change this force or that force and it takes time for the gs to catch up to the change in force. So the change in force here comes from the autothrottle going suddenly to zero. It does that because the lower temperature is associated with IAS shooting up suddenly because the cold air is denser.

So my curiosity here was whether the temp change could ever happen fast enough that the auttothrottle adjusting down wouldnâ€™t be soon enough to give enough time for the drag to slow the aircraft fast enough to avoid overspeed. If that makes any sense(?).

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In real life this wouldnâ€™t happen unless there was some sort of extraordinary weather phenomenon or weird geography when flying low level. In game maybe, if weather was presented to the app in such a way that it was an immediate temp switch. Never seen it happen though.

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