# How to compute the change in static temperature using only altitude?

I'm given an initial altitude of FL290 in ISA conditions, and the aircaft is said to climb to reach FL310. How can I use this to calculate the change in static temperature between the two flight levels?

I know that the formal term is the lapse rate and generally lapse rate decreases around 2 degrees Celsius per 1000 feet of climb.

Would I use a ISA temperature table to figure out the temperature for the initial altitude and then use the lapse rate to calculate the change in temperature when it climbs to reach FL310?

• This sounds a lot like a homework question. Did you Google how the temperature depends on altitude in ISA conditions? – Bianfable Nov 5 '19 at 18:03
• Could you include your current state of research? It will help us to know how we must answer – Manu H Nov 5 '19 at 18:23

## 1 Answer

I found the answer, it's literally looking at ISA temperature table of the initial FL. The lapse rate is 2degrees per 1000 feet of climb. You would see what the initial temperature is for your FL and then you minus the lapse rate for the climb that you are given. So in this case, the lapse rate would be 4degrees as it climbs 2000 feet.

• Why are you even looking up the initial temperature if you are only interested in the difference? The difference is simply the lapse rate times altitude difference no matter what the absolute values are, isn't it? – Jan Hudec Nov 5 '19 at 20:52
• Oh yes, you are right. I never thought about that approach. Thank you for sharing. – SillySquishy Nov 5 '19 at 22:26
• But careful! The same problem at altitudes above ~FL360 will have a different answer. – Zeus Nov 6 '19 at 1:38
• Yes, above FL360 the temperature remains constant, so there would be no change if we are comparing two altitudes above FL360 for temperature change. – SillySquishy Nov 6 '19 at 10:20