Hi. I was watching this video [**https://www.youtube.com/watch?v=bDoh8zQDT38**](https://www.youtube.com/watch?v=bDoh8zQDT38) of an old SL-10 mission, and I noticed that at 1:12 the air temperature goes down to -454F. Thing is, according to the altitude read the rocket should be right in the thermosphere, and at that height the atmospheric temperature shouldn’t be that low, so what’s going on? a malfunction, or is there something at play that I am not understanding?
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The thermosphere has an extremely low density. What atoms and molecules there are in the thermosphere can be very hot, moving at a very high average speed, but it is an average of a very small number of atoms. The thermometer is trying to measure the average thermal energy of the ambient atmosphere but it does so by measuring the amount of thermal energy imparted to its measuring apparatus (in this case probably a thermocouple).
Because the density of the thermosphere is so low the amount of energy it can add to the thermocouple is extremely small, so the temperature measured is very low. Each individual particle in the thermosphere is moving very fast and is “hot”, but there aren’t enough of them to heat up the thermocouple. If you stuck your hand out into the thermosphere it would *feel* cold.
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