Temperature drops about 6.5 °C for every 1,000 m of altitude.
The standard-atmosphere lapse rate in the troposphere. Real days vary (inversions exist), but it is the right first guess for a hike or a flight.
6.5 °C/km is the average through the whole troposphere. Dry air actually cools at 9.8 °C/km and saturated air nearer 5. And in a valley inversion on a still winter morning the gradient reverses entirely — it gets warmer as you climb, which is why the frost sits in the bottom.
At 2,000 m the rule says 275.1 K and the exact answer is 275.1 K — within half a percent. It holds to within 2% up to 11660 m, then drifts.
6.5 °C per kilometre is exact by definition up to the tropopause, and then it simply stops — the stratosphere is isothermal at −56.5 °C for the next 9 km. Keep applying the rule at cruising altitude and you are 6 K cold by 12 km and 30 K cold by 16. In Kelvin, because a relative error against −0 °C means nothing.
The rule against the exact answer, computed across the range. Inside the shaded band the shortcut is close enough to use; outside it, reach for the calculator.
Estimate with the rule, then check it against the calculator that models it properly.
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