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Why it is cooler at the top of a mountai...

Why it is cooler at the top of a mountain than at sea level?

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Pressure decreases with height . Therefore if hot air rises, it suffers adiabatic expansion.
From first lawof thermodynamic `Delta Q = Delta U + DeltaW implies DeltaU = - Delta W [because DeltaQ = 0]`
This cause a decrease in internal energy and hence a fall of temperature.
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