The air at the peak of even a small mountain is colder than the corresponding air in the valley. But why does air cool when it rises? Changes in air pressure at high altitude result in a process called adiabatic cooling, whereby no heat energy is actually changed or lost but the reduction in pressure results in a temperature drop.
The Conservation of Energy Principle
Energy cannot be created or destroyed. The principle of the conservation of energy states that if you have a closed system – i.e. no energy flows in or out – then the amount of energy in the system will always remain constant.
Energy can, however, be transferred from one type to another. For example, when oil is burned energy that is locked up inside hydrocarbon molecules is released as heat. In an automobile, this heat energy is converted to mechanical energy used to propel the car by an engine. No energy is created or lost, it is simply released and converted from latent energy in the fossil fuel to mechanical energy driving the car.
The First Law of Thermodynamics
The way in which a car engine converts heat energy to mechanical energy can actually help us to understand the process of adiabatic cooling and why air cools when it rises. It's an application of the first law of thermodynamics, which in itself is a derivative of the conservation of energy principle, stating that the amount of energy done in a system equals the amount of incoming heat minus the amount of work done.
In air, this is expressed as a relationship between temperature and pressure. If you add heat energy to a body of air it gets hotter and/or expands. In an internal combustion engine, this creates pressure – the air is trapped in a turbine and so cannot expand without exerting outward force. This force is used to drive the piston so that energy can be transferred out of the system in the form of mechanical energy.
The Adiabatic Lapse Rate
This transfer of energy out of the gas causes it to cool. This is adiabatic cooling – heat energy has not been transferred but the transfer of mechanical energy nevertheless results in a drop in temperature.
The same process can be observed in the atmosphere. Air further from the Earth's surface is under less pressure as there is a smaller weight of atmosphere above pressing down. As air rises the reduction in pressure allows it to expand and this expansion and loss of pressure results in a drop in temperature or around 1°C per 100m. This is known as the adiabatic lapse rate.
Thus the primary reason that air cools when it rises is down to the drop in pressure as it moves further from the surface of the Earth. Because air pressure is lower when standing on a mountain the surrounding atmosphere will be cooler than when standing in a valley. The rate of cooling relatively constant in dry air, 1°C per 100m, and this gradient – known as the adiabatic lapse rate – has a wide range of applications in meteorology.