CXXVIII. An elementary reasoned account of the general circulation of the atmosphere

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Abstract
The paper (which makes no claim to originality) gives an elementary survey of the physical causes believed to underly the more energetic features of the general circulation. The geographical distribution of temperature implies hydrostatically a strong fall of the pressure from the tropics to the poles at any fixed altitude in the upper troposphere. This produces geostrophically (in the main) the strong westerly winds found at all latitudes in that region. Simple estimates indicate why a typical wind speed is 20 m/see. The wind distribution near the ground, a secondary effect, is intelligible only if there is everywhere a poleward flow of west-east angular momentum (taken about the earth's axis). The required flow reaches a maximum of around 3 × 102s dyn cm, at latitude 30°. In the tropics and sub-tropics the angular momentum must flow against its own gradient. It can do so to the necessary extent, however, as a result of the poleward motions down pressure gradients in the upper troposphere which are needed tomaintain the energy of the general circulation, which is dissipated at a rate of around 5 × 103 ergs/cm2/sec. In the cyclone belt, the assumption behind this statement (that isobars in the upper troposphere lie east-west, on the whole) is constantly violated, but there the flow of angular momentum (down its own gradient) presents no difficulty, and is doubtless achieved by the cyclones themselves.

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