Summary: The differences between morning and evening pressures in a region of high pressure have been observed to be greater than normal differences. This was checked using Toronto station pressures. The effect is often explained as due to the expansion of warmed air, a direct result of increased insolation. Another explanation here suggested, is based on changes in the surface divergence in a high, which are shown to be an indirect result of insolation. The latter explanation is shown to lead to pressure differences of appreciable magnitude.

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*Published by permission of the Controller of Meteorological Services of Canada.