It is known that certain meteorological stations frequently record a mean sea level pressure which is lower than that which would be expected from the general run of the isobars by amounts up to about one millibar. These stations are located in over exposed situations, such as the tip of a promontory, where the wind speed may be appreciably in excess of the speed in an ideal exposure. In steady motion, the flow of wind over or round the promontory conforms with Bernoulli's equation, which gives a relation between pressure and velocity at points on the same stream line. It is shown that a reasonable increase of wind speed above normal produces a decrease of pressure of the right magnitude. The equation is applied in the first instance to steady frictionless motion, but it is shown that the results apply with little variation to' the How of surface air, as modified by friction.