AbstractThe ageostrophic component of the wind at 900 millibars in the cold air ahead of warm fronts has been found for sixty occasions between 1943 and 1949. The values show much variability both of magnitude and angular deviation from the isobars. It is found that this variability is not closely related to any of the following characteristics of the situation:—(i) The curvature of the isobars in the cold air.(ii) The isallobaric gradient ahead of the front.(iii) The pressure tendency at the place where the ageostrophic component is found.(iv) The refraction of the isobars at the front.(v) The temperature difference between the two air masses.(vi) The geostrophic wind at this place.For forty‐four of the cases the vector change of geostrophic wind along the trajectory over the preceding six hours has been obtained and it is found that there is a correlation coefficient of 0.88 between this quantity and the magnitude of the ageostrophic component.The line of best fit is4=0.78J′–1.0whereA andJ′ are the magnitudes of the ageostrophic vector and the vector change of the geostrophic wind respectively, both being in mi/hr.It is shown that this value ofA/J′ could arise with quasi‐geostrophic motion but the observed direction ofAis inconsistent with such motion.Finally, predictions made from the equations of motion in an exponentially growing pressure gradient without the quasi‐geostrophic assumption are shown to be in close agreement with t
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