AbstractUsing the observational material presented in an earlier paper (Miles 1949) it is found that the following relation fits the observations ahead of most of the warm fronts : [A] = 1.45IM– 1, where [A] is the magnitude of the ageostrophic wind at 900 mb andIMthe measured isallobaric wind as defined by Brunt and Douglas (1928), both in mi/hr.It is shown that when allowance is made for the fact thatIMis obtained from isopleths drawn from three‐hour tendencies the result compares closely with other observational relations which have been shown (Miles 1949) to be in agreement with theory.This and some further measurements from synoptic charts lead to the conclusion that in the majority of cases the measured isallobaric wind can give a useful first approximation to the ageostrophic wind at 900