Theory of airflow over mountains: II ‐ The flow over a ridge

Abstract

Abstract Three kinds of flow over unlevel ground are defined according to their scale ‐ aerodynamic, barostromatic, and geostrophic flow ‐ and it is seen that the practical difficulties in the way of obtaining the correct value of the relevant non‐dimensional numbers in model experiments are almost prohibitive. The flow over a ridge with varying degrees of static stability is calculated, and the vertical displacement is seen to depend very much on the details of the airstream. When the flow is oblique to the ridge the air is permanently displaced sideways from its original path; and if the original airstream contains shear, shear in almost any direction may appear over a ridge. This may be displayed in billow‐cloud formations. Diurnal variations in the length of lee waves are due to the development and disappearance of an adiabatic layer at the ground. The flow is calculated in a particular case and the temperatures recorded by radiosonde balloons are shown to contain spurious inversions as the balloon passes through the waves. The disappearance of the adiabatic layer at the ground makes orographic effects a maximum around sunset on sunny days; and the appearance of a statically neutral layer in middle levels, for instance by condensation, renders steady laminar flow impossible.