Orography is the term used to describe undulations on the surface of the Earth of any size and shape, from small hills to major mountain ranges that span continents.

Orography affects the weather in a variety of ways. These range from local effects, such as enhancement of precipitation and increased wind speeds over mountain summits, to large-scale effects on the global circulation, via a drag force exerted on the flow. This drag is associated with a number of phenomena such as the presence of a turbulent wake behind the hills, the blocking of the flow at low levels upwind of large mountain ranges and the generation of internal gravity-wave motions aloft. The latter can propagate large distances both downwind and above the mountains and if they overturn, or 'break', are one source of clear-air turbulence.

Understanding and representing the effects of orography (such as drag) in numerical weather prediction models is of crucial importance for weather forecasting and climate prediction since the phenomena usually occur on scales too small to be resolved by the models. Such effects need to be parametrized.

The role of the Orography Group is to improve our understanding of flow over hills and mountains and their parametrization in numerical weather prediction models. This is achieved through a combination of observations, both from field and laboratory experiments, and through analytical and numerical modelling studies.