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Chapter I: Introduction

What exactly do we mean when we speak of cyclogenesis? Is it just the creation of a low-pressure area in the lower or upper troposphere? Or is it the formation of a frontal system until the fronts are completely occluded?

Let's have a look at the definition given on Wikipedia:

Cyclogenesis is the development or strengthening of an area of low pressure in the atmosphere, resulting in the formation of a cyclone.

According to this definition, cyclogenesis consists of the formation of a low-pressure area, around which a cyclonic circulation forms. The definition also requires a transformation process that either creates a cyclonic circulation or strengthens one, until a cyclone forms.

A low-pressure region can have a variety of causes for forming. One of these causes is topography, which will be the main focus of this training module. We often observe the formation of a surface low near changes in orography, mainly at the leeward side (where the air descends) of a mountain range.

The term cyclogenesis, as most commonly used in meteorology, also means the development of fronts and cloud bands visible in satellite imagery and all the processes that are involved during the lifecycle of a cyclone. Meteorologists spend most of their time examining these important atmospheric phenomena.

In this module, we will first focus on the development of vortices in the wake of a mountain range such as the Alps. We will investigate the preconditions for the formation of such a low-pressure area—but most of all we are interested in analysing the conditions that are necessary for the leeward vortex to develop into a mid-latitude cyclone.