The largest fraction of what is called “weather” in the extratropics is delivered by the development and propagation of mid-latitude weather systems. These disturbances, commonly referred to as “lows” and “highs,” develop in response to a fundamental instability of the westerlies, known as baroclinic instability, that arises from the permanent but seasonally modulated pole-to-equator temperature contrast. The lows in this endless sequence of disturbances are known as mid-latitude cyclones.