1.2 Instability
1.2.1 CAPE

Convective storms form when rising bubbles or plumes of air move upward in the troposphere through an environment of denser, cooler(1) air. In this process, potential energy is converted into movement or kinetic energy. This potential energy is known as convective available potential energy or CAPE. CAPE tells us something about the strength of the convective updrafts that may form. However, updraft strength is not exclusively determined by CAPE but also by other factors (see below). The CAPE of a parcel of air that may flow into a storm's updraft can be calculated using data from a radiosonde or a numerical atmospheric model. These parcels of air can originate from different altitudes below the cloud base of the storm. They generally have different amounts of CAPE. A parcel has CAPE whenever it -at some point- becomes warmer than its environment when it is lifted. This occurs when heat is released in the parcel when water vapour in it condenses to liquid water or ice. As a result it may then cool less rapidly with height than its environment.

(1) To be precise, the density of a parcel (that has the same pressure as it's environment) is not only determined by its temperature but also by the amount of water vapor that it contains. For simplicity we will neglect this effect and assume that cooler parcels are always denser than warmer parcels. The effect can be accounted for by reading 'virtual temperature' wherever 'temperature' has been written.