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Cap — A layer of relatively warm, stable air aloft that suppresses or delays the development of thunderstorms. If the cap is broken by updrafts, thunderstorms are likely to develop. If the cap holds, thunderstorms fail to form. CAPE — Acronym for Convective Available Potential Energy. It is a measure, in joules per kilogram, of the amount of energy available for convection, or vertical movement of air. A CAPE of 1,000 or more is considered sufficient for severe thunderstorms, and may range up to 5,000 in extreme cases. Still, if the cap is strong enough, all the CAPE in the world won't break it. Core punching — The act of penetrating the middle of a thunderstorm. This is extremely dangerous and avoided by most chasers, who prefer to move around a thunderstorm rather than through it. Core-punching can bring chasers into torrential rain, high winds and possibly the path of a tornado before they see it. Dry line — A sharp cutoff between dry air and moist air. The dry line often develops over western Texas, Oklahoma or Kansas and is frequently a focal point for thunderstorm development. Mesocyclone — A large area of rotation within a thunderstorm. A mesocyclone often, but not always, signals the development of a tornado. MCS — Abbreviation for "Mesoscale Convective System." A mesoscale convective system, a cluster or line of thunderstorms that have linked to form a single weather system. Chasers prefer distinct, individual thunderstorms, so the formation of an MCS can signal the end of a chase. Supercell — A powerful thunderstorm, usually separated from other storms, that exhibits persistent rotation. Supercells commonly produce large hail, high winds and, sometimes, tornadoes. These are the storms most sought by chasers. Wall cloud — A rain-free cloud mass that lowers below a thunderstorm cloud. Wall clouds sometimes rotate within or beneath a mesocyclone. This can signal the imminent development of a tornado. |
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