Chapter 11

Chapter 11 - Chapter 11 Thunderstorm- cloud, or cluster of...

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Chapter 11 Thunderstorm- cloud, or cluster of clouds, that produces thunder, lightning, heavy rain, and sometimes hail and tornadoes. Associated with cumulonimbus clouds and unstable air. Overshooting Tops- can identify a thunderstorm where the top of a cloud may overshoot the tropopause and penetrate the stratosphere. Thunderstorms require warm, moist air, so they are most prevalent in regions with maritime Tropical (mT) air masses and in regions where mountains and frontal cyclones help lift air vertically. Areas with a position around the ITCZ and Africa have the most thunderstorm days. Within the US, Florida has the most thunderstorm days. Thunderstorms are triggered by various lifting mechanisms: Surface heating and along boundaries such as fronts. The Lifted Index is a stability index that is used to determine a numerical estimate of the severity of thunderstorm activity based on observations near and above the ground. It is computed by comparing the parcel and environmental temperatures at 500 mb. The more negative the index, the more likely thunderstorms will develop, with a –6 or less being when the chance of a severe thunderstorm is highest. Vertical Wind Shear, or the change of environmental wind speed and direction from the ground up, is another factor affecting thunderstorm type. Large amounts of shear cause thunderstorm clouds to tilt and become more severe storms. Nocturnal Low Level Jet and Capping Inversion are two other factors affecting thunderstorm growth and development. Thunderstorms can be classified by either their severity or their structure. The NWS (National Weather Service) defined a Severe Thunderstorm as a thunderstorm that produces one or more of the following: wind gusts of 50 knots or greater, hail 0.75 inches in diameter or larger, or a tornado. Thunderstorm Structure Thunderstorms are composed of basic building block referred to as Cells . A cell is a compact region of a cloud that has a strong vertical updraft. There are two categories of cells: Ordinary Cells (few kilometers or miles in diameter and exist for an hour or less) and Supercells (larger and can last for several hours). Multicell storms are composed of lines or clusters of thunderstorm cells, either ordinary or supercells or both. Ordinary Single Cell Thunderstorms are the most common, but the
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Chapter 11 - Chapter 11 Thunderstorm- cloud, or cluster of...

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