Thursday, March 08, 2007
TYPES OF THUNDERSTORMS
First of all three ingredients are needed for thunderstorms; moisture, instability and lift. Moisture is essentially the fuel for a thunderstorm. An unstable atmosphere is one where if a “blob” of air is forced to rise it will continue to rise, being warmer than the surrounding air. Lastly, something invariably will initiate rising motion, this is often referred to as the “kicker” or “trigger”. Sources of lift include; fronts, troughs, localized wind fluctuations interacting with topography and even daytime heating.
A regular thunderstorm, or what we often refer to as “garden variety storms” here in the weather center, go through three distinct stages. First is the towering cumulus stage. The developing storm is comprised mainly of updrafts. Next is the mature stage. The storm now has downdrafts too as well as rain, possibly hail, lightning, thunder and gusty winds. Eventually falling precipitation and evaporatively cooled air shut off in-flow and the storm begins to dissipate. This the last stage of a thunderstorm.
A severe thunderstorm by definition is one that has either wind gusts higher than 58mph and or ¾ inch hail in diameter and or a tornado. Two key additional ingredients that often take a thunderstorm into the realm of having severe criteria are a turning of winds with height which is also known as directional shear as well a lot of upper air support such as a jet streak imbedded in an already swiftly moving jet stream. The latter infers that not only the direction of the wind is changing with height but the speed is too. This is known as speed shear. Strong winds aloft allow the precipitation to fall out ahead of the storm leaving the in-flow of warm humid air intact. This can lead to long-lived rotating thunderstorms, which are often referred to as supercells, or formally as mesocyclones. Not all, but most tornados originate from these types of storms.
Things still look quiet over the next week with a couple of chances for rain and thunderstorms of the garden variety, though we’re just now starting to get into spring so we’ll continue to monitor all developments for the potential of severe weather.
A regular thunderstorm, or what we often refer to as “garden variety storms” here in the weather center, go through three distinct stages. First is the towering cumulus stage. The developing storm is comprised mainly of updrafts. Next is the mature stage. The storm now has downdrafts too as well as rain, possibly hail, lightning, thunder and gusty winds. Eventually falling precipitation and evaporatively cooled air shut off in-flow and the storm begins to dissipate. This the last stage of a thunderstorm.
A severe thunderstorm by definition is one that has either wind gusts higher than 58mph and or ¾ inch hail in diameter and or a tornado. Two key additional ingredients that often take a thunderstorm into the realm of having severe criteria are a turning of winds with height which is also known as directional shear as well a lot of upper air support such as a jet streak imbedded in an already swiftly moving jet stream. The latter infers that not only the direction of the wind is changing with height but the speed is too. This is known as speed shear. Strong winds aloft allow the precipitation to fall out ahead of the storm leaving the in-flow of warm humid air intact. This can lead to long-lived rotating thunderstorms, which are often referred to as supercells, or formally as mesocyclones. Not all, but most tornados originate from these types of storms.
Things still look quiet over the next week with a couple of chances for rain and thunderstorms of the garden variety, though we’re just now starting to get into spring so we’ll continue to monitor all developments for the potential of severe weather.
Posted by Ted Zarras at 5:55 PM
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