Thunderstorms and Tornadoes Chapter 15 Thunderstorms and Tornadoes EXTREME WEATHER PHOTOS Thunderstorms Violent local atmospheric disturbance that mostly last only 30 minutes but can create several dangerous phenomena Torrential rain usually intense but short in duration flash flooding and slope failure in mountainous terrain is often associated with this type of precipitation High wind Winds at the surface beneath a thunderstorm can reach well over 50 mph 80 kph Hail falling at several meters per second hail stones can result in extensive damage to crops and property in just a few minutes and can injure or kill Lightning every year in the U S lightning is responsible for an average of almost 100 deaths and 300 injuries Lightning frequently starts fires which threaten homes businesses and lives Power and communication outages caused by lightning as well as wind can result in large scale disruption of everyday activities Tornadoes Thunderstorms Ingredients for Cooking up the Storm All thunderstorm s require three ingredients for their formation Sources of Moisture A Lifting Mechanism and Instability Above and Right Life Cycle of a Thunderstorm Thunderstorms Ingredients for Cooking up the Storm Sources of moisture It is estimated that there are as many as 40 000 thunderstorm occurrences each day world wide most occurring in latitudes near the equator Late afternoon hours in Spring and summer bring the greatest risk of thunderstorms to most of North America Right The figure to the right shows the average number of thunderstorm days each year throughout the U S The greatest frequency of occurrence is in the southeastern states with Florida having the highest incidence Left Typical sources of moisture are large bodies of water such as the Atlantic and Pacific oceans as well as the Gulf of Mexico The southeastern U S has access to two moisture sources in the Atlantic ocean and the Gulf of Mexico which helps explain why there are so many thunderstorms in that region Thunderstorms Ingredients for Cooking up the Storm A Lifting Mechanis m a Convergent b Convective c Orographic d Frontal Thunderstorms Ingredients for Cooking up the Storm Cold air pushing under warm air along a cold front is a common lifting mechanism Below A squall line is a line of thunderstorm which typically forms along the leading edge of a cold front Thunderstorms Ingredients for Cooking up the Storm Atmospheric Instability Above Unstable air air parcel forced aloft cools slower than the surrounding environment and continues to rise because it is warmer and more buoyant than its surroundings Right Clouds of vertical development form Life Cycle of A Thunderstorm Cumulus Stage A cumulus cloud begins to grow vertically Air within the cloud is dominated by updraft Below Thunderstorms begin when a parcel of warm moist air rises and cools adiabatically until it reaches saturation and forms a fluffy cumulus cloud As this moisture condenses it releases heat and warms the air parcel more causing more uplifting Life Cycle of A Thunderstorm Mature Stage The storm is at its most intense stage of development and is now a cumulonimbus cloud The top of the cloud reaches the tropopause where upward motion is hindered and the cloud spreads out horizontally Right Mature thunderstorm cloud with typical anvil shaped cloud Life Cycle of A Thunderstorm Mature Stage Strong updrafts and downdrafts coexist The mature air mass thunderstorm contains heavy rain thunder lightning and produces wind gusts at the surface Right When the updrafts reach their maximum altitude precipitation begins to form The falling rain causes a drag on the air and a downdraft is created Life Cycle of A Thunderstorm Dissipating Stage The thunderstorm enters the dissipating stage as the supply of warm moist air is depleted Air currents within the convective storm are now mainly downdrafts Light rain and weak outflow winds may remain for a while during this stage before leaving behind just a remnant anvil top below Lightning Lightning occurs when thunderstorms concentrate positive electrical charges in the upper part of cumulonimbus clouds and negative charges in the lower part When the difference in the charge become great enough to overcome air resistance a sudden and visible violent electrical discharge occurs in the form of a lightning stroke The electrical discharge can be within clouds and or between the cloud and ground Left Cloud to ground lightning is initiated by the downward moving leader stroke Multiple cloud toground and cloud to cloud lightning strokes are observed during a night time thunderstorm at left Right Ground to cloud lightning is a lightning discharge between the ground and a cumulonimbus cloud from an upward moving leader stroke These two brothers didn t recognize the danger they were in Their hair is standing on end from a thunderstorminduced electrical charge The two brothers and their sister were killed moments later when lightning struck Lightning Images Thunder The lightning heats the surrounding air to about 30 000 degrees Celsius which expands at supersonic speeds creating the mighty crashes we recognize as thunder Lightning and thunder happen at exactly the same time but you see the lightning before you hear the thunder because light travels faster than sound Hail Hailstones appear when warm humid air in a thunderstorm rises rapidly into the upper atmosphere and freezes forming tiny ice crystals If the updraft is strong enough the ice crystals can rise and fall many times with the hail getting new coats of water which then freeze making the hail grow When the hail is too heavy to be sustained in the air by the updraft it falls to earth Flash Floods A short lived upstream flood characterized by rapidly rising high velocity water Tornadoes A tornado is a violently rotating funnel of air spawned from supercell thunderstorms 3 main atmospheric conditions must occur simultaneously to produce a tornado 1 a northerly flow of warm moist air from the Gulf of Mexico 2 a cold dry air mass rapidly moving southward from Canada or the Rocky Mountains Tornadoes 3 strong easterly jet stream Left Directional wind shear is the change in wind direction with height Left Speed shear is the change in wind speed with height These three air masses moving in different directions produce shearing conditions that give thunderstorm clouds a spin The rotational cell sags below the cloud base to form a distinctive slowly rotating wall cloud Strong tornadoes form
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