Geology 1350 Lecture 17 Outline of Last Lecture I. Middle lat cyclonesII. Weather forecastingOutline of Current Lecture I. ThunderstormsII. TornadoesCurrent LectureT-storms- warm moist air rises, unstable environment, near weather fronts, advance ofupper level troughs, over mountain slopes, mature> updrafts may stop at the troposphere where the cloud ice crystals are pushed horizontally by winds and form an anvil top, or they may overshoot further into the tropopause. Dissipating>Once downdrafts dominate updrafts, the storm ends as precipitation leaves the cloud faster than it is replenished by rising, condensing air, lower level cloud particles will evaporate leaving an isolate cirrus anvil top section.Basic building block to t-storms- cells: compact region of clouds having strong updrafts; ordinary cell: a few km in diameter exists less than an hour; Super cell: larger than a few kilometer last for several hoursOrdinary t storms(air mass t-storms)- short lived, rarely produce strong winds or large hails,cumulus> cumulonimbus-unstable atmosphere, vertical updrifts keep precipitation suspended,mature>intense phase-dry air comes in causes cold air from evaporation triggeringdowndrafts (side by side with updrafts) and falling rain and small hail, dissipatiion> dominated by downdrafts and evaporation, dissipate themselves because downdrafts cut off the storm’s fuel supply, estimated 100,000 t-storms that occur annually in US, about 10% become severe storm. Ordinary into severe T-storms? In addition to the conditions favor T-storm, Key factor for severe storm is the existence of strong vertical wind shear, changes in wind direction and/or speed between different heightsThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.Severe t-storms- wind gusts is greater than 50 knots or hail diameter greater than ¾ inches or a tornado, In ordinary storms the downdraft and falling precipitation cut off the updraft. In severe storms, winds aloft push the rain ahead and the updraft is not weakened and the storm can continue maturing.Multicell Storms- Cool downdrafts leaving a mature and dissipating storm, may relief summer heat, but they may also force surrounding, low-level moist air upward, therefore, dying storms often trigger new storms, and the successive stages may be viewed in the sky.Mesoscale Convective Complex-An organized mass, or collection, of thunderstorms that extends across a large region is called a mesoscale convective complex (MCC), With weak upper level winds (i.e, slow horizontal movements), such MCC's can regenerate new storms and last for upwards of 12 hours and may bring hail, tornadoes, and flash floods.Thunderstorm Movement- Middle troposphere winds often direct individual cells of a thunderstorm movement, but b/c dying storm downdrafts create new storms, the storm system tends to be right-moving relative to the upper level winds. In a figure upper level winds move storms to the northeast, but downdrafts generate new cells to the south, which eventually cuts off moisture to the old cell.Vertical wind shear- causes the strong updrafts to not remain vertical, but become tilted. The precipitation does not fall into updraft, allowing updrafts to maintain its strengths and storm to continue to manure.Squall Line-A Line of T-Storms,Squall lines identify major storms triggered by a cold front that may contain several severe thunderstorms, some possibly supercells, extending for more than1000 kilometers.Dryline Thunderstorms-Abrupt geographic changes from moist to dry dew-point temperature,called drylines, form in western TX, OK, and KS in the spring and summer. The diagram illustrates how cool cP air pushes hot and dry cT air, at the height of the central plains, over the warm moist mT air. Such mixing causes large scale instabilities and the birth of many supercell storms.Average Thunderstorm & Hail Days -Observed frequency in the pattern and occurrence of thunderstorms does not overlap with hail frequency, possibly because hail falling into the thick layer of warm Gulf air will melt before reaching the ground.Lightning & Thunder- Charge differences between the thunderstorm and ground can cause lightning strokes of 30,000°C, and this rapid heating of air will creates an explosive shock wave called thunder, which requires approximately 3 seconds to travel 1 kilometerLightning Stroke Development- Charge layers in the cloud are formed by the transfer of positive ions from warmer hailstones to colder ice crystals. Negative charges at the bottom ofthe cloud induce an area of positive charges at the surface. Those charges are dense on protruding objects (e.g. trees). – When the negative charge near the bottom of the cloud is large enough to overcome the air's resistance, a stepped leader forms.Types of Lightning– Nearly 90% of lightning is the negative cloud-to-ground type described earlier, but positive cloud-to-ground lightning can generate more current and more damage. Distant, unseen lightning is often called heat lightning. A tornado strikes IN & KY; as family sleeps; 20 dead A category 3 tornado, with winds ranging from 158 mph to 206 mph stroke Evansville IN on Sunday (Nov. 5) night, killing at least 20 people.Tornado-A rapidly rotating column of air often evolve through a series of stages, from dust-whirl, to organizing and mature stages, and ending with the shrinking and decay stages. Average tornado characteristics: Rotation: counterclockwise Diameter: 300-600 m (~0.5 – 1 mile) Speed of movement: 20-70 knots Lifetime: a few minutes Average path length: 7 km (~ 4miles) -Tornado Occurrence-Tornadoes from all 50 states of the U.S. add up to more than 1000tornadoes annually, but the highest frequency is observed in tornado alley of the Central Plains. Nearly 75% of tornadoes form from March to July, and are more likely when warm humid air is overlain by cooler dryer air to cause strong vertical lift.Tornado Wind Speed – Suction Vortices & Damage -As the tornado moves along a path, the circular tornado winds blowing opposite the path of movement will have less speed. For example, if the storm rotational speed is 100 knots, and its path is 50 knots, it will have a maximum wind of 150 knots on its forward rotation side. A system of tornadoes with smaller whirls, or suction vortices, contained within the tornado is called a multi-vortex tornado. Damage from