Presentation on theme: "Air Mass Thunderstorms"— Presentation transcript:
1 Air Mass Thunderstorms mT air often becomes unstable in spring and summer, when it is warmed from below by the heated land surface which causes thunderstorms to occur more frequentlyStrong preference for midafternoon, when surface temperatures are highest.
2 Cumulus StageBuoyant thermals produce fair weather cumulus clouds that may exist before evaporatingWater vapor is moved from the surface to greater heightsThe release of latent heat allows surges of warm air to rise higher than the last, adding to the height of the cloudBergeron process begins producing precipitation once the cloud passing beyond freezing levelOnce the precipitation accumulated in the clouds is too great, a downdraft is initiatedEntrainment intensifies the downdraft because the air is cool and heavy and importantly, it is dry
3 Mature Stage Most active period of a thunderstorm The downdraft leaves the base of the cloud, precipitation is releasedUpdrafts exist side by side with downdrafts and enlarge the cloudUpdrafts spread laterally and produce the anvil top made up od ice-laden cirrus cloudsGusty winds, lightning, heavy precipitation, and sometimes small hail
4 Dissipating stageDominated by downdrafts and entrainment of the cool, dry airThe cooling effect of falling precipitation and influx of colder air aloft mark the end of the thunderstorm20% moisture leaves the cloud as precipitation, the remaining 80% evaporates back into the atmosphere
5 OccurrenceAir near the mountain slope is heated more intensely than air at the same elevation over the adjacent lowlandsMany thunderstorms are not generated solely by surface heating. For example, many Florida thunderstorms are triggered by the convergence associated with sea to land airflowNear the equator, thunderstorms commonly form in the association with the convergence along the equatorial low.
6 SEVERE THUNDERSTORMSCapable of producing heavy downpours and flash flooding, as well as strong, gusty, straightline winds; large hail; frequent lightning; and tornadoesTo officially classify as severe, must have winds in excess of 93 km/hr or produce hailstones larger than 1.9 cm in diameter or generate a tornadoAnnually about 10,000 storms reach severe status in the United States
7 Key factors why thunderstorms persist Strong vertical wind shear - Changes in wind direction and/or speed at different heightsUpdrafts that provide the storm with moisture do not remain vertical but become tiltedPrecipitation that forms high in the cloud falls into the downdraftUpdrafts are sufficiently strong that the cloud top is able to push into the stable lower stratosphere, called overshootingBeneath the cumulonimbus tower, denser cool air spreads out along the ground like a wedge , forcing warm, moist surface air in to the thunderstorm
8 Gust frontOutflowing cool air of the downdraft acts as a “mini cold front” which advances in to the warmer surrounding air, called a gust frontRoll clouds form as warm air is lifted along the leading edge of the gust frontThe advance of the gust front can provide the lifting needed for the formation of new thunderstorms many kilometers away
9 Supercell thunderstorm An estimated 2000 – supercell thunderstorms occur annuallyConsist of a single, very powerful cell that at times can extend to heights of 20 km and persist for many hoursDiameters range between 20 & 50 kmIf the surface flow is from the south or southwest and the winds aloft increase in speed and become more westerly with height, the updraft may rotateThis column of cyclonically rotating air is called the mesocycloneTornadoes often from hereHuge quantities of latent heat are needed to sustain a supercellAn inversion layer a few kilometers above the surface helps keep the lower troposphere warm and moisture richThe presence of an inversion aides the production of a few large thunderstorms by inhibiting the formation of many smaller ones
10 Squall lines & Mesoscale Convective complexes (mcc) A relatively narrow band of thunderstorms that develops in the warm sector of a midlatitude cycloneUsually 100 – 300 km in advance of the cold front and may stretch for 500 km or moreConsists of many individual cells in various stages of developmentForms when the divergence and resulting lift created by the jet stream is aligned with a strong, persistent low- level flow of warm, humid air from the southA narrow zone along which there is an abrupt change in moisture is called a drylineA dryline forms when cT air from the southwestern US is pulled into the warm sector of a midlatitude cycloneMost frequently develop in the western portions of Texas, Oklahoma, and KansasConsists of many individual thunderstorms organized into a large oval to circular clusterCovers an area of at least 100,000 square kmThe usually slow-moving complex may persist for 12 hrs or moreTend to form most frequently in the Great PlainsThe transformation of afternoon air-mass thunderstorms into an MCC requires strong low-level flow of very warm, moist airNew thunderstorms tend to develop near the side of the complex that faces the incoming low-level flow of warm, moist air
12 Lightning and Thunder What causes lightning? When liquid and ice particles above the freezing level collide, it builds up large electrical fields in the clouds. Once these electric fields become large enough, a giant "spark" occurs between them (or between them and the ground) like static electricity, reducing the charge separation. The lightning spark can occur between clouds, between the cloud and air, or between the cloud and ground.
13 Lightning and Thunder Lightning strokes: The flash lasts a few tenths of a second.It is what we see and it contains multiple strokes.The leader is the ionized air, which forms a conductive path.A step leader extends earthward in a short, nearly invisible burst.The return stroke extends upward from ground to cloud.How does the lightning "know" where to discharge—or strike?The electric field "looks" for a doorknob. Sort of. It looks for the closest and easiest path to release its charge. Often lightning occurs between clouds or inside a cloud.But the lightning we usually care about most is the lightning that goes from clouds to ground—because that's us!
14 Lightning and Thunder Thunder: The air is heated quickly to as much as 33,000°C.It expands explosively, which produces sound waves that travels at 330 mps.If lightning is more than 20 km away, thunder is not heard.heated air expands explosively, creating a shockwave as the surrounding air is rapidly compressed. The air then contracts rapidly as it cools. This creates an initial CRACK sound, followed by rumbles as the column of air continues to vibrate.If we are watching the sky, we see the lightning before we hear the thunder. That is because light travels much faster than sound waves. We can estimate the distance of the lightning by counting how many seconds it takes until we hear the thunder. It takes approximately 5 seconds for the sound to travel 1 mile.