• 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 frequently

Strong preference for midafternoon, when surface temperatures are highest.

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CUMULUS STAGE

• Buoyant thermals produce fair weather cumulus clouds that may exist before evaporating

• Water vapor is moved from the surface to greater heights

• The release of latent heat allows surges of warm air to rise higher than the last, adding to the height of the cloud

• Bergeron process begins producing precipitation once the cloud passing beyond freezing level

• Once the precipitation accumulated in the clouds is too great, a downdraft is initiated

• Entrainment intensifies the downdraft because the air is cool and heavy and importantly, it is dry

MATURE STAGE

• Most active period of a thunderstorm

• The downdraft leaves the base of the cloud, precipitation is released

• Updrafts exist side by side with downdrafts and enlarge the cloud

• Updrafts spread laterally and produce the anvil top made up od ice-laden cirrus clouds

• Gusty winds, lightning, heavy precipitation, and sometimes small hail

DISSIPATING STAGE

• Dominated by downdrafts and entrainment of the cool, dry air

• The cooling effect of falling precipitation and influx of colder air aloft mark the end of the thunderstorm

• 20% moisture leaves the cloud as precipitation, the remaining 80% evaporates back into the atmosphere

OCCURRENCE

• Air near the mountain slope is heated more intensely than air at the same elevation over the adjacent lowlands

• Many thunderstorms are not generated solely by surface heating. For example, many Florida thunderstorms are triggered by the convergence associated with sea to land airflow

• Near the equator, thunderstorms commonly form in the association with the convergence along the equatorial low.

SEVERE THUNDERSTORMS• Capable of producing heavy

downpours and flash flooding, as well as strong, gusty, straightline winds; large hail; frequent lightning; and tornadoes

• To 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 tornado

• Annually about 10,000 storms reach severe status in the United States

KEY FACTORS WHY THUNDERSTORMS PERSIST

• Strong vertical wind shear - Changes in wind direction and/or speed at different heights

• Updrafts that provide the storm with moisture do not remain vertical but become tilted

• Precipitation that forms high in the cloud falls into the downdraft

• Updrafts are sufficiently strong that the cloud top is able to push into the stable lower stratosphere, called overshooting

• Beneath the cumulonimbus tower, denser cool air spreads out along the ground like a wedge , forcing warm, moist surface air in to the thunderstorm

GUST FRONT• Outflowing cool air of the downdraft acts as a

“mini cold front” which advances in to the warmer surrounding air, called a gust front

• Roll clouds form as warm air is lifted along the leading edge of the gust front

• The advance of the gust front can provide the lifting needed for the formation of new thunderstorms many kilometers away

SUPERCELL THUNDERSTORM

• An estimated 2000 – 3000 supercell thunderstorms occur annually

• Consist of a single, very powerful cell that at times can extend to heights of 20 km and persist for many hours

• Diameters range between 20 & 50 km

• If 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 rotate

• This column of cyclonically rotating air is called the mesocyclone

• Tornadoes often from here

• Huge quantities of latent heat are needed to sustain a supercell

• An inversion layer a few kilometers above the surface helps keep the lower troposphere warm and moisture rich

• The presence of an inversion aides the production of a few large thunderstorms by inhibiting the formation of many smaller ones

SQUALL LINES & MESOSCALE CONVECTIVE COMPLEXES (MCC)

Squall Lines• A relatively narrow band of

thunderstorms that develops in the warm sector of a midlatitude cyclone

• Usually 100 – 300 km in advance of the cold front and may stretch for 500 km or more

• Consists of many individual cells in various stages of development

• Forms 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 south

• A narrow zone along which there is an abrupt change in moisture is called a dryline

• A dryline forms when cT air from the southwestern US is pulled into the warm sector of a midlatitude cyclone

• Most frequently develop in the western portions of Texas, Oklahoma, and Kansas

MCC• Consists of many individual

thunderstorms organized into a large oval to circular cluster

• Covers an area of at least 100,000 square km

• The usually slow-moving complex may persist for 12 hrs or more

• Tend to form most frequently in the Great Plains

• The transformation of afternoon air-mass thunderstorms into an MCC requires strong low-level flow of very warm, moist air

• New thunderstorms tend to develop near the side of the complex that faces the incoming low-level flow of warm, moist air

LIGHTNING AND THUNDER

LIGHTNING AND THUNDERWhat 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.

LIGHTNING AND THUNDERLightning 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!

LIGHTNING AND THUNDERThunder:

• 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.