Thunderstorms and Tornadoes

what causes thunderstorms to develop

dynamic thermal instability of an air mass causes thunderstorms to develop. more specifically from when warm, humid air that is lifted in a conditionally unstable environment. Air must be lifted past LFC to trigger thunderstorms. Rising air is buoyant from latent heat release when water vapor condenses

thunderstorms generate

lightning and thunder

-gusty winds

-heavy rain

-hail

-possibly tornadoes

Conditional instability prevails when

moist air has an ELR between the dry and wet adiabatic rates

localized convective lifting

Unequal surface heating causes localized pockets of air to

rise. Cause single-celled or ordinary celled thunderstorms

comvergence

Converging horizontal air flow results in upward movement. Often associated with low pressure. Lifting mechanism to generate thunderstorm.

Orographic Lifting occurs when

elevated terrains, such as mountains, act as barriers to the flow of air.

Adiabatic cooling can generate clouds and copious precipitation. Many of the wettest places in the world are located on windward mountain slopes.

Lifting mechanism to generate thunderstorms.

Frontal Lifting

Masses of warm and cold air collide, producing fronts. Cooler, denser air acts as a barrier over which the warmer, less dense air rises. This process is called frontal lifting.

Lifting mechanism to generate severe thunderstorms.

Wind shear is the change in wind speed and direction with increasing height above the surface.

Greatest proportion of storms and lightning strikes occur where

Warm air

– Abundant moisture

– Instability

• Found in tropics year round

• Largely warm season in mid- latitudes

Equator= year round thunderstorms

closer to equator= more often

thunderstorms in the US are most frequent in

mT air of east Gulf coast n Florida region(warm air coming off Atlantic + Gulf. Convergence). East of Rockies in Colorado and New Mexico is next.(orographic Lifting)

Two types of thunderstorms

Air-mass thunderstorms and severe thunderstorms

air-mass thunderstorms

associated with warm, humid air that rises in unstable environments, cumulonimbus clouds, and mT air masses. Also called ordinary cell or ordinary thunder storm. pop up showers with a life cycle of an hour.

severe thunderstorms

may produce high winds, hail, flash floods, and tornadoes.

-They are the result of uneven heating(of land n ocean), frontal lifting, and diverging winds.

severe, possibility of severe loss of life.

Air mass thunderstorms- mT air mass

stages of development

-cumulus

-mature

-dissipating

single cell events, short lived (less than an hour). predictable life cycle.

Cummulus stage of air mass thunderstorms

Largely a product of uneven surface heating and convergence. A cumulonimbus tower develops as water vapor moves from the surface to greater heights. This phase is dominated by updrafts. where rain is suspended until they become too ‘heavy’ to support. (look for a pic w/ Hella red arrows-updrafts)

Updrafts

suspend raindrops until they become too ‘heavy’ to support [red lines]

downdrafts

are further aided by entrainment (influx of cool, dry air) [blue lines]

Mature stage of an air mass thunderstorm

As downdraft leaves the base of the cloud it caries precipitation w/ it n triggers the mature stage. cool downdrafts exist as well as updrafts. Formation of anvil top.

-This is the most intense phase(happens in span of an hour). This results in gusty winds, lightning, heavy rain and possibly small hail. Occasional micro-bursts which are sudden, powerful, downward busts of air 

microbursts

sudden powerful downward bust of air

Dissipation stage of air mass thunderstorms

rain turns updraft into a downdraft which causes storm to be cut off from rising, moist air. This stage is dominated by downdrafts n entertainment causing evaporation.

Occurrence

Mountainous regions, such as the Rockies and the Appalachians, experience a greater number of air-mass thunderstorms. lifting associated w/ mid-latitude cyclones. convergence of air-masses.

Severe thunderstorms

Heavy downpours

– Flash flooding

– Straight line wind gusts

– Hail, lightning

– Tornadoes (have tornadoes, have severe thunderstorm)

– Wind shear

– Can overshoot (enter stratosphere)

– Downdraft preceding (gust front)

For National Weather Service to classified as severe: 1 of these must be satisfied

winds access of 58 miles per hour, produce hail larger than 1 inch, generate a tornado. severe thunderstorms can last for hours to days. Wind shear allows for updrafts to continue to feed moisture to the cloud because it is tilted.

wind sheer

a relatively abrupt change in wind direction and/or wind speed with increasing height above Earth’s surface. Wind shear allows for updrafts to continue to feed moisture to the cloud because it is tilted.

Over Shooting Top

where updrafts are strong enough so the top pushes into the stable lower stratosphere.

Gust fronts

leading edges of down draft or outflow boundary.

straight line winds

travel behind the gust front. Can cause damage identical to a tornado.

roll clouds or shelf clouds are called

arcus clouds

roll clouds/shelf clouds from

at leading edge of storm.

roll cloud is

a horizontal tube-shaped cloud. often form between the cold downdrafts n warm inflow of air.

shelf cloud

from in similar environments to roll clouds but are low, wedge-shaped clouds.

squall lines

narrow bands of thunderstorms. Looks like a line with a bulge on radar.

Mammatus skies sometimes precede squall lines. cT air is pulled into

the warm sector of a midlatitude cyclone. These can also form along a dryline,

where there is an abrupt change in moisture. Commonly create straight-line winds,

heavy rain, and lightning. Small hail or tornadoes can form.

Mesoscale convective complexes (MCCs)

An MCC consists of many individual thunderstorms. It is organized into a large oval to circular cluster. They cover an area of at least 100,000 km2. It is a slow-moving complex that may last for 12 hours or more. MCCs tend to form mainly in the Great Plains. warm air moved in, forced where cold air wants to sit. collide form small bunches of thunderstorms. Often happens at nt.

Supercells

DONT HAVE ALL THINGS FOR THIS< ASK GIRL They consist of a single, powerful cell that can extend to heights of 20 km (65,000 ft) or more. Massive cumulonimbus clouds can measure 20–50 km (12 to 30 miles) in diameter. Single-celled structure that is complex and varies between one structure to the next. Upper portion has often forms large anvil that is blown downstream by upper-level winds.

• Overshooting tops

• Mammatus clouds may form

Vertical speed shear is the difference in

air velocity between the surface and midlevel

Intense rotating updraft causes

vertical directional shear which is the change in wind direction with height from the ground.

Mesocyclone

Vertical winds may cause the updraft to rotate, which forms a column of cyclonically

rotating air. Tornadoes often form. (not every supercell spawns a tornado, not every tornado comes from a supercell.

Thunderstorms are only classified after

thunder is heard. Because lightning produces thunder, lightning must also be present. Lightning is a dis-charge of electrical energy.

sheet lightning may occur

because it produces bright but diffused illumination in parts of the cloud

-Between cells in the same cloud

-Between two clouds

-Cloud-to-ground lightning

what causes lightning

Separation of electric charge in the formation of large clouds. Lightning equalizes

electrical differences. Produces a negative flow of current from region of negative to positive charge or vice versa. not exactly sure what causes charge seperation.

theory for causes of charge seperation

Hail and rain tend to have a warmer surface than ice crystals. When warm rain or hail collides with colder ice, electrons transfer to hail. Rain drops or hail is bigger and heavier and settles toward the bottom of the cloud. Smaller ice crystals are lofted to the top.

lightning strokes parts

the flash, leader, step leader, and return stroke

the flash

(total discharge) lasts a few tenths of a second. It is what we see and it contains multiple strokes.

The Leader

the ionized air, which forms a conductive path

Step Leader

extends earthward in a short, nearly invisible burst

Return Stroke

extends upward from ground to cloud.

Thunder

The air is heated quickly to as much as 33,000°C. It expands explosively, which produces sound waves that travel at 330 mps (1000 fps). If lightning is more than 20 km away, thunder is not heard. the more time between the lightning n the thunder, the further away it is.

Tornadoes (twisters, cyclones)

these are violent windstorms w/ a rapidly rotating column of air, or vortex that extends from a cumulonimbus cloud. Pressures within tornadoes can be as much as 10% lower than immediately outside the storm. It may consist of single or multiple vortices.

Multiple-vortex tornadoes

Many stronger tornadoes have smaller intense whirls inside called suction vortices that orbit the center of the larger tornado circulation. Diameter of 10 m. Form and die <1 min. Responsible for most of the narrow, short swaths of extreme damage that occur along tornado track.

development of a mesocyclone is important precondition to

tornado development

Mesocyclones need

vertical wind sheer

for tornadoes, wind sheer produces

a rolling motion about a horizontal axis. Strong updrafts tilt the axis nearing vertical. Mesocyclone is wide, short, and rotates slowly at first. It gets stretched and speed up. Slender and rapidly spinning vortex emerges from the bottom of the cloud – funnel cloud.

Tornado climatology

-Squall lines

-Cold fronts

-Where cP and mT meet

-Midwest United States

profile of a tornado

Average diameter 150–600m. Travels ~45 kph. Path about 26 km long. Most travel to the NE. Exist between <3 min to >3 hours. Wind speeds between <150 kph to >500 kph.

tornados are assessed/classified based on

damage down, not wind speed.

EF-0

Light. Some damage to siding and shingles

EF-1

Moderate. Considerable roof damage. Winds can uproot trees and overturn single-wide mobile homes. Flagpoles bend.

EF-2

Considerable. Most single-wide mobile homes destroyed. Permanent homes can shift off foundations. Flagpoles collapse. Softwood trees debarked.

EF-3

Severe. Hardwood trees debarked. All but small portions of houses destroyed

EF-4

Devastating. Complete destruction of well-built residences and large sections of school buildings.

EF-5

incredible. Significant structural deformation of mid- and high-rise buildings

tornado watches 

alert the public.  Tornadoes are possible and conditions are favorable. They usually cover an area of about 26,000 km2. Watches can last 3 hours or longer. watches for large areas. lie entire states ect. look out, tornado could happen, just stay alert. taco watch/taco warning

tornado warning

issued when a tornado is actually sighted or conditions are just rt. There is a high probability of imminent danger. They are usually for a much smaller area. Warnings are in effect for a much shorter period, usually 30–60 minutes. for hyper specific places. like a road, or town. taco watch/taco warning

doppler radar

This radar measures the motion and speed of the wind. Two or more units are optimal for more accurate forecasting. Tornadoes have hook-shaped echoes.