Thunderstorm

1
Thunderstorm

• Any cloud or cluster of clouds that produces
  thunder
• Often produce heavy rain, hail and tornadoes
• Associated with tall cumulonimbus clouds
• Great amounts of energy are released when
  saturated air rises rapidly
• Anvil flat top of the thunderstorm cloud
• Overshooting Top An area where cloud rises
  above the anvil appearing like a fist above the
  anvil

2
Ingredients for a Thunderstorm

• Moisture we need a cloud! (mT air mass)
• Instability a rising parcel needs to be buoyant
• Lift some means to get the parcels rising
  (orographic lift, frontal lift, convergence,
  etc.)
• Florida has more thunderstorms than any other
  state (warm, moist air is plentiful)
• Colorado New Mexico are second because of the
  sloping terrain (lift)

3
Factors Affecting Thunderstorm Growth and
Development

• To get more than a garden-variety thunderstorm
  (one that produces tornadoes)
• LOTS of instability
• Vertical wind shear (change in direction or speed
  of the wind with height
• Stability Indicies a single number that is used
  to determine how unstable the atmosphere is

4
Lifted Index

• LI T500 TP500
• The warmer the parcel is compared to the
  environment, the more negative the LI
• The warmer the parcel, the greater the buoyancy
  and the stronger the updraft.
• LI of 0 to -3 Air is marginally unstable
• LI -9 or less Extreme instability

5
Other Severe Factors

• Nocturnal Low-Level Jet low-level jet of air
  (850 mb) that brings moisture and warm air
  northward at night to feed nocturnal storms
• Capping Inversion Hot, dry air at 700 mb that
  prevents convection until the air beneath is the
  warmest possible

6
Types of Thunderstorms

• Severe Thunderstorms
• Produces wind gusts 50 knots (58 mph)
• Produces hail . ¾ inches in diameter (dime)
• Tornado
• Cell a compact region of a cloud that has a
  strong vertical updraft
• Two basic categories of thunderstorm cells
• Ordinary Cells
• Supercells

7
Ordinary Single-Cell Thunderstorm

• Few miles in diameter
• Lasts for less than an hour
• Air Mass Thunderstorm forms in mT air mass
• Three stages of its life cycle
• Cumulus Stage all updraft, cloud building
• Mature Stage precipitation falls at the
  surface, most lightning, rain and hail occurs,
  cloud has an updraft and a downdraft
• Dissipating Stage dominated by downdraft

8
Multicell Thunderstorm

• Composed of several individual single-cell
  storms, each at a different stage of development
• Two basic types
• Squall Line
• MCC

9
Squall Line

• Storms arranged in a line or band
• Lasts for 6-12 hours
• Can expand across several states
• Wind shear tilts the updraft and separates
  updraft and downdraft
• Gust front cold outflow forces warm air to rise
  at leading edge of the line
• Shelf cloud Sloping, low-level cloud formed on
  gust front
• Often form along cold fronts

10
MCC

• Mesoscale Convective Complex
• Composed of multiple single-cell storms
• Covers a large area 40,000 square miles
• Lasts for more than 6 hours
• Cloud tops are cold (
• Tend to form under ridges if divergence aloft
• New cell development on a preferred flank
• Supplies the bulk of growing season rains to the
  farm belts of the US and Canada

11
Supercell Thunderstorm

• Dominant severe weather producer
• Requires very unstable environment and lots of
  wind shear
• Wind shear creates vorticity in the horizontal
  direction
• Updrafts tilt this vorticity into the vertical
• Mesocyclone Rotating updraft of a supercell

12
Tornadoes

• Definition of a Tornado
• Violently-rotating column of air
• In contact with the ground
• Attached to a cumulonimbus cloud
• Usually less than 1 mile wide and live shorter
  than a half hour
• Most damaging tornadoes form out of supercell
  thunderstorms
• Appear many times from a wall cloud an
  isolated lowering beneath cloud base
• Multiple Vortex Tornado tornado composed of
  several smaller suction vortices rotating about
  a common center
• Funnel cloud a circulation that does not extend
  all the way to the ground

13
Radar Observations

• Certain features on radar can warn of a tornadic
  thunderstorm
• Hook Echo an appendage on the south or
  southwest part of the thunderstorm caused by
  rain, and perhaps debris, wrapping around a
  low-level mesocyclone
• TVS tornado vortex signature intense wind
  shear detected on Doppler radar

14
Fujita Scale

• Tornado damage rated from F0 to F5
• F0 very little damage
• F5 houses totally demolished
• Only 1 of tornadoes are rated F4 or F5, but they
  account for 70 of the deaths
• The Fujita scale is a damage scale, so the actual
  F-rating is determined after the event by a
  damage survey team
• The wind speeds associated with the F ratings are
  only approximations

15
Tornado Distribution

• Majority of tornadoes are observed in the US
• Tornado Alley Texas, Oklahoma, Kansas and
  several other states in the Plains
• Majority happen in late-spring (May/June)
• Secondary maximum occurs in November along the
  Gulf coast
• Most occur between 3 p.m. and 7 p.m.

16
Tornado Safety

• Get in a well-built structure
• Get to the center of the lowest floor
• Keep away from windows
• Get under a sturdy piece of furniture
• If no basement, pick a closet or bathroom
• Cover up with a blanket
• If outside, get inside!
• If driving, drive at right angles to the
  tornados path (towards the south, if possible)
• If no time to maneuver, get out and lie flat in a
  ditch
• DONT GET UNDER AN OVERPASS!!

17
Waterspout

• Narrow spinning funnels of rising air that form
  underneath clouds
• Not a legitimate tornado form under modest
  cumulus clouds and not a cumulonimbus (remember
  definition of tornado)
• Weaker than tornadoes
• Many people define a water spout as a tornado
  over water. NO! A tornado over water is called
  a tornado!

18
Lightning

• Huge electrical discharge
• Can travel from cloud to cloud, from cloud to
  ground, or remain in the same cloud
• A lightning bolt is typically composed of a
  series of flashes
• Kills 85 people on average every year in the US
• Air is a great insulator (prevents electricity
  from passing within it)
• Because of it, electrical discharges must be BIG!
• Majority of lightning occurs in Florida Gulf
  coast

19
Steps in a Lightning Strike

• Charge Separation
• Opposite charge collects at the surface
  underneath the cloud
• Pilot leader initial path of negative charge
  towards ground
• Stepped leaders sequence of downward movement
  of electrons
• Return Stroke surge upward of protons that
  meets the stepped leader aloft (BRIGHT FLASH)
• Dart Leader subsequent movement of electrons
  from other parts of the cloud downward (other
  flashes)

20
Lightning Safety

• Stay indoors during lightning
• When indoors, stay off corded telephones and away
  from electrical devices
• When indoors, dont take a bath!
• If outside, get in your car (metal roof)
• If outside and hair stands on end, get down like
  a baseball catcher with feet TOGETHER
• Give CPR immediately to victims (call 911)

21
Flash Floods and Flooding

• Flood a substantial rise in water that covers
  areas not usually submerged
• Occur when water flows into a region faster than
  it can be absorbed, stored, or removed
• Caused by rain over a long duration or high
  intensity (or lots of snow melting)
• Greatest weather-related threat to life in US
• Flash flood a sudden local flood in a short
  duration, can be caused by thunderstorms

22
Hail

• Large balls or lumps of ice
• Begin as small hail particles
• Grow by accretion of supercooled droplets
• Rings are observed in the ice
• Contrary to the text, hail stones dont loop
  repeatedly in the cloud rings are due to
  different types of freezing
• Requires a strong updraft
• Hailshaft curtain of hailstones falling from a
  cloud
• Hail swath a path of ground covered with hail
• Damages crops and property
• Mostly occurs in the high plains of the US