Chapter11

Thunderstorm: 

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

Ingredients for a Thunderstorm: 

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)

Factors Affecting Thunderstorm Growth and Development: 

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

Lifted Index: 

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

Other Severe Factors: 

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

Types of Thunderstorms: 

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

Ordinary Single-Cell Thunderstorm: 

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

Multicell Thunderstorm: 

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

Squall Line: 

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

MCC: 

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 (< -40˚ F) 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

Supercell Thunderstorm: 

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

Tornadoes: 

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

Radar Observations: 

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

Fujita Scale: 

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

Tornado Distribution: 

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.

Tornado Safety: 

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 tornado’s path (towards the south, if possible) If no time to maneuver, get out and lie flat in a ditch DON’T GET UNDER AN OVERPASS!!

Waterspout: 

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!

Lightning: 

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

Steps in a Lightning Strike: 

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)

Lightning Safety: 

Lightning Safety Stay indoors during lightning When indoors, stay off corded telephones and away from electrical devices When indoors, don’t 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)

Flash Floods and Flooding: 

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

Hail: 

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 don’t 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