9.3. Tornadoes Flashcards
Tornadoes
rapidly rotating vortex of air which extends from a cumulonimbus cloud to the ground
Characteristics of Tornadoes
- the most violent atmospheric storms
- usually more intense than hurricanes but are much smaller in size and short-lived, so although the damage is high, the area affected is much smaller
- 80% of occurences in USA
- Wind speed reach 500km/h (>tropical storm) lasts few minutes
- occur during late spring (May, June, July) as there is still cold air and warm air up North
- occur in the late afternoon (as land is heated up)
Conditions of Tornadoes
- conditions are ideal with winds at different altitudes blowing in different directions and at different speeds - a condition called wind shear
- occur during large thunderstorms
Geographical and temporal distribution
- since tornadoes result from the interaction of contrasting air masses, they are mainly found in mid-latitude regions where tropical and polar air meet as part of the global atmospheric circulation
- they can also develop from the cumulonimbus cloud development around hurricanes, or in other situations where there are strong vertical contrasts of temperatures, winds and moisture
- most tornadoes are caused by supercell storms
- can occur anywhere except antarctica
- Tornadoes are a constant hazard in the USA, with 80% off the world’s reported occurrences and almost all of the violent tornadoes. The hazard is widespread, with the area of the Great Plains (which has 5 or more tornadoes per year) being called ‘Tornado Alley’
Supercells
a severe thunderstorm with updrafts and downdrafts that are in near balance, allowing the storm to maintain itself for several hours supercells often produce large hail, powerful downpours, very strong winds and sometimes tornadoes
- different from normal thunderstorms because they rotate;
Mesocyclone
an area of vertical atmospheric rotation in supercell
thunderstorms, which signals the threat of a possible tornado.
Formation of Tornadoes
- NOT FULLY UNDERSTOOD YET * -_-
- form from supercell thunderstorms
1) Cool, dry air comes from Canada and mountains from west and meets warm, moist air comes from Mexico at the Tornado alley (centre of USA)
2) A wind results (when two winds at different levels and speeds above the ground blow together in a location)
3) The faster moving wind begins to spin and roll over the slower wind. As it rolls on, it speeds up and grows in size
4) At this stage, it is a horizontal wind spinning and rolling like a cylinder
5) With more warm air rising, the spinning air encounters more updraft. The winds spin faster, vertically upwards, and gains more momentum.
6) The rising warm air causes instability and a thunderstorm forms (cumulonimbus cloud)
7) A supercell forms as upper winds tilt the rotating updraft called a mesocyclone. This allows the storm to keep growing as warm air is sucked into the storm away from the cool downdraft.
8) A downdraft brings a funnel-shaped rotating funnel of air (mesocyclone) to the ground. When the pointed part of the tornado touched the ground from the cloud (touchdown).
9) Updraft grows in intensity, creating a low pressure near the surface. This pulls the mesocyclone down as a funnel cloud
Fujita-Pearson Scale
Used to classify tornadoes using 3 independent scales: wind speed, path length (PL) and path width (PW)
- The frequency with which tornado occurs ↓ down the scale
- F-0 to F-5
- Developed into the Enhanced Fujita scale
- EF-0 to EF-5
F-0 on the Fujita-Pearson Scale
Light Damage (<117km/h)
- Some damage to chimneys; branches broken
off trees; shallow-rooted trees pushed over;
sign boards damaged
F-3 on the Fujita-Pearson Scale
Severe damage (254–332km/h) - Roofs and some walls torn off well-constructed houses; trains overturned; most trees in forests uprooted; heavy cars lifted off ground and thrown
F-5 on the Fujita-Pearson Scale
Incredible damage (419–512km/h) - Strong-frame houses lifted off foundations and swept away; automobile-sized missiles fly through the air in excess of 100m; trees debarked
Tornado Hazards
1) High winds –> flying / falling debris
- At least 1000 tornadoes hit the US yearly and on average they killed around 60 people/year from flying or falling debris
- people could be lifted up
2) Damage to infrastructure + nature
- Falling trees/ collapsing parts of building and infrastructure
3) Intense precipitation –> Massive hail and rain storms
- When the funnel cloud has grown to its extent and is vertical, the tornado is in it most damaging +mature stage
4) Burden power lines - secondary hazard
- Power failure/ lack of communication
Preparing for Tornadoes
1) Listen to the radio during the tornado season
- A tornado watch is issued by the National Weather Service when weather conditions make tornadoes likely
2) Tornado warnings
3) Education of what to do during Tornadoes
4) Storm shelters
- tornado resistant buildings - steel doors, secure roofs
- underground storm shelters next to or in basement
Tornado Prediction and Monitoring
1) The US National Oceanic and Atmospheric Administration (NOAA)
2) The US National Weather Service
The US National Oceanic and Atmospheric Administration (NOAA) in Prediction and Monitoring Tornadoes
- satellite imagery
- when the satellite imagery shows the imagery right for Tornadoes to form, NOAA sends out a Tornado Watch Announcement to the media and arranges for a satellite to send back image of the area every 5 minutes, instead of the normal 30 mins
- another satellite at western USA acquires imagery at one minute intervals
