Tornadoes

What is a tornado?

A violently rotating column of air pendant from or underneath a cumuliform cloud, often visible as a funnel cloud. Must be in contact with the ground. A funnel over water is called a waterspout.

Why might a tornado funnel not be visible?

Precipitation blocking the view, or low relative humidity. Funnels may also not appear to be in contact with the ground even when they are.

3 stages of a single-cell convection thunderstorm

1. Early: rising cumulus clouds (~6 km), warm air updraft. 2. Mature: cumulonimbus builds (~12 km, −40°C), strong downdrafts produce rain. 3. Dissipating: downdrafts dominate, cloud mass shrinks.

What causes the tilt and anvil shape of a thunderstorm?

Vertical wind shear in the updraft causes the storm to tilt, producing the elongated anvil shape at the top.

What is a supercell thunderstorm?

A special thunderstorm that persists for hours due to its organized internal structure. Several km in diameter with strong updrafts; track is forced right of average tropospheric winds. Also produces tornadoes, hail, rain, and a wall cloud.

Conditions needed for a supercell to form

Vertical wind shear ≥ 20 m/s in the lowest 6 km, strong mid-level winds (jet stream). Requires: (1) low-altitude warm humid Gulf of Mexico air (~22°C) moving north; (2) mid-altitude cold dry air aloft (~8 km, >80 km/h) from Canada or Rockies. These air masses set up shearing and spin.

What is a mesocyclone and how does it form?

The rotating updraft of a supercell. Vertical shear creates horizontal vorticity; the supercell tilts this into vertical (cyclonic/counterclockwise) rotation. A streamwise vorticity current is eventually lifted into the updraft, triggering tornadogenesis.

Mesocyclonic vs. non-mesocyclonic tornadoes

Mesocyclonic: most common, occur in supercells, associated with a mesocyclone. Non-mesocyclonic (landspouts): ~20% of all tornadoes, form in any severe weather, weak vertical shear, weak funnels, vorticity from storm winds not Coriolis, horizontal shear starts near ground.

EF0 and EF1 damage

EF0 (105–137 km/h): minor — peels roof surfaces, breaks branches, pushes over shallow trees. EF1 (138–177 km/h): moderate — roofs severely stripped, mobile homes overturned, windows broken.

EF2 and EF3 damage

EF2 (178–217 km/h): considerable — roofs torn off, foundations shifted, mobile homes destroyed, cars lifted. EF3 (218–266 km/h): severe — entire stories destroyed, malls damaged, trains overturned, trees debarked, heavy cars thrown.

EF4 and EF5 damage

EF4 (257–322 km/h): devastating — well-built homes levelled/swept away, large objects thrown. EF5 (>322 km/h): incredible — strong homes swept off foundations, steel-reinforced concrete critically damaged, vehicles thrown ~1 mile.

Where are tornadoes concentrated globally / what is Tornado Alley?

Concentrated in North America. Tornado Alley: Texas, Oklahoma, Kansas, Colorado, S. Dakota, Minnesota, Iowa. Oklahoma gets ~7 km of tornado track/year per 10,000 mi².

Tornado statistics in Canada

60–80 reported tornadoes/year; ~2 deaths/year; $10s of millions damage/year; total insured and uninsured damage >$300M.

Who is most at risk of dying in a tornado?

Elderly people, mobile home residents, people in rooms with windows, and people without any warning.

Edmonton 1987 tornado

EF4; winds up to 460 km/h; track 40 km long, up to 1 km wide, ~1 hr duration; 27 killed, 300+ injured; only the second worst tornado in Canadian history; triggered development of a provincial emergency public warning system.

What is a waterspout?

A tornado funnel that forms over water rather than land — still a rotating column of air pendant from a cumuliform cloud.