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Tornadoes: The Tempestuous Atmosphere (Tornado Hazards (Tornado Warnings…
Tornadoes: The Tempestuous Atmosphere
Overall Characteristics
Violently rotating column of air extending between a cloud and the surface of the earth
Highest wind speeds of any weather phenomenon
70% of Earths tornadoes occur in the Great Plains of the Central U.S.
Travel up to 100 km/hr, wind speeds up to 500 km/hr, core vortex less than 1km wide
Can form hundreds of metres high in the atmosphere and may never touch the ground
General circulation of Atmosphere
Fronts
: sloping surfaces or boundaries separating air masses with different temperature and moisture content. They can trigger severe weather, violent storms
With a
cold front
, a cold air mass moves in and under a warm air mass, lifting the warm air mass up. Vertical storm clouds, heavy rain or snow, and thunderstorms can result
With a
warm front
, warm air flows up and over cold air mass which can result in widespread clouds and light to moderate precipitation over a broad area
Jet stream:
narrow bands of high-velocity (200km/hr) wind flowing from west to east at high-altitudes
Subtropical Jet:
occurs at 30 degrees latitude
Polar Jet :
more powerful and occurs at about 60 degrees north latitude while changing paths
A
trough
of low pressure (concave northward bend) forms the core of a
cyclone
A
ridge
of higher pressure (convex northward bend) forms the core of an
anticyclone
Mid-latitude cyclones
A tornado is a rapidly rotating column of air from a large thunderstorm
Thunderstorms: a continuous supply of rising, warm, and moist air is required to make a cloud mass grow. At mature stage:
upper-level precipitation begins when ice crystals and water drops become too heavy for updrafts to support
The falling rain pulls in cooler and dryer air causing downdrafts (sinking air)
Updrafts and downdrafts blow side by side, which creates gusty winds, heavy rain, thunder, lightning, and hail
Dissipating stage: downdrafts drag in so much cold and dry air that the updrafts fuelling the thunderstorm are cut off
Downburst:
violent downdrafts of cold air with rain and during during mature stage. Dangerous to airplanes, associated with horizontal wind shear(wind shift from head to tail winds)
Air mass thunderstorms
: most common and result from air convection. They can form at low altitudes all year long and tend to form late in afternoon during summer at mid-latitiudes
Supercell Thunderstorms
: result from frontal collisions of air masses at mid-latitudes
Necessary Ingredients
1) A low-altitude flow of warm,moist air
2) A middle-altitude, cold and dry air mass moving at speeds higher than 80km/hr
3) High-altitude, jet stream winds moving at speeds above 250km/hr
These three air masses moving in different directions set up shearing conditions and cause a thunderstorm cloud to spin
How tornadoes form in North America
1) The low-altitude, northernly flow of warm, moist air comes from the Gulf Mexico
2) The middle-altitude, cold and dry air mass moving at speeds higher than 80km/hr comes from the Rocky Mountains
3) High-altiitude, jet stream winds moving at speeds above 250km/hr are present
The rising Gulf air is spun one way by the mid-altitude cold air flow then spun another way by the jet stream, creating a
corkscrew
effect
The warm air is rising on the leading side of the system (middle updraft)
The cold air is descending on the trialing side of the system (rear downdraft)
Tornadoes form between middle updraft and rear downdraft
Assessing the wind speeds
Fujita scale: six-point intensity ranking (F0 to F5) based on structural damage and increasing wind speeds
Enhanced Fujita scale: Six-point intensity ranking (EF0 to EF5) but differentiates structural damage depending on the building materials used or types of trees present at site
Tornado Hazards
High speed winds
Winds throwing debris like bullets/shrapnel
Increased air pressure inside of buildings that causes roofs to blow up and walls to fall out
Deaths declining in recent decades: greater risk to old people, mobile home residents
Safe rooms can be used to minimize human casualties, or interior closets and bathrooms
Tornado Warnings
a tornado is believed to be present in the warning area
can be based on Doppler radar signature, or storm spotters
only a few hours in advance
Tornado Watch
States that conditions are favourable for a tornado to form
may be given as much as 12 hours in advance
Tornado Forecasting
based largely on the recognition of the patterns conducive to tornado formation,
Often generalized forecasts for regions, with probabilities of events