They begin life as ghosts,
gently coursing through a solitary existence,
but slowly, their gentility turns to rage.
They grow larger and larger, hurling and twisting,
它们变得越来越大 咆哮着 缠绕着
and desperately reaching down from the sky.
And what began as an invisible shade
is turned into a monster.
Tornadoes are powerful spinning columns of air
that stretch from the ground to the clouds.
Most are relatively weak,
but the few that grow into large events
are extremely violent and cause immense destruction.
Tornadoes occur on six of the seven continents.
The country with the most tornadoes based on land size
is the United Kingdom,
with an average of about 33 tornadoes reported each year
But the country with the greatest overall number of tornadoes
and the most intense is the United States,
with over 1,000 reported annually.
Tornadoes, no matter where they occur,
are classified as either supercell tornadoes,
which form within supercells, the most powerful class of thunderstorms,
or non-supercell tornadoes which are smaller and weaker,
and form within non-supercell storms.
There are many theories surrounding the formation of tornadoes.
One key component they share
is the presence of both high and low pressure air in a given space.
Air particles from the area of high pressure
move toward an area of low pressure,
a movement that creates wind.
such as waterspouts and landspouts,
begin when cool high-pressure air
and warm low-pressure air are present,
particularly near ground level.
As air particles move horizontally
from the high pressure area to the low pressure area,
wind begins to pick up.
Winds blowing at different speeds
and in different directions and altitudes
begin to blow cyclically.
In the case of non-supercell tornadoes,
they turn into an upright spinning vortex.
But to create supercell tornadoes,
the circumstances are slightly different.
Violent supercell storms draw warm low-pressure air
up to a higher altitude,
leaving behind cool high-pressure air near the ground.
Air particles attempting to bring
the two levels of air pressure into balance
creates wind that blows vertically.
The wind increases and starts to blow in a cyclical fashion,
creating a pipe of wind that rolls along the ground.
In both cases, an upward current of wind called an updraft
provides the final ingredient for creating a tornado.
In a budding non-supercell tornado,
an updraft stretches its vertical vortex until it reaches the clouds.
To create a supercell tornado,
an updraft lifts the rolling pipe of wind upward
until it stands upright.
Then it pulls condensation from the skies
and into the spinning vortex.
As soon as the vortices, supercell or non-supercell,
connect the ground to the clouds,
they are officially classified as tornadoes.
All tornadoes are rated based on a system called
the Enhanced Fujita Scale.
The Enhanced Fujita, or EF, Scale,
classifies tornadoes from a rating of EF0 to EF5.
The rating is based on a number of factors,
such as the damage a tornado causes
and the Doppler radar estimates of its wind speeds.
EF0 tornadoes are the weakest,
with the wind speeds between 65 to 85 miles per hour.
EF5 tornadoes are the strongest,
with the wind speeds exceeding 200 miles per hour.
One of the strongest tornadoes recorded
occurred in Oklahoma City, Oklahoma in 1999.
Born from a supercell thunderstorm,
the EF5 tornado had a wind speed of over 300 miles per hour.
It resulted in 36 fatalities, injured nearly 600,
and caused about $1 billion in damages.
While tornadoes cannot be prevented,
measures are being taken to protect communities.
Meteorologists closely monitor storm fronts
in high-risk areas
and try to forecast possible tornadic events.
In doing so, they help mitigate damages to neighborhoods
and save countless lives,
even in the face of one of nature’s most formidable.
They begin life as ghosts,