BIOMES OF THE EARTH - GRASSLANDS Part 4 ppt

GRASSLAND CLIMATES 69

the cloud, and the speed or direction of the wind may change

at different heights inside the cloud. This process produces

wind shear, a force that exists when the wind at a particular

height blows across the path of the wind below it and at a

greater speed. Wind shear sets the column of rising air ro￾tating, so the air is spiraling upward. The rotation begins in

the upper part of the cloud, below the level of wind shear.

The rotating center of the cloud is then known as a mesocy￾clone. The word cyclone describes air that rotates in the same

direction as the Earth—counterclockwise in the Northern

Hemisphere. Most mesocyclones rotate cyclonically (coun￾terclockwise), but the reason for this is unclear and occasion￾ally there are mesocyclones that turn in a clockwise direction

(anticyclonically).

Gradually more and more of the inside of the cloud begins

to turn, and the rotation extends downward. At this stage the

mesocyclone is up to five miles (8 km) across. Eventually the

rotation may extend to the air immediately below the cloud.

Air that is drawn into the up currents now starts turning as it

approaches the cloud, so the mesocyclone consists of air that

is spiraling upward to where it is swept into the anvil and

removed.

Because air is being removed, the atmospheric pressure

inside the mesocyclone is low, and as air enters the spiral its

pressure drops. The reduction in pressure allows the air to

expand, causing it to cool, and its water vapor condenses.

Condensation in the rotating air beneath the cloud base

makes it look as though the cloud itself is descending. Its

rotation is clearly visible from a distance, and fragments of

cloud can be seen moving across it.

The rotation continues to extend downward, and as it does

so it becomes narrower. Visible because of the condensation

it produces, the rotating column of air extends below the

storm cloud as a funnel cloud, widest at the top and tapering

toward the lower end. Air accelerates as it enters the spiral

and the wind speed is greatest around the core of the funnel.

The acceleration is due to a property of spinning objects.

When it spins, an object possesses angular momentum that is

proportional to its mass, speed of rotation (called its angular

velocity), and radius of rotation. Its angular momentum

remains constant, so if one of its components changes, one

or more of the others changes to compensate. This is called

the conservation of angular momentum. Air cannot alter its

mass, but as it approaches the center of the funnel, its radius

of spin decreases and consequently its angular velocity

increases in proportion. It means that the wider the funnel,

the greater the wind speeds around the center.

If the funnel touches the ground it becomes a tornado—

called a “cyclone” or a “twister” in some parts of the United

States. Tornadoes sweep up dust and other debris to produce

a dark cloud around the base of the funnel. As this material is

carried upward and into the cloud, the tornado darkens. All

tornadoes are dangerous. Even a mild one will lift debris and

hurl it out of the spiral with great force, and all but the

mildest tornadoes are capable of demolishing small buildings

and throwing trailer homes and cars around as if they are

toys.

Tornadoes can happen anywhere and at any time, but they

are more likely in some places and at some times. More than

half of all tornadoes occur in spring. The season begins in

February in the Gulf states. In March and April there are

often tornadoes in Georgia and Florida. The greatest number,

however, occur in May and June across the Great Plains. A

belt extending from northern Texas and the Texas Panhandle

through Oklahoma and Nebraska suffers more tornadoes

than any other part of the country—or of the world. It is

known as “Tornado Alley.”

70 GRASSLANDS