Download Atmospheric Motion

Atmospheric Motion
Pressure Gradient Force- The difference in atmospheric pressure will cause air to move
from high pressure to low pressure. This is something like water flowing down hill or
seeking its own level.
Coriolis Force- This is a deflection of the wind caused by the spinning earth. The
deflection is to the right in the Northern Hemisphere and to the left in the Southern
Hemisphere. The result is that air moving out of a high pressure cell or into a low
pressure cell will develop a rotation.
Gravity- The principal effect of gravity can be seen in cold air drainage. As the earth’s
surface cools at night, cold air will flow down hill. Thus the valleys are always cooler
than the hillsides above them.
Friction - Friction is only important for air in contact with the earth’s surface in the
planetary boundary layer (PBL). This is about 1 km thick. The principal effects are in
rough terrain, forests, and downtown urban canyons.
As the air
rises it cools
and so slows
down.
Air pushed away by rising
column
Convection cell
Hot air expands
becomes less
dense and rises
Hot
Cool air sinks. As it hits the
ground it spread out. To the
left to replace the rising air
Cold
Hadley Cell Circulation and the Trade Winds
Prevailing Wind Belts of Earth. The earth is encircled by several broad prevailing wind
belts, which are separated by narrower regions of either subsidence or ascent. The
direction and location of these wind belts are determined by solar radiation and the
rotation of the earth. The three primary circulation cells are known as the: Hadley cell;
Ferrel cell; and Polar cell.
On or near the equator, where average solar radiation
is greatest, air is warmed at the surface and rises. This
creates a band of low air pressure, centered on the
equator known as the intertropical convergence zone
(ITCZ). The Intertropical Convergence Zone draws in
surface air from the subtropics. When this subtropical
air reaches the equator, it rises into the upper
atmosphere because of convergence and convection.
It attains a maximum vertical altitude of about 14
kilometers (top of the troposphere), and then begins
flowing horizontally to the North and South Poles.
This rising air comprises one segment of a circulation
pattern called the Hadley Cell (see diagram below).
The Hadley cell eventually returns air to the surface
of the earth, near 30 deg N and S.
The descending portion of the Hadley Cell produces a band of high air pressure at these
latitudes called the subtropical high. From this zone, the surface air travels in two
directions. Winds are generated between the subtropical high and the equatorial band of
low pressure (ITCZ), as air moves from high surface pressure toward low surface
pressure. These winds are deflected from east to west as they travel toward the equator by
the coriolis force, and are called the Trade Winds or the Tropical Easterlies. The other
portion of the surface air moves towards the poles from the subtropical high zone. This
air is also deflected by the Coriolis force, producing the Westerlies.
Jet streams and Rossby waves
Jet streams are fast flowing, narrow air currents found at the tropopause, the transition
between the troposphere (where temperature decreases with height) and the stratosphere
(where temperature increases with height)
They are located at 10-15 kilometers above the surface of the Earth.
They form near boundaries of adjacent air masses with significant
differences in temperature, such as the polar region and the warmer
air to the south.
The path of the jet typically has a meandering shape, and these meanders are one
manifestation of Rossby waves.
Rossby waves propagate westward with respect to the flow in which
they are embedded, which translates to a slower eastward migration
across the globe than smaller scale short wave troughs. The major jet
streams are westerly winds (flowing west to east) in the Northern
Hemisphere. During the summer, easterly jets can form in tropical
regions, typically in a region where dry air encounters more humid air at
high altitudes. Low level jets can form wherever low level winds are
squeezed.
Questions
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9.
what is Coriolis Force
How Gravity affect atmospheric motion
How does Friction affect atmospheric motion
How does Gravity Pressure affect atmospheric motion
Explain Jet streams
How does the three cell model work
Draw the three cell model
What are trade winds
Explain Rossby waves
Exam questions
(a)
What are jet streams?
(b)
Examine the factors influencing the global pattern of surface and upper air
circulation.
a)
Outline the main causes of air movement.
(b)
Discuss the application and limitations of the tri-cellular model.