By: Anna Mae Cababaros
MS EST- 1
By: Anna Mae Cababaros
MS EST- 1
- refers to the weight of the
Earth's atmosphere pressing
down on everything at the
• Another way to define
air pressure is that it is
the force exerted
against a surface by
the continuous collision
of gas molecules.
• Average air pressure at
sea level is about 1
kilogram per square
centimeter, or 14.7
pounds per square inch.
• If air pressure
decreases, the column of
mercury falls; if the
pressure increases, the
• Standard atmospheric
pressure at sea level
equals 29.92 inches of
this instrument has a partially evacuated chamber that changes
shape, compressing as atmospheric pressure increases, and
expanding as pressure decreases.
•Wind is the result of horizontal differences in
•If Earth did not rotate and there were no
friction, air would flow directly from areas of
higher pressure to areas of lower pressure.
•However, because both factors exist, wind is
controlled by a combination of:
–(1) the pressure-gradient force,
–(2) the Coriolis force, and
• The pressure-gradient force is the primary
driving force of wind that results from
pressure changes that occur over a given
distance, as depicted by the spacing of
isobars, lines drawn on maps that connect
places of equal air pressure.
•The spacing of isobars indicates the amount of
pressure change occurring over a given
distance, expressed as the pressure gradient.
• The Coriolis force produces a
deviation in the path of wind due to
Earth's rotation (to the right in the
Northern Hemisphere and to the left
in the Southern Hemisphere).
• The amount of Coriolis deflection also
increases with wind speed.
Is always directed at
right angles to the
direction of airflow.
Affects only wind
direction, not wind speed.
Is affected by wind
Is strongest at the
poles and weakens
nonexistent at the
Friction with Earth’s Surface
• It acts to slow the movement of air.
• Friction, which significantly influences airflow near Earth's
surface, is negligible above a height of a few kilometers.
• Winds in which the Coriolis force is exactly equal and
opposite to the pressure gradient force are called
• Geostrophic winds flow in a straight path, parallel to the
isobars, with velocities proportional to the pressure-
• Winds that blow at a constant speed parallel to curved
isobars are termed gradient winds.
Highs and Lows
Lows (Cyclone) – centers of
Highs (Anticyclone) – high-
Winds move from higher pressure to lower
pressure and are deflected to the right or left
by Earth’s rotation.
In a Northern Hemisphere surface
anticyclone, winds blow outward and
Regardless of the hemisphere, friction causes
a net inflow (convergence) around a cyclone
and a net outflow (divergence) around an
General Circulation of the Atmosphere
The underlying cause
of wind is unequal
heating of earth’s
The atmosphere acts
as a giant heat-transfer
Ocean currents also
contribute to the global
Series of Prevailing Winds:
Polar Regions- Polar easterlies
Near the Equator- Easterly Trade winds
Middle Latitude- Westerlies
The flow is divided into six cells, three in each regions:
a)Equatorial Low (Doldrums)- the air that flows up from the equator
forms an equatorial zone of low pressure, a region of calm.
b) Subtropical High (Horse latitudes)- the region of light winds.
c) Westerlies- the north-ward flowing air current turns right.
d) Northeast trade winds-the southward air also deflected to the
e) Polar easterlies- the air aloft gradually moves , continues to cool
and descends at the polar region. There it cools further at the surface,
and flows southward.
This force causes the
winds in the Northern
hemisphere to move from
the East towards the West
below the subtropical
high, and from the West
towards the East above
the subtropical high.
Above the subtropical highs in the Northern Hemisphere,
and below the subtropical highs in the Southern
Hemisphere, winds blow from the West, towards the East.
The subpolar lows
generally cause the
winds above 60 °
latitude to move from
the East towards the
• small-scale winds produced
by a locally generated
• it is caused either by topographic effects
or variations in surface composition in the
All winds are produced for the same reason:
Pressure differences that arise because of
temperature differences that are caused by
unequal heating of Earth’s surface.