Natural forces of atmosphere. Pressure Gradient force, Coriolis force and Geostrophic wind.

1. PRESSURE GRADIENT FORCE, CORIOLIS FORCE AND
GEOSTROPHIC WIND

2. PRESSURE
GRADIENT FORCE

3. PRESSURE
• Pressure is defined as the force per unit area. It is proportional to the
the rate of collision between the molecules and the wall.
• Increased pressure can result from a) Increased density ,b)
Increased temperature.
• Air constantly moves from high to low pressure.

4. IN THE DIAGRAM BELOW, THE PRESSURE AT POINT "X"
INCREASES AS THE WEIGHT OF THE AIR ABOVE IT INCREASES.
THE SAME CAN BE SAID ABOUT DECREASING PRESSURE,
WHERE THE PRESSURE AT POINT "X" DECREASES IF THE
WEIGHT OF THE AIR ABOVE IT ALSO DECREASES.
Thinking in terms of air molecules, if the number
of air molecules above a surface increases, there are
more molecules to exert a force on that surface and
consequently, the pressure increases. The opposite
is also true, where a reduction in the number of air
molecules above a surface will result in a decrease
in pressure. Atmospheric pressure is measured with
an instrument called a "barometer", which is why
atmospheric pressure is also referred to as
barometric pressure.

5. PRESSURE GRADIENT
• The variation of heating from one locality to another is the initial
factor that produces movement of air or wind. The most direct
path from high to low pressure is the path along which the
pressure is changing rapidly. The rate of change is called
pressure gradient.
• Variation of air pressure over earth’s surface are determined
from barometric readings at hundred of weather stations. Which
is in the surface weather map is called “isobars”.
• The spacing of isobars indicate the amount of pressure change
occurring at a given distance and is expressed as “pressure
gradient”.

6. PRESSURE GRADIENT
• So the pressure gradient can be defined ass a change in
pressure over a given distance i.e.
PRESSUREGRADIENT=∆P/distance
= Phigh-Plow/distance
The magnitude of pressure gradient can be
addressed by nothing but the spacing of
the isobars-
-if the isobars are close together
the pressure gradient is large.
-if the isobars are far apart the
reassure gradient is small.

7. HORIZONTAL PRESSURE GRADIENT
• when surface under one air column is heated: o air
column expands, following P = ρ Rd T , example: height at
which 500 mb is reached: 5500 m, say (before heating)

8. HORIZONTAL PRESSURE GRADIENT
after heating and expansion: height at which 500 mb
pressure is reached is now higher up, at 5600 m, say (this is
like a hill of air above the heated spot)
at 5500 m the pressure is now less than 500 mb.

9. HORIZONTAL PRESSURE GRADIENT
• Gradual poleward decrease in mean temperature .
• Denser air at higher latitude.
• More rapid decrease of pressure with height .

10. HORIZONTAL PRESSURE GRADIENT
• The horizontal pressure gradient is the driving force of
wind.
• with a horizontal pressure gradient created in this way, air
can start to flow from higher pressure to lower pressure.
• It has both magnitude and direction.
• Its magnitude is determined from the spacing of the
isobars. And the direction of force is always from higher to
lower pressure.
• At the right angels to the isobars. Give rise to pressure
gradient force.

11. VERTICAL PRESSURE GRADIENT
• The vertical pressure gradient is usually in, or near ,
balance with gravity.
• For the vertical pressure gradient the upward and
downward flow in the Atmosphere is comparatively
slow.(with the exception of localized updrafts and
downdrafts)

12. WHY THE AIR IS NOT ESCAPE INTO
SPACE?
• The airflow is from areas of higher pressure to
lower pressure. Air pressure is highest near
earth surface and get progressively lower
as we move upward. but the air not accelerate
and escape to space because due to gravity.
Which acts the opposite direction to
the vertical pressure gradient. The important balance
that is usually maintained between these two
opposite forces is called HYDROSTATIC
EQULIBRIUM.

13. HYDROSTATIC EQULIBRIUM

14. PRESSURE GRADIENT FORCE
• Pressure gradient force is the force that moves air from an
area of high pressure to an area of low pressure. The
velocity of the wind depends upon the pressure gradient. If
the pressure gradient is strong , the wind speed is high. If
the pressure gradient is weak, the wind speed is light.

15. PRESSURE GRADIENT
FORCE

16. CORIOLIS FORCE

17. CORIOLIS FORCE
• The Coriolis force is an inertial force (also called a fictitious force) that
acts on objects that are in motion relative to a rotating reference frame.
• In a reference frame with clockwise rotation, the force acts to the left of
the motion of the object. In one with anticlockwise rotation, the force acts
to the right.
• Though recognized previously by others, the mathematical expression for
the Coriolis force appeared in an 1835 paper by French scientist Gaspard-
Gustave de Coriolis, in connection with the theory of water wheels.
• Early in the 19th century, the term Coriolis force began to be used in
connection with meteorology. Deflection of an object due to the Coriolis
force is called the 'Coriolis effect'.

18. • The weather map consists of high and low pressure system.as expected,
the air moves from high to low pressure but not cross the isobars at right
angles as the pressure gradient force directs. This deviation is the result
of earth’s rotation which is called as Coriolis force.
• It is essentially an extension of the centrifugal force in case of an object
or air. All the air on rotating earth feels a centrifugal force that is
perpendicular to earth rotation axis. But if the air is also moving so that it
changes either the total speed or the distance from axis or both, then the
centrifugal force is change as it is equal to V2/R . This change of the total
centrifugal force and appears as a new force to the observer that deflect
motion horizontally , perpendicular to motion . This deflecting force is
called Coriolis force
•

19. WHICH WAY DOES THE CORIOLIS
FORCE DEFLECT MOTION?
• The Coriolis force deflects movements to the right angle of the direction of
the movement. The Coriolis Force affects only wind direction, not wind
speed
• In northern hemisphere it deflect movement to right and in southern
hemisphere this rule is reverse because the earth’s rotation is clockwise
as viewed at south pole.
• The magnitude of Coriolis force per unit mass is proportional to the
latitude and the speed of movement of air parcel. This can be written
mathematically as follows: CF= -
+fV
• where V is the wind speed ,f is the Coriolis parameter which is written
as follows
• f=2*Earth’s rotation rate(ω)*the sin of the latitude(sinф)

21. THE CORIOLIS FORCE IS ZERO IN
EQUATOR AND HIGHEST AT POLE -WHY?
• As we moves towards pole, angular momentum must be
conserved due to radius on axis of rotation is decreasing.
So that we have to increase our tangential velocity to keep
angular momentum constant. As we know faster the speed
, the larger the Coriolis force.
• The Coriolis parameter is given by f=2*Earth’s rotation
rate(ω)*the sin of the latitude(sinф)
• Where ω=rotation of earth & ф= latitude. By applying this
we see f is greatest when sinф=1 which means sin 90°=1
or ф=90°.

22. GEOSTROPHIC WIND

23. GEOSTROPHIC WIND
• A nonmoving parcel of air has no starting point to move. The Coriolis
force is not work on it. Underneath the influence the pressure
gradient force which is always directed perpendicularly to the
isobars, the air parcel begun to accelerate directly toward the area of
low pressure.
• As some as the flow begins the Coriolis force comes into play and
causes a deflection to the right for winds in the NH.
• As parcel continuous to accelerate the Coriolis force intensified.
Thus the increased speed results further deflection.
• The wind turn and following parallel to the isobars.

24. • When this occur the pressure gradient force is balanced by
opposing Coriolis force.
• As long as the force remain balance the resulting wind continue
to flow parallel to the isobars at a constant speed.

25. • •Geostrophic motion occurs when there is an exact balance between the PGF and
the Co, and the air is moving under the the action of these two forces only.
• It implies 1) No acceleration •e.g., Straight, parallel isobar
2) No other forces •e.g., friction
3)No vertical motion •e.g., no convergence

26. GEOSTROPHIC WIND

27. • Mathematically, let's see how Vg is calculated. If we equate the PGF and
Coriolis force, then we can solve for Vg, which in this case replaces the
actual wind (V), previously used in the Coriolis equation.
• Vg is simply the pressure gradient force divided by the Coriolis parameter.
• So,

28. Thank you…
PARTHA PRATIM KANDAR
SWAGATA CHAKRABORTY