Lecture notes
Laws of Horizontal Motion
- Module
- Earth Systems
- Institution
- The University Of Birmingham (UBir)
notes on the 4 forces that affect the wind.
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Laws of horizontal motion.1. Pressure gradient force
2. Coriolis force
3. Centripetal force
4. Frictional force
These 4 factors affect the wind. The wind speed is a product of the balance of
these forces.
There will always be areas of high pressure and areas of low pressure. Fact
that pressure varies on the earth’s surface. Difference in pressure can have 2
causes; thermal/heating affect (amazon rainforest, intense heating gives low
pressure along the equator). Mechanically by the wind.
Mean sea level pressure chart – isobars; joins together locations of equal
atmospheric pressure.
1. Pressure gradient is the change in pressure divided by the distance of 2
points. dp/dn = change in pressure / horizontal distance (pascal per metre)
Movement from high pressure to low pressure.
Calculating the pressure gradient force from the pressure gradient. This contains
rho, the air density. PGF = -1/p x dp/dn (- sign as air is blowing from high to low
pressure) 1mb = 100Pa
At rho increases, PGF decreases. Pressure changes faster, then it will be windier.
Do question sheet.
2. Coriolis force is
due to the earth’s
rotation on air
movement.
The deflective force
per unit mass is; f =
2wvsin(theta)
F = Coriolis force
W = angular velocity
of the earth
Must be in RADIANS
1 radian = 57.3o
360o = 2pi radians
So, 6.28 radians = 360o
W = 2pi / (60x60x24) = 7.2722 x 10-5rad/s
V = velocity of the mass
Theta = latitude
Sin0 = 0 (equator)
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