Chapter 6


-          Wind is air in motion that arises from a variety of forces.

-          Wind direction is the direction from which the wind is blowing.

-          Wind associated with Low Pressure systems (Cyclonic, counterclockwise in NH) or High Pressure systems (Anti-cyclonic, clockwise in NH).

-          Forces are characterized by direction and magnitude.  The overall force is the Net Force.

-          Newton’s Second Law of Motion states that a force exerted on an object (or a parcel of air) of a given mass causes the object to accelerate in the direction of the applied force.    F=MA (sum of forces=mass x acceleration).

-          The Momentum of an object is its mass multiplied by its velocity.


-          Five different forces combine to move air:

o       Gravitational Force

§         Directed downward perpendicular to the ground and is approximately equal to the mass times the gravitational acceleration (9.8 m/s2).

o       Pressure Gradient Force (PGF)

§         Force resulting from pressure differences over distances in a fluid.

§         A Pressure Gradient is a change in pressure over a distance.

§         Always pushes from High Pressure to Low Pressure.

§         Strong winds almost always result from large pressure gradients.

§         Isobaric (constant pressure) maps show that the spacing between lines of constant height (isoheights) is proportional to the PGF.  (strongest winds are where spacing is at a minimum).

o       Coriolis Force

§         Force caused by the rotation of the planet which describes the observed deflection of moving objects caused by the observer’s moving frame of reference. 

§         The magnitude of this force is determined by the wind speed and latitude.

§         Deflects objects to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

§         Changes the direction, but not the overall speed of anything moving with respect to the ground.

§         Arises from the fact that at different locations the Earth’s surface moves at different speeds while rotating (slower at Poles, faster at Equator) due to distance needed to travel over the same orbit time.

§         Coriolis force is zero at the Equator and when the wind speed is zero.  It increases as you go higher in latitude and wind speed increases.

o       Friction

§         Caused by the flow of wind over the roughness of the Earth’s surface.

§         The roughness of surface and the speed of the wind determine the magnitude of frictional force.

§         Always acts in direction opposite to movement.

o       Centrifugal Force/Centripetel Acceleration

§         Centrifugal Force is an apparent force felt by objects in a turning frame of reference that seems to push the objects Outward from the center of the turn.

§         Centripetal Acceleration is the change of velocity of an object moving in a curved path where the acceleration is directed Toward the center of the curved path.

§         Centrifugal Force and Centripetal Acceleration are equal and opposite of each other and increase as the speed of the object increases.

-          The Pressure Gradient Force acts to cause the wind, and the Coriolis, Centrifugal, and Frictional Forces react to the wind.


-          Winds arise as a result of different balanced combinations of these forces:

o       Hydrostatic Balance  

§         A balance of gravitational force and vertical PGF. 

§         PGF + GF = 0

o       Geostrophic Balance

§         A balance between PGF and Coriolis Force.  Requires wind must blow in a straight line.

§         PGF + CF = 0

§         Geostrophic Wind- horizontal wind created by balance of horizontal PG and Coriolis Forces.    Low pressure always on the left of in NH (Buys Ballot’s Law)

§         Buy Ballot’s Law explains that the Wind blows Clockwise around High Pressure and Counterclockwise around Low Pressure in the NH, opposite in SH.

o       Gradient Balance

o       Three way balance between horizontal PGF, Coriolis Force, and Centrifugal Force.

§         PGF + CF + Cent. F = 0

o       Gradient Wind is wind that results from this balance.

o       Around a Low Pressure, Centrifugal Force and Coriolis Force are in same direction.

o       Around a High Pressure, Centrifugal Force and PGF are in same direction.

o       Winds around a high pressure will be stronger than a low pressure when spacing between isobars is the same.

o       Guldberg-Mohn Balance

o       Three way balance between horizontal PGF, Coriolis Force, and Frictional Force.

§         PGF + CF + FF = 0

o       This balance explains why cloudy, wet weather is associated with low pressure and sunny, dry weather is associated with high pressure.

-          Energy imbalances are the trigger for atmosphere and ocean circulations.

-          Thermal Wind is the relationship between the vertical changes in the geostrophic wind and the horizontal temperature gradient.  It is a combination of hydrostatic and geostrophic balances.

o       Thermal Wind results in winds being more Westerly as you go up towards the Poles where it’s colder, and as you go up in altitude.

-          A Sea Breeze is the result of uneven heating during the daytime between the land and adjacent water and is driven initially by pressure gradients created by this unequal heating.

-          A Land Breeze is a wind that blows offshore from land to water during nighttime in the vicinity of large bodies of water.

-          Atmospheric motions span an enormous range of space and time:

o       Microscale- such as a cumulus cloud, circulations less than 1 km.  Coriolis is negligible.

o       Mesoscale- such as thunderstorms, fronts, from 1 km-a few hundred kilometers, Coriolis force becomes more important with growing size.

o       Synoptic Scale- such as mid latitude low-pressure systems, ~1000 km, Geostrophic Balance is dominant

o       Planetary Scale- much greater than 1000 km, Geostrophic balance is dominant.