WIND

DEFINITION:

Is The Movement Of Air Relative To The Surface Of The Earth.

IMPORTANT METEOROLOGY TERMS:

  1. Pressure Gradient Force:
    • Is The Force Which Results When There Is A Difference In Pressure Across A Surface.
    • A Difference In Pressure Across A Surface Then Implies A Difference In Force, Which Can Result In An Acceleration According To Newton’s Second Law, If There Is No Additional Force To Balance It.
    • The Resulting Force Is Always Directed From The Region Of Higher-pressure To The Region Of Lower-pressure.
    • When A Fluid Is In An Equilibrium State (I.e. There Are No Net Forces, And No Acceleration), The System Is Referred To As Being In Hydrostatic Equilibrium.
    • In The Case Of Atmospheres, The Pressure Gradient Force Is Balanced By The Gravitational Force, Maintaining Hydrostatic Equilibrium.
    • In Earth’s Atmosphere, For Example, Air Pressure Decreases At Increasing Altitudes Above Earth’s Surface, Thus Providing A Pressure Gradient Force Which Counteracts The Force Of Gravity On The Atmosphere.
    • To Learn More: Wikipedia
  2. Coriolis Force:
    • The Coriolis Effect Is Caused By The Rotation Of The Earth And The Inertia Of The Mass Experiencing The Effect.
    • Because The Earth Completes Only One Rotation Per Day, The Coriolis Force Is Quite Small, And Its Effects Generally Become Noticeable Only For Motions Occurring Over Large Distances And Long Periods Of Time, Such As Large-scale Movement Of Air In The Atmosphere Or Water In The Ocean.
    • Such Motions Are Constrained By The Surface Of The Earth, So Only The Horizontal Component Of The Coriolis Force Is Generally Important.
    • This Force Causes Moving Objects On The Surface Of The Earth To Be Deflected To The Right (With Respect To The Direction Of Travel) In The Northern Hemisphere And To The Left In The Southern Hemisphere.
    • The Horizontal Deflection Effect Is Greater Near The Poles And Smallest At The Equator, Since The Rate Of Change In The Diameter Of The Circles Of Latitude When Travelling North Or South, Increases The Closer The Object Is To The Poles. Rather Than Flowing Directly From Areas Of High Pressure To Low Pressure, As They Would In A Non-rotating System, Winds And Currents Tend To Flow To The Right Of This Direction North Of The Equator And To The Left Of This Direction South Of It.
    • This Effect Is Responsible For The Rotation Of Large Cyclones (See Coriolis Effects In Meteorology).
    • To Explain This Intuitively, Consider How An Object That Moves Northwards From The Equator Has A Tendency To Maintain Its Greater Speed At The Equator (Rotating Around Towards The Right As You Look At The Sphere Of The Earth), Where The “Horizontal Diameter” Is Larger, And Therefore Tends To Move Towards The Right As It Passed Northwards Where The “Horizontal Diameter” Of The Earth (The Rings Of Latitude) Is Smaller, And The Linear Speed Of Local Objects On The Earth’s Surface At That Latitude Is Slower.
    • To Learn More: Wikipedia
  3. Geostrophic Wind:
    • Is The Theoretical Wind That Would Result From An Exact Balance Between The Coriolis Effect And The Pressure Gradient Force.
    • This Condition Is Called Geostrophic Balance.
    • The Geostrophic Wind Is Directed Parallel To Isobars (Lines Of Constant Pressure At A Given Height).
    • This Balance Seldom Holds Exactly In Nature.
    • The True Wind Almost Always Differs From The Geostrophic Wind Due To Other Forces Such As Friction From The Ground. Thus, The Actual Wind Would Equal The Geostrophic Wind Only If There Were No Friction And The Isobars Were Perfectly Straight. Despite This, Much Of The Atmosphere Outside The Tropics Is Close To Geostrophic Flow Much Of The Time And It Is A Valuable First Approximation.
    • Geostrophic Flow In Air Or Water Is A Zero-frequency Inertial Wave.
    • To Learn More: Wikipedia
  4. Gradient Wind:
    • Is A Balance Of The Pressure Gradient Force, Centrifugal And Coriolis.
    • A Geostrophic Wind Becomes A Gradient Wind When The Wind Begins Flowing Through Curved Height Contours.
    • The Curving Motion Introduces A Centrifugal Force.
    • Veering Wind:
    • Is A Wind That Turns Clockwise With Height.
    • A Veering Wind Is Associated With Warm Air Advection And Dynamic Lifting.
    • The Magnitude Of Warm Air Advection Is A Function Of Wind Speed And The Pre-existing Thermal Gradient.
    • Weak Winds Will Result In Weak Advection. Winds Often Veer Ahead Of Cold Fronts.
  5. Backing Wind:
    • Is A Wind That Turns Counter-clockwise With Height.
    • A Backing Wind Is Associated With Cold Air Advection And Dynamic Sinking.
    • Winds Back Behind Cold Fronts.

HOW DOES THE WIND HAPPEN:

  1. Wind Is Caused By Differences In The Atmospheric Pressure. When A Difference In Atmospheric Pressure Exists, Air Moves From The Higher To The Lower Pressure Area, Resulting In Winds Of Various Speeds.
  2. The Two Major Driving Factors Of Large-scale Wind Patterns Are:
    • The Differential Heating Between The Equator And The Poles.
    • The Rotation Of The Planet.
  3. Outside The Tropics And Aloft From Frictional Effects Of The Surface, The Large-scale Winds Tend To Approach Geostrophic Balance.
  4. Near The Earth’s Surface, Friction Causes The Wind To Be Slower Than It Would Be Otherwise. Surface Friction Also Causes Winds To Blow More Inward Into Low Pressure Areas.

HOW TO DETERMINE WIND SPEED AND DIRECTION FROM CHART:

1. WIND SPEED:

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2. WIND DIRECTION:

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