Wind & Waves , Isobars , The Pressure Gradient Force,Coriolis Force,The Beaufort Wind Scale


What is wind?

  • Wind is the horizontal movement of the air across the surface of the earth.
  • The direction from which it blows and its speed are its important characteristics.
  • Wind blows FROM.  Southwesterly wind means wind blowing from the SW to NE.
  • Easterly wind means coming from 090° and blowing towards 270°’
  • Wind speed is expressed in Knots or meters/second. Conversion m/second X 2 = knots.

What Causes Wind

  • Winds are caused due to flow of air from High (HP) to Low Pressure areas (LP) in the atmosphere. 
  • On the surface of the earth, LP ares are created by vertical rise of an air parcel due to sun’s heating or Jet stream effect.
  • On land, parcel of air rises mostly during the day and summer.
  • At sea, parcel of air rises mostly during night and  winter, when the sea is warmer than the air above it.
  • Air from surrounding areas converges into LP area to fill up the voids which causes the winds.

Types of wind system

There are three types of wind systems

  • Permanent Systems         
    • Hedley and Ferrell cells  causing Westerlies and Trades
  • Periodic Systems
    • Monsoons caused by during hemispherical summers
  • Local conditions.
    • Land & Sea breeze
    • Katabatic and Anabatic winds
    • Thunderstorms         


  • For wind prediction, isobars first drawn on a chart in the shore Meteorological office..
  • Isobar is a smooth line on a map joining places of same atmospheric pressure at a given instant.
  • By international convention, Weather maps are normally drawn at 0000, 0600, 1200 & 1800 (All UTC).
  • Isobars curve gently without sudden changes of direction except at ‘Fronts’
  • Isobars do not cross or meet because one place can not have different values of atmospheric pressure at the same time.
  • Isobars undergo constant change with time. Weather maps are generally available every six hours (UTC Time) and isobars will be different in each succeeding weather map.
  • Isobars may be open as shown on the left or closed on the right.

How are the High and Low Pressure areas created?

This is repetition of what you have studied earlier. Recapitulate.

  • Area of High and Low atmospheric pressure are created primarily because of uneven heating of the earth by the sun.
  • If the entire earth was not inclined by 23 ½ ° and consisted of a homogenous mass, either only land or only sea, the equator will always be hotter than the poles and all surface winds on the earth will blow from poles to the equator and then from equator to the poles in the higher atmosphere in one simple gigantic circulation. 
  • With its inclination, if the earth was a homogenous, it would have a pressure pattern with :-      
  • Two bands of high pressures at Poles and at 30°N & S                              
  • One band of low pressure each at 60°N & S and a common band at the equator.
  • In NH & SH, Westerlies blow from West to East & trade winds blow from East to west.
  • The earth is non-homogenous. Its land and sea have differing heating characteristics, which results in pressure and wind pattern which alters with season where            
    • Lows develop over the land mass in summer and Highs in winter.
    • Seas continue to follow the generic pattern of a permanent low over the equator called ITCZ, Permanent Highs over 30°N & S & Permanent low over 60°N & S.

Characteristics of wind

  • Like any vector, wind has two characteristics – Direction and Velocity.
  • Both, direction and velocity, should be a direct function of the pressure difference between the two.
  • But in reality, blows in a direction and velocity which is a resultant of the interaction of following forces
    • Pressure Gradient Force.
    • Coriolis Force
    • Cyclostrophic force
    • & Surface Friction

The Pressure Gradient Force.

  • The change ofpressure over unit distance at right angles to the isobars is termed as theHorizontal Pressure Gradient.
  • The Gradient issteep when the isobars are close together and strong winds are expected toblow.
  • The gradient isslack or small when isobars are far apart and winds or lower speed areexpected.
  • When a horizontal pressure gradient exists, a force called ‘Pressure Gradient Force’ acts on air and moves it from HP to LP at right angles to the isobar.
  • However for a number of reasons, the actual air motion at the surface is seldom in the direction of  or at the speed of related this force.
  • One of the reasons for this variation is the Coriolis or the Geostrophic force.

Equation of the Coriolis Force

  • Coriolis force for unit mass is 2Ω*sin φ*v
    • where Ω is the angular velocity of the earth,
    • φ the latitude
    • and v is the speed of the air.
    • It’s interpretation is
      • Velocity of earth’s is about 900 miles an hour from west to east. It decreases pole wards and becomes zero at the Poles.
      • At equator, sin φ = 0, and the Coriolis force = 0, whereas at the poles (sin φ = 90) it is maximum. 
      • Slowly blowing winds will be deflected only a small amount, while stronger winds will be deflected more.

Coriolis Force                                                                                               

  • It is so named because it relates to the rotation of the earth about its axis @ 15°per hour.
  • This rotation causes an air particle to be deflected to its right of its line of motion in the NH and to the left of its line of motion in the SH.
  • It has following characteristics
  • The CF always acts at right angles to the direction of wind.
  • The CF is directly proportional to the wind velocity.
  • The CF is zero at the equator and increases towards the poles .
  • It makes wind deflect to the right in the NH and to the left in the SH.
  • The deflection is greater in higher latitudes and it also increases as wind velocity builds up.

The Beaufort Wind Scale  

  • This scale was devised in 1805 by Commander Francis Beaufort (Later Admiral and Sir).
  • It is used for estimating wind speed at sea by observing the sea surface.
  • Because of the inherent inaccuracies in such estimations, the scale is gives Wind Force and not Wind Speed. 
  • Use of this wind scale became mandatory for log entries in the Royal Navy.
  • IMO accepted this wind scale and adopted world wide.
  • This scale should only be used for estimating winds in open and deep waters away from the land where the wind has considerable fetch.
  • If used near the land, it normally results in underestimation of the wind.