What Causes weather


Movement of air causes Weather

  • At its most basic level, one can say that weather is caused by movement of air from one place on the earth to another. Rain, storm, thunderstorm, snow, ice, clouds or fog – are all caused by the movement of air around us.
  • The air may move horizontally, which we call as wind or breeze at sea level or jet streams at higher altitudes.
  • It also moves vertically up and down, which are known as updrafts and downdraft.
  • We all have felt wind and heard the wind, but where does one see the up and down draft? Have you seen a bird called kite flying up and down in the air? It uses the up and down drafts all the time.
  • This movement of air mostly happens in the lowest layer of atmosphere called the troposphere.How does weather affect ships?
    • Movement of wind causes the surface layers of seas and oceans to move. As this movement is not uniform over the entire extent of the sea, waves appear on the surface of the sea.
    • The waves grow larger as wind builds up and cause discomfort and problems to the ships in the form of roll, pitch, surge, heave and yaw.
    • When winds sustain over long period, the deeper parts of ocean also begin to move which causes the Swells. Stronger the winds, higher the sea and swell, greater the danger to ships.
    • In extreme rough weather, ships can be damaged, can break or capsize and sink.
    • This in nutshell is the weather and its deleterious effects on ships at sea. .Weather at LP and HP Areas
      • For a mariner, weather means wind and visibility.
      • Weather at LP areas is always rougher than HP area.
      • The speed of air as it begins to flow from an HP area is gentle in the beginning.
      • It picks up speed as it approaches LP area and is fastest in the LP area.
      • So winds in LP area are always stronger and seas always rougher than in HP area.
      • Air rising at LP area condenses and forms fog, clouds and precipitation.
      • Air sinking at HP area is free of moisture so no clouds or precipitation occur.
      • The visibility at LP area therefore is always poorer than at an HP area.Why do HP and LP Happen?
        • LP and HP areas occur because of Variation in Heating of the Earth by the sun.
        • Basic Postulate Quantum of  energy given to the earth by the sun is same, be it
        • morning or noon, poles or equator or land or sea. All rays of sun falling on earth surface have same heat content. It is approximately 250 W/m2. But surface of the earth heats up differently because of following 6 reasons.
          1. Angle of incidence of sun’s rays which cause:-
            • Daily Variation
            • and Seasonal Cariation
          2. Extent of atmosphere
          3. Reflectivity of the surface.
          4. Differing heating characteristics of Land and sea
          5. Hemispherical variation in distribution of land & SeaDaily Variation
            • Due to the inclination of sun’s rays hitting the earth, noon is always hotter than the Mornings and evenings.
            • Normally, highest temperature of the day occurs at 1400hrs local time.
            • Lowest temperature of the day occurs at about 0400hrs.
            • Thus there is one Maxima and one Minima a day and hence this variation is called a “Diurnal Temperature Variation.


            Seasonal Variation

            At school level, it has been taught to students that seasonal variation happens because Earth’s axis of rotation is inclined by 23½° with sun’s axis.


            Because of tilt of axis, the sun appears to be moving between tropic of Cancer and tropic of Capricorn every year as below:-

            • On top of Equator on 20th March and then move North.
            • On top of Tropic of cancer on 21st June then move South
            • Again on top of Equator on 22 September and continue South
            • On top of Tropic of Capricorn on 21st December and then move North.


            Why are Poles colder than Equator?

            As seen in the diagram above, rays from sun arrive at the earth parallel to each other.

            During Northern summer, rays falling on NH are more vertical than rays falling on SH.

            SP does not get any sun at all during this period. Exactly opposite happens during Southern Summer. On an average, during the annual passage of the sun, Equator gets maximum heat per unit area as compared to higher latitudes, with Poles receiving least amount.


            This variation in heating by the Sun is represented by three Zones shown on page 13 of the text book “Marine Meteorology” by Captain Subramanian.

            • The Torrid Zone
            • The Temperate Zone
            • The Frigid Zone
            • The Zones