Weather Phenomenon

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 Water Vapour

  • Zero to four per cent by volume of the atmosphere.
  • Three forms – gaseous, liquid and solid.
  • Continuous exchange of water between the atmosphere through processes of evaporation, transpiration, condensation and precipitation.
Precipitation
  • Resistance of the air fails to hold them against the force of gravity, they fall on to the earth’s surface. Release of Moisture.
  • Rainfall – Form of water.
  • Snowfall - Form of fine flakes of snow and is called snowfall. Temperature lower than 0o C.
  • Sleet & Hail - Frozen raindrops and refrozen melted snow-water. layer of air with the temperature above freezing point overlies a subfreezing layer near the ground.
  • Hailstones - Drops of rain after being released by the clouds become solidified into small rounded solid pieces of ice. Rainwater passing through the colder layers.
Humidity
  • Humidity - Water vapour present in the air.
  • Specific humidity of an air pocket is not affected by temperature & pressure changes
  • Absolute Humidity - The actual amount of the water vapour present in the atmosphere.
  • Relative Humidity - The percentage of moisture present in the atmosphere as compared to its full capacity at a given temperature. (Grater over the ocean compare to Continent)
  • Both absolute & Relative Humidity vary with temperature
Saturation
  • The air containing moisture to its full capacity at a given temperature.
Dew Point
  • Temperature at which saturation occurs in a given sample of air.
Evaporation
  • Evaporation is a process by which water is transformed from liquid to gaseous state.
  • Temperature at which the water starts evaporating is referred to as the latent heat of vapourisation.
  • Movement of air replaces the saturated layer with the unsaturated layer.
  • Greater the movement of air, the greater is the evaporation.
Condensation
  • Transformation of water vapour into water is called condensation.
  • Sublimation - If it directly condenses into solid form.
  • In free air, condensation results from cooling around very small particles termed as hygroscopic condensation nuclei.
  • Influenced by Volume of air, temperature, Pressure & Humidity.
  • Take Place
  • When the temperature of the air is reduced to dew point with its volume remaining constant.
  • when both the volume and the temperature are reduced.
  • when moisture is added to the air through evaporation
  • Forms - dew, frost, fog and clouds.
Dew
  • Water droplets on cooler surfaces of solid objects (Stone, Grass).
  • The ideal conditions for its formation are clear sky, calm air, high relative humidity, and cold and long nights.
  • Dew point is above the freezing point.
Frost
  • Forms on cold surfaces when condensation takes place at or below freezing point (0o C).
  • Form of minute ice crystals instead of water droplets.
Fog & Mist
  • fog is a cloud with its base at or very near to the ground.
  • Smog - fog is mixed with smoke.
  • Mists - frequent over mountains as the rising warm air up the slopes meets a cold surface.
Clouds
  • Mass of minute water droplets
  • Tiny crystals of ice formed by the condensation in free air at considerable elevations.
  • Dust & Salt Particles (act as hydroscopic nuclei) Responsible for Cloud Formation (High Concentration of Dust particle found in Sub-tropic & temperate region)
  • Cirrus
    • Thin and detatched clouds. Altitude: 8,000 – 12,000m.
    • White in Colour.
  • Cumulus
    • Cloud look like cotton wool & Flat base.
    • Altitude: 4000m – 7000m.
  • Stratus
    • Layered clouds covering large portions of the sky.
    • Formed either due to loss of heat or the mixing of air masses with different temperatures.
  • Nimbus
    • Form at middle levels or very near to the surface. Black or Dark gray.
    • Shapeless masses of thick vapour.
Combination of Basic Types
  • High Clouds – cirrus, cirrostratus, cirrocumulus.
  • Middle Cloud – Altostratus & Altocumulus.
  • Low Cloud – Stratocumulus & Nimbostratus.
  • Extensive Vertical Development – Cumulus and Cumulonimbus.
Types of Rainfall
Conventional Rainfall
  • Air on being heated, becomes light and rises up in convection currents. Cumulous clouds are formed.
  • Heavy rain with thunder & lightening but not last long.
  • Common in summer or in the hotter part of the day. Region of the equatorial and interior parts of the continents.
Orographic or Relief Rainfall
  • Saturated air mass comes across a mountain, it is forced to ascend and as it rises, it expands; temperature falls, and the moisture is condensed.
  • Windward slope: more rainfall. Leeward slope (Rain-shadow region): Rainless or Dry.
Cyclonic Rainfall
World Distribution of Rainfall
  • Rainfall Decreasing – Equator to Poles.
  • Coastal area more rainfall than interior of continent.
  • Rainfall in Ocean more than land.
  • 35 o N & 40 o of Equator, Rain heavy on east coast & decrease on west.
  • 45 o N & 60 o S of Equator, due to the westerlies, the rainfall is first received on the western margins of the continents and it goes on decreasing towards the east.
  • Mountains run parallel to the coast, the rain is greater on the coastal plain, on the windward side and it decreases towards the leeward side.
Major precipitation regimes of the world
  • Equatorial belt, over 200 cm per annum & Interior continental - 100 - 200 cm per annum.
  • Coast area moderate rainfall.
  • The central parts of the tropical land and the eastern and interior parts of the temperate lands between 50 - 100 cm per annum.
  • Rain shadow zone of the interior of the continents - < 50 cm per annum.
  • Seasonal distribution of rainfall provides an important aspect to judge its effectiveness.

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