Chapters :

Thunderstorm – 01

THUNDERSTORM Thunderstorms are storms ranging several kilometres in diameter, created by the rapid lifting of moist and warm air, as a result of which a dense vertical tower of the cloud is created. Thunderstorms are associated with strong winds, hail, lightning, tornadoes, thunder and heavy rain. WHAT CAUSES THUNDERSTORMS? Air carrying water droplets are lifted vertically into the atmosphere due to unequal warming of the surface of the Earth. As a result of this lift, the air condenses and latent heat is released with the expansion resulting from a decrease in pressure (with the increase in height). These condensed droplets freeze and fall back to the ground along with hail, and lightning From severe thunderstorms sometimes spiralling wind descends like a trunk of an elephant with great force, with very low pressure at the centre, causing massive destruction on its way. Such a phenomenon is called a Tornado. Tornadoes generally occur in middle latitudes. The tornado over the sea is called water sprouts INDIAN CLIMATE – SEASONS
  • The cold weather season or winter season,
  • The hot weather season or summer season,
  • The south-west monsoon season or Rainy season, and
  • The season of the retreating monsoon or cool season.
  • November – March. January is the coldest month.
  • Sun’s apparent path is to the south of equator.
  • Clear sky, pleasant weather, low temperature, low humidity, high range of temperature, cool and slow north-east trade winds.
  • The diurnal range of temperature, especially in interior parts of the country, is very high
  • They cause light rain in the Indus-Ganga plains and snowfall in the Himalayan belt.
  • After the passage of the disturbance, widespread fog and cold waves lowering the minimum temperature by 5° to 10°C below normal are experienced.
  • Fog lowers visibility and causes great inconvenience for transportation
  • This is due to low sea surface temperature and exit of ITCZ farthest south.
  • The storms which are born in the Bay of Bengal strike Tamil Nadu and bring heavy rainfall.
  • Some of them cross the southern peninsula over to the Arabian Sea.
  • March to June.
  • High temperature and low humidity are the chief characteristics.
  • Sometimes referred to as pre-monsoon period.
TEMPERATURE IN SUMMER SEASON High sun’s insolation due to apparent movement of sun between the equator and the Tropic of Cancer.


Water is present in the atmosphere in three forms namely – gaseous, liquid and solid.  The moisture in the atmosphere is derived from water bodies through evaporation and from plants through transpiration. Thus, there is a continuous exchange of water between the atmosphere, the oceans and the continents through the processes of evaporation, transpiration, condensation and precipitation. Water vapour present in the air is known as humidity The actual amount of the water vapour present in the atmosphere is known as the absolute humidity It is the weight of water vapour per unit volume of air and is expressed in terms of grams per cubic metre. The ability of the air to hold water vapour depends entirely on its temperature The percentage of moisture present in the atmosphere as compared to its full capacity at a given temperature is known as the Relative Humidity The air containing moisture to its full capacity at a given temperature is said to be saturated. It means that the air at the given temperature is incapable of holding any additional amount of moisture at that stage. The temperature at which saturation occurs in a given sample of air is known as dew point.


The amount of water vapour in the atmosphere is added or withdrawn due to evaporation and condensation respectively.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. The transformation of water vapour into water is called Condensation.  Condensation is caused by the loss of heat.If it directly condenses into solid form, it is known as sublimation.In free air, condensation results from cooling around very small particles termed as Hygroscopic Condensation Nuclei. Particles of dust, smoke and salt from the ocean are particularly good nuclei because they absorb water.  Condensation also takes place when the moist air comes in contact with some colder object and it may also take place when the temperature is close to the dew point. Condensation, therefore, depends upon the amount of cooling and the relative humidity of the air. condensation the water vapour or the moisture in the atmosphere takes one of the following forms — dew, frost, fog and clouds. Forms of condensation can be classified on the basis of temperature and location.  Condensation takes place when the dew point is lower than the freezing point as well as higher than the freezing point.


When the moisture is deposited in the form of water droplets on cooler surfaces of solid objects (rather than nuclei in air above the surface) such as stones, grass blades and plant leaves, it is known as dew.  The ideal conditions for its formation are clear sky, calm air, high relative humidity, and cold and long nights. For the formation of dew, it is necessary that the dew point is above the freezing point.
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