Chapters :

Thunderstorm – 02


Frost forms on cold surfaces when condensation takes place below freezing point (00C), i.e. the dew point is at or below the freezing point. The excess moisture is deposited in the form of minute ice crystals instead of water droplets. The ideal conditions for the formation of white frost are the same as those for the formation of dew, except that the air temperature must be at or below the freezing point.


 When the temperature of an air mass containing a large quantity of water vapour falls all of a sudden, condensation takes place within itself on fine dust particles. So, the fog is a cloud with its base at or very near to the ground.  Because of the fog and mist, the visibility becomes poor to zero. In urban and industrial centres smoke provides plenty of nuclei which help the formation of fog and mist. Such a condition when fog is mixed with smoke, is described as smog. The only difference between the mist and fog is that mist contains more moisture than the fog. Mists are frequent over mountains as the rising warm air up the slopes meets a cold surface. Fogs are drier than mist and they are prevalent where warm currents of air come in contact with cold currents. Fogs are mini clouds in which condensation takes place around nuclei provided by the dust, smoke, and the salt particles.


The heat energy reflected, absorbed and radiated back into the space equals the energy received by the earth. Incoming radiation and the outgoing radiation pass through the atmosphere. The earth maintains its optimum temperature. Consider that the insolation received at the top of the atmosphere is 100 per cent. While passing through the atmosphere some amount of energy is reflected, scattered and absorbed. Only the remaining part reaches the earth surface


The interaction of insolation with the atmosphere and the earth’s surface creates  heat which is measured in terms of temperature. While heat represents the molecular movement of particles comprising a substance, the temperature is the measurement in degrees of how hot (or cold) a thing (or a place) . Distribution of Temperature Distribution of temperature varies both horizontally and vertically. 
  1. Horizontal Distribution of Temperature
  2. b. Vertical Distribution of Temperature
Horizontal Distribution of Temperature Distribution of temperature across the latitudes over the surface of the earth is called horizontal distribution of temperature. On maps, the horizontal distribution of temperature is commonly shown by isotherms. Isotherms are line connecting points that have an equal temperature at mean sea level.

Factors Affecting the Horizontal Distribution of Temperature Latitude: 

The angle formed by the solar radiation to the ground iscalled ‘angle of incidence’.  The solar radiation passes vertically along the equator. The angle of incidence decreases from equator towards the poles. The area heated by the solar radiation increases towards the poles and therefore, temperature decreases from the equator to the poles.

Distribution of land and water:

Land is heated and cooled at a faster rate due the conduction process whereas water is heated and cooled at slower rate due to convection process.

Water takes 2.5 times of heat energy to heat a unit area compared to land. Thus, the land will have higher temperature than the water in summer and vice versa during the winter. So more land mass in northern hemisphere (15.28C) leads to higher average temperature than the southern hemisphere.

Ocean currents:

Warm ocean currents carry warm water from the tropical region towards the poles and increase the temperature while cold ocean currents carry cold water from Polar Regions and reduce the temperature along the coasts.

Prevailing winds:

Warm winds like trade wind and westerly, that carry higher heat energy, increase the temperature while cold polar easterlies carry lower heat energy from polar region reduces the temperature.


The cloudy sky obstructs the solar radiation from the sun to earth and reduces the temperature. But the clear sky during the day allows more solar radiation to reach the earth’s surface and increases the temperature. Meanwhile clear sky at night allows more terrestrial radiation to escape. For example ,the tropical hot deserts experience higher temperature at day and lower temperature at night. Nature of the surface: The reflection from surface varies based on the nature of land cover. The more reflection from the snow surface leads to low temperature accumulation. But the dense forest, which reflects less heat energy and absorbs more heat energy, leads to higher temperature. Mountain barriers: If a wind or air mass blows towards the mountain, it influences the distribution of temperature on either side of the mountain. Factors Affecting the Vertical Distribution of Temperature We all know that the temperature decreases with increasing altitude from the surface of the earth. The vertical decrease in temperature of troposphere is called as ‘Normal Lapse Rate’ or ‘vertical temperature  gradient’ at which the temperature reduces at the rate of 6.5 8C per 1000 meter of ascent.  This is influenced by the following factors:
  1. Amount of terrestrial radiation reaching the altitude 
  2. Density of air to absorb the heat energy at higher altitude. 
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