Page 119 - SST Class 07
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lower than the pressure over the land. So the air
                                                                    solid, liquid or gaseous state. The humidity level
            blows from the land towards the sea and is called       varies from place to place and time to time
            as land breeze .                                        depending upon the temperature conditions. If
                                                                    temperature is high, the rate of evaporation will
                                                                    be high and humidity too will be high. Similarly,
                                  Return
                                  Return Flow Flow
                                                                    low temperature will result in low humidity.
                                                                    The air in the equatorial regions has more
                                                        cool
                 warm             sea b                 cool        moisture than in other areas of the world. It has
                 warm
                                  sea breezereeze
                                                                    more moisture in summer season in comparison
                                                                    of winter season. The amount of moisture
                                 Sea breeze
                                                                    decreases at higher places. But we do not find
                                                                    moisture in the cold air of the
                                  Return Flow Flow                  polar region.
                                  Return
                                                                    When the air is full of water
                                                                    vapour, it is called a humid
                 cool             lan                  warm         day . We can measure humidity
                                                       warm
                                  land breezed breeze
                 cool
                                                                    with   an    instrument    called
                                                                                                          Hygrometer
                                                                    hygrometer  .
                                 Land breeze
            Local Winds                                              Precipitation
            These winds blow only during a particular period        Precipitation is the process by which condensed
            of the day or year in a small area. Loo, chinook,       water falls down to the earth’s surface. Rainfall,
            foehn and harmattan are some local winds.               snowfall, hail or sleet are together known as
                                                                    precipitation. Rainfall is the most important form
            Loo :  This hot and dry wind blows over the             of precipitation . When tiny water droplets join
            Northern Plains of India in the summer months of
                                                                    together, they grow in size until they are unable to
            May and June.
                                                                    float in the air. Then they fall as rainfall or
            Chinook :  This warm and dry wind blows along           snowfall. Huge masses of water droplets are called
            the eastern side of the Rocky mountains in North        clouds . When they fall on the earth in a liquid
            America. It melts the snow, so only it is called        form, it is called rain .
            chinook or the snow-eater.
                                                                    Types of Rainfall
            Foehn :  This warm and dry wind blows across the        Rainfall  is   broadly   categorised    into   three
            Alps in winter and helps in ripening grapes in
                                                                    types—convectional rainfall, relief rainfall and
            France.
                                                                    cyclonic rainfall.
            Harmattan :   This cool and dry wind blows over         Convectional Rainfall :    Convectional rainfall is
            North-West Africa and brings succour from heat.
                                                                    quite common in equatorial and tropical areas. As
                  F ind Outind Out                                  these regions are hot, the air coming into contact
                                                                    with the earth’s hot
                                                                                                  Sun heats the   Warm air rises,   Rain can
            Find out that how wind plays an important role in       surface heats up and          land and air   cools and condenses   then occur
                                                                                                         forming clouds
                                                                                                  above
            our lives.                                              rises       vertically
             Humidity                                               producing a vertical
                                                                    air current. The air
            Water vapour is an important part of the                cools down at the
            atmosphere. Its presence in the air determines the
                                                                    higher altitude and
            moisture content and is called humidity           or    rapid   condensation     Sea           Land
            moisture . Moisture is found in the atmosphere in
                                                                    occurs. The clouds           Convectional rainfall
                                                                                                The Atmosphere
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