Topic outline

    • Winds, Storms and Cyclones

      Q48. High speed winds are accompanied by reduced air pressure. Prove the statement with help of an experiment.

      Ans. Crumple a small piece of paper into a ball of size smaller than the mouth of an empty bottle. Hold the empty bottle on its side and place the paper ball just inside its mouth. Now try to blow on the ball to force it into the bottle. When we blow into the mouth of the bottle the air near the mouth has higher speed. This decreases the pressure there. The air pressure inside the bottle is higher than near the mouth. The air inside the bottle pushes the ball out.

      Image from NCERT

      Q49. Explain with the help of an activity that an increase in the speed of air, results in the drop of pressure.

      Ans. Take two balloons of approximately equal size. Put a little water into the balloons. Blow up both the balloons and tie each one to a string. Hang the balloons 8–10 cm apart on a cycle spoke or a stick. Blow in the space between the balloons. We will observe that the freely suspended balloons move toward each other. The reason is, when high speed air moves through the gap in between the balloons, the pressure of air falls in the gap. Thus, air at high pressure the sides of balloons forces them inward.

      Image from NCERT


      Q50. Write an experiment to show that on heating the air expands and occupies more space.

      Ans. Take two paper bags or empty paper cups of the same size. Hang the two bags in the inverted position on the two ends of a metal or wooden stick. Tie a piece of thread in the middle of the stick. Hold the stick by the thread as in a balance. Put a burning candle below one of the bags. We will see that the balance of the bags gets disturbed.  This happen as the warm air rises up, it pushes the bag above the candle.

      Image from NCERT

      Q51. How technologies help people to be protected from cyclone?

      Ans. These days we are better protected. In the early part of the last century, coastal residents may have had less than a day to prepare or evacuate their homes from an oncoming cyclone. The world today is very different. Thanks to satellites and radars, a Cyclone alert or Cyclone watch is issued 48 hours in advance of any expected storm and a Cyclone warning is issued 24 hrs in advance. The message is broadcast every hour or half hour when a cyclone is nearer the coast. Several national and international organisations cooperate to monitor the cyclone-related disasters.


      Q52. What are characteristics of Tornadoes?

      Ans. Characteristics of Tornadoes are:

                                  i.        A tornado is a dark funnel shaped cloud that reaches from the sky to the ground.

                                 ii.        Most of the tornadoes are weak. A violent tornado can travel at speeds of about 300 km/h.

                                iii.        Tornadoes may form within cyclones. The diameter of a tornado can be as small as a metre and as large as a km, or even wider.

                                iv.        The funnel of a tornado sucks dust, debris and everything near it at the base (due to low pressure) and throws them out near the top.

      Q53. Write a note on structure of a cyclone.

      Ans. Structure of a cyclone

                                i.        The centre of a cyclone is a calm area. It is called the eye of the storm. A large cyclone is a violently rotating mass of air in the atmosphere, 10 to 15 km high.

                               ii.        The diameter of the eye varies from 10 to 30 km. It is a region free of clouds and has light winds.

                              iii.        Around this calm and clear eye, there is a cloud region of about 150 km in size. In this region there are high-speed winds (150–250 km/h) and thick clouds with heavy rain.

                              iv.        Away from this region the wind speed gradually decreases.


               Q54. Take a boiling tube. Stretch a balloon tightly over the neck of the tube. You can use a tape to make it tight. Pour some hot water in a beaker. Insert the boiling tube with the balloon in the hot water. Observe for 2–3 minutes for any change in shape of the balloon. Take the tube out, let it cool down to the room temperature. Take some ice-cold water in another beaker and place the tube with the balloon in cold water for 2–3 minutes. Observe the change in the shape of the balloon. Think and try to answer:
                              i.        What makes the balloon inflated when the boiling tube is placed in hot water?
                             ii.        Why is the same balloon deflated when the tube is kept in cold water?
                            iii.        Can we infer from the first observation that air expands on heating?
                            iv.        Can you now state what happens to the air in the boiling tube when it cools down?

      Image from NCERT


                                i.        Before the air in the balloon is heated, the molecules inside are creating as much pressure as the molecules outside, meaning that the balloon stays at equilibrium and neither expands nor contracts. When they get heated, however, the inside molecules start moving with more force. They create more pressure, causing the balloon to expand outward until the pressure equalizes.

                               ii.        When the inflated balloon is placed in cold water, the cold water lowers the overall temperature of the air inside the balloon. The decrease in temperature causes the air molecules to move more slowly, with less energy and air pressure reduces. Since the molecules have lower energies, their collisions with the walls of the balloon are insufficient to keep the balloon inflated.

                              iii.        Yes, this shows that air expands on heating.

                              iv.        The overall temperature of the air inside the balloon decreases. The decrease in temperature causes the air molecules to move more slowly, with less energy and air pressure reduces.


    • Download to practice offline.