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www.GOALias.blogspot.comtowards each other. How could thishappen? This could happen if thepressure of air between the balloonswere somehow reduced. The pressureoutside the balloons would then pushthem towards each other.In Activity 8.4 you saw that when youblew over the paper strip, it wentupwards. Again, this could happen ifblowing over the paper reduced the airpressure above the strip.We see that the increased windspeed is, indeed, accompanied by areduced air pressure.Can you imagine what would happenif high-speed winds blew over the roofsof buildings? If the roofs were weak, theycould be lifted and blown away. If youhave any such experience, share it withyour friends.Let us try to understand how windsare produced, how they bring rain andhow they can be destructive sometimes.You already know that when airmoves, it is called wind. Air moves fromBalloon tied overthe neck of theboiling tubeBoiling tubeimmersed inhot waterFig. 8.6 The shape of the balloon in hot and cold waterthe region where the air pressure ishigh to the region where the pressureis low. The greater the difference inpressure, the faster the air moves. Buthow are the pressure differences createdin nature? Is the difference intemperature involved? The followingactivities will help you to understandthis.8.3 AIREXPANDSON HEATINGBoiling tubeimmersed inice-cold waterActivity 8.5Take a boiling tube. Stretch a balloontightly over the neck of the tube. Youcan use a tape to make it tight. Poursome hot water in a beaker. Insert theboiling tube with the balloon in the hotwater. Observe for 2–3 minutes for anychange in shape of the balloon. Take thetube out, let it cool down to the roomtemperature. Take some ice-cold waterin another beaker and place the tubewith the balloon in cold water for 2–3minutes. Observe the change in theshape of the balloon.Think and try to answer:What makes the ballooninflated when the boiling tubeis placed in hot water?Why is the same balloondeflated when the tube is keptin cold water?Can we infer from the firstobservation that air expandson heating? Can you now statewhat happens to the air in theboiling tube when it coolsdown?, SWINDSINDS, , STORMSAND CYCLONES8383