Contents
The study of Physics Topics involves the exploration of matter, energy, and the forces that govern the universe.
Explain How High Speed Winds Are Accompanied By Reduced Air Pressure
Fast moving air creates a region of low pressure. Since moving air is called wind, therefore, we can also say that fast blowing wind creates a region of low pressure. Now, ‘fast wind’ means ‘high speed wind’ and ‘low pressure’ means ‘reduced air pressure’.
So, a yet another way of saying this is that high speed winds are accompanied by reduced air pressure. We will now describe some activities which will show that fast moving wind creates a region of low air pressure (or reduced air pressure).
Activity 5
Take two balloons of equal size. Fill a little water into both the balloons (to make them slightly heavier). Inflate both the balloons by filling air into them with mouth (or pump) and tie their mouths properly with strong threads. Hang the two inflated balloons about 10 cm apart on a stick with the help of threads tied to their mouths [see Figure (a)].
If we blow air (or wind) in the space between the two balloons from our mouth, we will see that the two balloons come closer [see Figure 10(b)], This can be explained as follows : When we blow air (or wind) between the balloons, then the fast moving air (or fast moving wind) creates a region of low air pressure in the space between the two balloons.
The air pressure on the outside of the balloons being higher, pushes the two balloons towards each other and makes them come closer. So, this activity shows that the fast moving air (or fast moving wind) creates a region of low air pressure (or reduced air pressure).
Activity 6
Take a piece of paper and crumple it to make a small ‘paper ball’. Hold an empty glass bottle on its side (horizontally) and place the small paper ball in the neck of the bottle just inside its mouth (see Figure). Let us now blow air into the bottle from our mouth.
When we blow air into the bottle, the paper ball kept in the neck of the bottle remains unaffected, the paper ball does not go inside the bottle (see Figure).
This can be explained as follows : When we blow air into the mouth of the bottle, then the air in the neck of bottle has high speed. This fast moving air reduces the air pressure in the neck of the bottle. The air pressure inside the bottle being higher, constantly pushes the paper ball out and does not allow it to go inside the bottle. So, this activity also shows that increased wind speed is accompanied by a reduced air pressure.
The fast moving air (or high speed wind) blowing over an object can also lift the object up by producing a region of low pressure above it. This will become clear from the following activity.
Activity 7
Hold a strip of paper about 20 cm long and 3 cm wide just below your lips and blow air over its surface from your mouth. We will find that the strip of paper lifts up (see Figure).
This can be explained as follows : When we blow air over the surface of paper strip, the fast moving air creates a region of low air pressure above the paper strip.The air pressure below the paper strip, being higher, pushes the paper strip upwards and lifts it up. This activity shows that fast moving air is accompanied by low air pressure (or reduced air pressure). In other words, increased wind speed is accompanied by a reduced air pressure.
We have just learnt that when air is blown over a strip of paper, then the strip of paper is lifted up’ (because the fast moving air reduces the pressure above the paper strip and higher air pressure acting from below the paper strip lifts it up).
If high speed winds blow over the roofs of houses, they will reduce the air pressure above the roofs. And if the roofs of houses are weak, then higher air pressure from below will lift up the roofs which can then be blown away by the fast winds. Thus, weak roofs of houses (like the tin roofs or thatched roofs) can be lifted and blown away by high speed winds during a wind-storm.
Wind Speed and Wind Direction
Wind speed and wind direction play an important role in the formation of storms (such as thunderstorms, cyclones and tornadoes). It is, therefore,mecessary to measure wind speed and wind direction at a place. The wind speed is measured by an instrument called anemometer (see Figure).
The most common type of anemometer consists of 3 or 4 cups mounted on a rod which can rotate freely. The cups catch the wind and rotate, The greater the speed of wind, the faster the cups rotate. At the base of the anemometer is a scale. The speed of wind (in kilometres per hour) is read from this scale. The anemometer is installed on a tall mast where the wind blows freely around it.
The wind can blow from different directions. The direction of wind at a place is found by using an instrument called wind vane (see Figure). We can find the direction from which wind is blowing by looking at the wind vane. The arrow of wind vane rotates freely. The arrow of wind vane moves until it points to the direction from which the wind is blowing. If the arrow points to the south, it shows that the wind is blowing from the south direction (see Figure).
High Speed Winds Can be Destructive
We have just studied that winds bring us rains. This is a useful part played by winds. But high speed winds (called strong winds) can also be destructive sometimes. High speed winds create certain situations in nature which create disasters and pose a threat to Houses, other buildings and standing crops are also destroyed in these disasters.
Wind plays an important role in the formation of thunderstorms, cyclones and tornadoes which can cause disasters. We will now discuss the formation of thunderstorms, cyclones and tornadoes, and their effects. Before we do that, we should know the meaning of the terms : storm, lightning and thunder. These are discussed below.
(i) Storm :
A period of violent weather with strong winds, and usually rain (or snow), is called a storm. A storm is the disturbed state of the earth’s atmosphere which affects the surface of earth and all the objects on it. Storm is called ‘toofari in Hindi.
(ii) lightning :
A natural flow of high voltage electricity within a cloud (or between a cloud and the earth) which appears as a bright flash of light in the sky, is called lightning. Please note that for a cloud to become electrically charged and produce lightning, its temperature must be close to the freezing point of water (which is 0°C). We will explain it further when we discuss the formation of a thunderstorm after a while. Lightning is usually accompanied by thunder.
(iii) Thunder :
The loud sound which is heard a little after a lightning flash is seen in the sky is called thunder. Thunder is produced by lightning as follows : The high voltage electric spark of lightning heats the air in the atmosphere to a very high temperature.
This extreme heating causes the air to expand at an ‘explosive rate’ producing a loud sound called thunder. Thus, it is lightning which causes thunder. As we will study after a while, lightning and thunder are the two characteristic features of a thunderstorm.