The Earth is surrounded by a cover of air called atmosphere. It extends to a few hundred kilometers above sea level. Just as certain sea creatures live at the bottom of ocean, we live at the bottom of a huge ocean of air. Air is a mixture of gases. The density of air in the atmosphere is not uniform. It decreases continuously as we go up.
What causes atmospheric pressure?
Atmospheric pressure acts in all directions. Soap bubbles expand till the pressure of air in them is equal to the atmospheric pressure. Why the soap bubbles so formed have spherical shapes? Can you conclude that the atmospheric pressure acts on a bubble equally in all directions?
A balloon expands as we fill air into it. In what direction does the balloon expands? The fact that atmosphere exerts pressure can be explained by the simple experiment.
Take an empty tin can with a lid. Open its cap and put some water in it. Place it over flame. Wait till water begins to boil and the steam expels the air out of the can. Now place the can under tap water. The can will squeeze due to atmospheric pressure. Why?
When the can is cooled by tap water, the steam in it condenses. As the steam changes into water, it leaves an empty space behind it. This lowers the pressure inside the can as compared to the atmospheric pressure outside the can . this will cause the direction to collapse from all directions. This experiment shows that atmosphere exerts pressure in all directions.
The fact can also be demonstrated by collapsing of an empty plastic bottle when air is sucked out of it.
Measuring Atmosphere Pressure
At sea level, the atmospheric pressure is about 101,300 Pa or 101,300 Nm-2. The instruments that measure atmospheric pressure are called barometers. One of the simple barometers is a mercury barometer. It consists of a glass tube 1 m long closed at one end. After filling it with mercury, it is inverted in a mercury trough. Mercury in the tube descends and stops at a certain height. The column of mercury held in the tube exerts pressure at its base. At sea level the height of mercury column above the mercury in the trough is found to be about 76 cm. Pressure exerted by 76 cm of mercury column is nearly 101,300 Nm-2 equal to atmospheric pressure. It is common to express atmospheric pressure in terms of the height of mercury column. As the atmospheric pressure at a place does not remains constant,hence, the height of mercury column also varies with atmospheric pressure.
Mercury is 13.6 times denser than water. Atmospheric pressure can hold vertical column of water about 13.6 times the height of mercury column at a place. Thus, the sea level, vertical height of water column would be 0.76 m 13.6=10.34 m. Thus, a glass tube more than 10 m long is required to make a water barometer.
Applications of atmospheric pressure
The atmospheric pressure decreases as we go up. The atmospheric pressure on mountains is lower than at sea level. At a height of about 30 km, the atmospheric pressure becomes only 7 mm of mercury which is approximately 100 Pa. It would become zero at an altitude where there is no air. Thus, we can determine the altitude of a place by knowing the atmospheric pressure at that place.
Atmospheric pressure may also indicate s change in the weather. On a hot day, the above the Earth becomes and expands. This causes a fall of atmospheric pressure in that regions. On the other hand, during cold chilly nights, this above the Earth cools down. This causes an increase in atmospheric pressure.
The changes in atmospheric pressure at a certain place indicate the expected changes in the weather conditions of that place. For example, a gradual and average drop in atmospheric pressure means a low pressure in a neighbouring locality. Minor but rapid fall in atmospheric pressure indicates s windy and showery condition in the nearby regions. A decrease in atmospheric pressure is accompanied by breeze and rain. Whereas a sudden fall in atmospheric pressure often followed by a steam, rain and typhoon to occur in few hours time.
One the other hand, an increasing atmospheric pressure with a decline later on predicts an intense whether conditions. A gradual large increase in the atmospheric pressure indicates a long spell of pleasant weather. A rapid increase in atmospheric pressure means that it will soon be followed by a decrease in the atmospheric pressure indicating poor weather ahead.