What is Atmospheric Pressure?

ATMOSPHERIC PRESSURE

Atmospheric Pressure: The earth is surrounded by a cover of air called atmospheric. It extend to 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 mixture of gasses. The density of air in the atmosphere is not uniform. It decreases continuously as we go up.

Atmospheric pressure acts in all directions. A balloon expand as we fill air into it. In what direction does the balloon expand? The fact that atmosphere exerts pressure can be explain by simple experiment.

EXPERIMENT 

Take an empty thin can with  a lid. Open its can and put some water in it. Place it over flame. Wait till water being to boil and the steam expels the air out of the can. Move it from the flame, close the can firmly by its cap. Know place the can under the 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 can to collapse from all direction. 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 ATMOSPHERIC PRESSURE

At sea level, the atmospheric pressure is about 101,300 Pa or 101,300 N/m2 . The instrument that measuring atmospheric pressure are called barometer. One of the simple barometer is mercury barometer. It consist of a glass tube 1m long closed at one end. After filling it with mercury, it is inverted in mercury through. Mercury in the tube descends and stooped at a certain height. The column of the mercury held in the tube exerts pressure as it base. At sea level the height of the 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 Pa equal to atmospheric pressure. It is common to express atmospheric pressure in terms of height of mercury column. As the atmospheric pressure at a place does not remain constant, hence, the height of mercury column also varies with atmospheric pressure.

        Mercury is 13.6 time is denser than water. Atmospheric pressure can hold vertical column of water about 13.6 times the height of mercury column at a place. Thus, at sea level, vertical  height of water column would be 0.76 m * 13.6 = 10.34 m. Thus a glass tube is more than 10 m long is required to make a water barometer.

VARIATION IN ATMOSPHERIC PRESSURE

         The atmospheric pressure decreases as we go up. The atmospheric pressure on mountain 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 1000 Pa. It would become zero at an attitude where there is no air. Thus, we can determine the attitude of place by knowing the atmospheric pressure at the place. Atmospheric pressure may also indicate a change in the weather. On a hot day, air above the Earth becomes hot and expends. This called a falls of atmospheric pressure in that region. 

         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 mean a low pressure in a neighbor locality. Minor but rapid fall in atmospheric pressure indicates a windy and showery condition in the nearby region. A decrease in atmospheric pressure is accompanied by breeze and rain. Whereas a sudden fall in atmospheric pressure often followed by storm, rain and typhoon to occur in few hours time.

        On the other hand, an increasing atmospheric pressure with a decline later on predicts an intense weather conditions. A gradual large increase in the atmospheric pressure indicates a long spell of pleasant weather. A rapid increase in the atmospheric pressure mean it will soon be followed by a decrease in the atmospheric pressure indicating a power weather ahead.