Physics - Heat & Thermo Question with Solution | TestHub
THE ATMOSPHERIC LAPSE RATE
For small volumes of gas, according to KINETIC THEORY OF GASES, all parts of the gas are at the same temperature. But for huge volumes of gas like atmosphere, assumption of a uniform temperature throughout the gas is not valid. Different parts of the atmosphere are at different temperature. Apart from the surface of the earth, variations also occur in temperature at different heights in the atmosphere.
The decrease in temperature with height called the ATMOSPHERIC LAPSE RATE is similar at various locations across the surface of the Earth. By analyzing the data collected at various locations, it is found that average global lapse rate is –6.7 °C/km.
The linear decrease with temperature only occurs in the lower part of the atmosphere called the TROPOSPHERE. This is the part of the atmosphere in which weather variation occurs and our planes fly. Above the troposphere is the stratosphere, with an imaginary boundary separating the two layers. In the stratosphere, temperature tends to be relatively constant.
Absorption of sunlight at the Earth's surface warms the troposphere from below, so vertical convection currents are continually mixing in the air. As a parcel of air rises, its pressure drops and it expands. The parcel does work on its surrounding, so that its internal energy and, therefore, its temperature drops. Assume that the vertical mixing is so rapid as to be adiabatic and the quantity T·P^((1–γ)/γ) has a uniform value through the layers of troposphere.
Mechanical equilibrium of the atmosphere requires that the pressure decreases with altitude according to ( mean atmospheric density; mean molecular weight of air; gravitational acceleration and Universal gas constant) Assume free fall acceleration to be uniform throughout, if value of acceleration due to gravity on earth would have been larger than then atmospheric lapse rate would have been
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or . The lapse rate ; a larger g gives a larger lapse rate.
