INSOLATION (Solar Radiation)_02

(b) Heating and cooling of the Atmosphere

Sun is the ultimate source of atmospheric heat and energy, but its effect
is not direct. For example, as we climb a mountain or ascend in the
atmosphere, temperature become steadily lower, rather than higher, as
we might expect. This is because the mechanism of heating the
atmosphere in not simple. There are four heating processes directly
responsible for heating the atmosphere. They are :
(i) Radiation
 (ii)Conduction
 (iii) Convection and
(iv) Advection.

(i) Radiation : Radiation is the process by which solar energy reaches
the earth and the earth loses energy to outer space. When the source
of heat transmits heat directly to an object through heat waves, it is known as radiation process. In this process, heat travels through the empty space. The vast amount of heat energy coming to and
leaving the earth is in the form of radiation. The following facts
about radiation are worth noting.
(i) All objects whether hot or cold emit radiant energy continuously.

(ii) Hotter objects radiate more energy per unit area than colder objects.

(iii) Temperature of an object determines the waves length of radiation.

Temperature and wave length are inversely related. Hotter the object
shorter is the length of the wave.

(iv) Insolation reaches the earth’s surface in short waves and heat is
radiated from the earth in long waves.

You will be amused to know that atmosphere is transparent to short
waves and opaque to long waves. Hence energy leaving the earth’s
surface i.e. terrestrial radiation heats up the atmosphere more than the
incoming solar radiation i.e. insolation.
(ii) Conduction: When two objects of unequal temperature come in contact
with each other, heat energy flow from the warmer object to the cooler
object and this process of heat transfer is known as conduction. The
flow continues till temperature of both the objects becomes equal or
the contact is broken. The conduction in the atmosphere occurs at zone
of contact between the atmosphere and the earth’s surface. However,
this is a minor method of heat transfer in terms of warming the
atmosphere since it only affects the air close to the earth’s surface.
(iii) Convection: Transfer of heat by movement of a mass or substance from
one place to another, generally vertical, is called convection. The air of
the lower layers of the atmosphere get heated either by the earth’s
radiation or by conduction. The heating of the air leads to its expansion.
Its density decreases and it moves upwards. Continuous ascent of heated
air creates vacuum in the lower layers of the atmosphere. As a
consequence, cooler air comes down to fill the vacuum, leading to
convection. The cyclic movement associated with the convectional
process in the atmosphere transfer heat from the lower layer to the upper
layer and heats up the atmosphere.
(iv) Advection: Winds carry the temperature of one place to another. The
temperature of a place will rise if it lies on the path of winds coming
from warmer regions. The temperature will fall if the place lies on the
path of the winds blowing from cold regions. This process of horizontal
transport of heat by winds is known as advection. MORE