AGE 101 LECTURE SERIES
The Accretion process occurs in clouds containing a mixture of water droplets and ice particles. Snow
grains, ice pallets or hail are formed as droplets fuse on ice particles. This takes place in clouds of –
100 C to – 300 C. It takes place where there is large liquid water content. In this kind of process, rainfall
may cease to occur even when raindrops have grown large enough to come down gravitationally
because the occurrence of the rain depends on the changing air conditions from the surface. For
example, if temperature rises considerably near the earth’s surface and the air become unsaturated,
the snow flakes found in that particular cloud may melt to raindrops. These raindrops may evaporate
in drier air.
Atmospheric Motion
The atmospheric system is characterized by a ceaseless air motion on a greater variety of spatial and
temporal scales. These movements range from minor short-lived events of a few square centimeters
or less to a major seasonal motions encompassing a continental extent or greater. Air mass moves
both horizontally and vertically. There are various factors that that control air motion. These are:
pressure gradient force, coriolis force, frictional force, geostrophic winds and centripetal
acceleration. Together these factors are referred to as the laws of horizontal motion.
The Pressure Gradient Force (Pf)
This is the amount of pressure change occurring over a given distance. It has vertical and horizontal
components but the vertical component is more or less in balance with the force of gravity. The force
that generates wind results from the horizontal component of the Pf. Thus horizontal difference in
pressure due to thermal or mechanical causes results in horizontal air motion. When air is subjected
to greater pressure on one side than on the other, the imbalance produces a force that is directed
from regions of high pressure to towards regions of low pressure. It is the pressure differences that
cause wind to blow and the greater the differences the higher the wind speed. The underlying cause
of these pressure differences is simply unequal heating of the earth’s land–sea surfaces by the sun.
Temperature variations creates pressure differences and hence the wind. The greater the
temperature differences, the stronger the pressure gradient and the resultant winds. Therefore,
horizontal pressure can generate winds of gale intensity. The spacing of isobars indicates the amount
of pressure change occurring over a given distance and it is expressed as the pressure gradient. Thus,
closely spaced isobars indicate a steeper pressure gradient and high wind speed and vice-versa.
3
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, AGE 101 LECTURE SERIES
Pressure gradient force is always directed at right angles to the isobars and has both magnitude and
direction.
The Coriolis Force
This is the deflective force of earth’s rotation of all free moving objects, including the atmosphere
and ocean. Wind does not cross isobars at right angles as pressure gradient force directs. There is a
deviation as a result of earth’s rotation and has been named as the coriolis effect. All free moving
objects, including wind, are deflected to the right of their path of motion in the northern hemisphere
and to the left in the southern hemisphere. The magnitude of the deflective force is dependent on the
latitude; it is strongest at the poles and decrease to zero at the equator. This is because it work is in
proportion to the sine of latitude. Thus, the deflection is affected by the angle of the latitude. Since
equator lies at zero degrees and the sine is zero, there is no deflection. At the poles where the angle
is 90 degrees and the sine of 900 is one the deflection is at the maximum. As you move away from the
equator towards high latitudes Coriolis force increases in intensity. The amount of coriolis deflection
increases with increase in wind speed. This results because faster winds cover greater distances than
does the slower winds in the same period. The force is responsible for the present arrangement of
the planetary wind system. The effect of the rotation of the earth diverts the wind into gigantic
whirling systems, or vortices, that are aligned more or less in latitudinal belts. The resulting zonal
flow patterns have prevailing winds with strong easterly and westerly components. Thus the Coriolis
force accounts for the easterly and westerly components of the wind. On a non-rotating earth, all
types of wind would be straight. Thus the wins could not have been labeled because they could be
straight.
The Friction
Whenever air moves relatively close to the earth’s land–water surface, frictional drag tends to slow
the air and alter the direction of the movement. The force of friction is greatest over rough surfaces.
It is common to experience strong winds over the sea or the prairie but much weaker winds in the
forest. The effect of friction is mostly pronounced near the surface; it becomes relatively insignificant
at elevation greater than 600m above the earth’s surface. The layer of frictional influence is known
as the planetary boundary layer. However, this height varies from time to time. On clear days, when
the surface of the earth is strongly heated by the sun, the turbulent mixing of the air in the lower
atmosphere slows the air aloft while the movement near the earth surface is speed up. In this case
4
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The Accretion process occurs in clouds containing a mixture of water droplets and ice particles. Snow
grains, ice pallets or hail are formed as droplets fuse on ice particles. This takes place in clouds of –
100 C to – 300 C. It takes place where there is large liquid water content. In this kind of process, rainfall
may cease to occur even when raindrops have grown large enough to come down gravitationally
because the occurrence of the rain depends on the changing air conditions from the surface. For
example, if temperature rises considerably near the earth’s surface and the air become unsaturated,
the snow flakes found in that particular cloud may melt to raindrops. These raindrops may evaporate
in drier air.
Atmospheric Motion
The atmospheric system is characterized by a ceaseless air motion on a greater variety of spatial and
temporal scales. These movements range from minor short-lived events of a few square centimeters
or less to a major seasonal motions encompassing a continental extent or greater. Air mass moves
both horizontally and vertically. There are various factors that that control air motion. These are:
pressure gradient force, coriolis force, frictional force, geostrophic winds and centripetal
acceleration. Together these factors are referred to as the laws of horizontal motion.
The Pressure Gradient Force (Pf)
This is the amount of pressure change occurring over a given distance. It has vertical and horizontal
components but the vertical component is more or less in balance with the force of gravity. The force
that generates wind results from the horizontal component of the Pf. Thus horizontal difference in
pressure due to thermal or mechanical causes results in horizontal air motion. When air is subjected
to greater pressure on one side than on the other, the imbalance produces a force that is directed
from regions of high pressure to towards regions of low pressure. It is the pressure differences that
cause wind to blow and the greater the differences the higher the wind speed. The underlying cause
of these pressure differences is simply unequal heating of the earth’s land–sea surfaces by the sun.
Temperature variations creates pressure differences and hence the wind. The greater the
temperature differences, the stronger the pressure gradient and the resultant winds. Therefore,
horizontal pressure can generate winds of gale intensity. The spacing of isobars indicates the amount
of pressure change occurring over a given distance and it is expressed as the pressure gradient. Thus,
closely spaced isobars indicate a steeper pressure gradient and high wind speed and vice-versa.
3
This study source was downloaded by 100000889810302 from CourseHero.com on 10-01-2025 12:53:03 GMT -05:00
https://www.coursehero.com/file/251788852/Condensation-LECTURE-SERIES-AGE-101-pages-2pdf/
, AGE 101 LECTURE SERIES
Pressure gradient force is always directed at right angles to the isobars and has both magnitude and
direction.
The Coriolis Force
This is the deflective force of earth’s rotation of all free moving objects, including the atmosphere
and ocean. Wind does not cross isobars at right angles as pressure gradient force directs. There is a
deviation as a result of earth’s rotation and has been named as the coriolis effect. All free moving
objects, including wind, are deflected to the right of their path of motion in the northern hemisphere
and to the left in the southern hemisphere. The magnitude of the deflective force is dependent on the
latitude; it is strongest at the poles and decrease to zero at the equator. This is because it work is in
proportion to the sine of latitude. Thus, the deflection is affected by the angle of the latitude. Since
equator lies at zero degrees and the sine is zero, there is no deflection. At the poles where the angle
is 90 degrees and the sine of 900 is one the deflection is at the maximum. As you move away from the
equator towards high latitudes Coriolis force increases in intensity. The amount of coriolis deflection
increases with increase in wind speed. This results because faster winds cover greater distances than
does the slower winds in the same period. The force is responsible for the present arrangement of
the planetary wind system. The effect of the rotation of the earth diverts the wind into gigantic
whirling systems, or vortices, that are aligned more or less in latitudinal belts. The resulting zonal
flow patterns have prevailing winds with strong easterly and westerly components. Thus the Coriolis
force accounts for the easterly and westerly components of the wind. On a non-rotating earth, all
types of wind would be straight. Thus the wins could not have been labeled because they could be
straight.
The Friction
Whenever air moves relatively close to the earth’s land–water surface, frictional drag tends to slow
the air and alter the direction of the movement. The force of friction is greatest over rough surfaces.
It is common to experience strong winds over the sea or the prairie but much weaker winds in the
forest. The effect of friction is mostly pronounced near the surface; it becomes relatively insignificant
at elevation greater than 600m above the earth’s surface. The layer of frictional influence is known
as the planetary boundary layer. However, this height varies from time to time. On clear days, when
the surface of the earth is strongly heated by the sun, the turbulent mixing of the air in the lower
atmosphere slows the air aloft while the movement near the earth surface is speed up. In this case
4
This study source was downloaded by 100000889810302 from CourseHero.com on 10-01-2025 12:53:03 GMT -05:00
https://www.coursehero.com/file/251788852/Condensation-LECTURE-SERIES-AGE-101-pages-2pdf/
