Transcript Factors Affecting Wind - School of Engineering

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Factors Affecting Wind
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We discussed the upward movement of air and its importance in cloud formation. We
now turn our attention to horizontal movement - the phenomenon we call wind.
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What causes air to move horizontally? Air flows from areas of higher pressure to areas
of lower pressure. Because unequal heating of Earth’s surface continually generates
these pressure differences, solar radiation is the ultimate driving force of wind.
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Wind is controlled by a combination of forces:
1) The pressure-gradient force (PGF)
2) The Coriolis force
3) Friction
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Once the air starts to move, the Coriolis force and friction come into play, but only to
modify the movement - not to produce it.
Pressure-Gradient Force: the force that generates wind
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The force that generates winds results from horizontal pressure differences.
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Variations in air pressure over Earth’s surface are determined from barometric readings
taken at hundreds of weather stations. These pressure data are shown on surface weather
maps by means of isobars. Isobars are lines connecting places of equal air pressure.
The spacing of the isobars indicates the amount of pressure change occurring over a
given distance and is expressed as the pressure gradient.
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Thus closely spaced isobars indicate a steep pressure gradient and strong winds; widely
spaced isobars indicate a weak pressure gradient and light winds.
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Note that the pressure gradient force is always directed at right angles to the isobars.
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The underlying cause of pressure differences observed on the daily weather map is
simply unequal heating of Earth’s land-sea surface.
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To illustrate how temperature differences can generate a pressure gradient and thereby
create winds, we look at a common example: the sea breeze.
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The Sea Breeze
Figure LT 6-10a shows a vertical cross section of a coastal
location just before sunrise. In the present dscussion we
assume that temperatures and pressures do not vary
horizontally at any level. This assumption is shown by the
horizontal pressure surfaces that indicate the equal pressure
at equal heights. Because there is no horizontal variation in
pressure (zero pressure gradient), there is no wind.
• After sunrise the unequal rates at which land and water heat
will initiate pressure differences and, therefore, airflow.
Surface temperatures over the ocean change only slightly
on a daily basis. On the other hand, land surfaces and the
air above can be substantially warmed during a single
daylight period.
• Heating the land warms the air. The air expands and “lifts”
the isobars. This warming does not by itself produce a
surface pressure change. However, a given pressure surface
aloft does become elevated over the land compared to over
the ocean. The resultant horizontal pressure gradient aloft
causes the air aloft to move from the land toward the ocean.
• The warmer air aloft moves over the ocean, cools and sinks.
This creates a surface high-pressure area over the ocean,
where the air is collecting. Now the pressure near the
ocean surface is greater than near the land surface. Thus a
horizontal pressure gradient is created that causes air near
the surface to move from the ocean to the land. (sea
breeze).
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Coriolis Force: How does the Earth’s rotation modify the Wind?
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Wind does not cross the isobars at right angles as the pressure-gradient force directs.
All free-moving objects, including wind, are deflected to the right of their path of
motion in the Northern Hemisphere and to the left of their path of motion in the
Southern Hemisphere.
The reason for this deflection is the Coriolis force: Fcoriolis  m2Ω  u
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where m is the mass and u is the velocity vector of a fluid parcel, and  is the rotation
vector of the Earth.
The magnitude of the Coriolis force is:
Fcoriolis  m 2 Ω u sin 
 mfu
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where  is the latitude, f = 2 sin is called the Coriolis parameter, and u is the
magnitude of the velocity.
The Coriolis force written in vector form clearly indicates that
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It is directed at right angles to the direction of air flow.
It affects only wind direction, not the wind speed.
Its magnitude is affected by wind speed (the stronger the wind, the greater the deflecting force).
Its magnitude increases from zero at the Equator to a maximum at the poles.
The Coriolis force thus has the effect of deflecting air flow. It also has the effect of
deflecting ocean currents.
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Coriolis effect.
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Coriolis deflection of winds blowing eastward.
Coriolis deflection of winds blowing in different directions:
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• Northern Hemisphere: All free-moving objects, including wind, are deflected to the
right of their path of motion.
• Southern Hemisphere: All free-moving objects, including wind, are deflected to the left
of their path of motion.
Geostrophic Wind
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A few kilometers above the surface the effect of friction is negligible, and thus the
Coriolis force is responsible for balancing the pressure-gradient force and thereby
directing airflow.
To illustrate, consider an air parcel initially at rest at ‘Starting point’ in Fig LT 6-13.
Since our parcel is at rest, the Coriolis force acting on it is zero; only the pressuregradient force acts on it. Under the influence of the pressure-gradient force, which is
always directed perpendicularly to the isobars, the parcel begins to accelerate directly
toward the area of low pressure. As soon as the flow begins, the Coriolis force starts to
act and causes a deflection to the right of the path of motion (since our parcel is in the
Northern Hemisphere). As the parcel continues to accelerate, the Coriolis force
intensifies. Thus, the increased speed results in further deflection. Eventually the wind
turns so that it is flowing parallel to the isobars. When this occurs the pressure-gradient
force and Coriolis force balance and the airflow is said to be in geostrophic balance.
Winds generated by this balance are called geostrophic winds.
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Curved Flow and the Gradient Wind
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Isobars are not generally straight; instead, they make broad sweeping curves and
occasionally connect to form roughly circular cells of either high or low pressure. Thus
winds around cells of high or low pressure follow curved paths in order to parallel the
isobars. Winds of this nature, which blow at a constant speed parallel to curved isobars,
are called gradient winds.
How do the Coriolis force and pressure-gradient force combine to produce gradient
winds?
Fig LT 6-15a shows the gradient flow around a centre of low pressure. As soon as the
flow begins, the Coriolis force causes the air to be deflected. In the Northern
Hemisphere the resulting wind blows counterclockwise about a low.
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Gradient Wind - continued…
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Because the Coriolis force deflects the winds to the left in the Southern Hemisphere, the
flow is reversed there - clockwise around low pressure centres and counterclockwise
around high pressure centres.
It is common practice to call all centres of low pressure cyclones and the flow around
the cyclonic. Cyclonic flow has the same direction of rotation as Earth:
counterclockwise in the Northern Hemisphere and clockwise in the Southern
Hemisphere.
Centres of high pressure are frequently called anticyclones and exhibit anticyclonic
flow (opposite that of Earth’s rotation).
Whenever isobars curve to form elongated regions of low and high pressure, these areas
are called troughs and ridges, respectively. The flow about a trough is cyclonic; the
flow around a ridge is anticyclonic.
Referring back to Fig LT 6-15a, we see that in flow about a low pressure centre, the
inward-directed pressure-gradient force is opposed by the outward-directed Coriolis
force plus an outward centrifugal force, which is generated by motion along a curved
path. Thus the pressure-gradient force must exceed the Coriolis force to keep the path
curved (parallel to the isobars) - that is, to produce the gradient wind.
The opposite situation exists in anticyclonic flows, where the inward-directed Coriolis
force must balance the combined pressure-gradient force and centrifugal force to
produce the gradient wind.
Despite the importance of the centrifugal force in establishing curved flow aloft, near
the surface, friction comes into play and greatly overshadows this much weaker force.
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Surface Winds
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Friction has an important effect on wind only within the first few kilometers of Earth’s
surface.
Friction acts to slow the movement of air. By slowing air movement, friction also
reduces the Coriolis force, which is proportional to wind speed. This alters the force
balance in favour of the pressure-gradient force with the outcome that there is a
movement of air at an angle across the isobars toward the area of lower pressure.
In a cyclone pressure decreases inward. Thus friction causes a net flow toward its
centre. Therefore, the resultant winds blow into and counterclockwise about a surface
cyclone.
In an anticyclone the oppose is true: the
pressure decreases outward and thus friction
causes a net flow away from the centre. Therefore,
the resultant winds blow outward and clockwise
about a surface anticyclone.
In whatever hemisphere, friction causes a net
inflow (convergence) around a cyclone and a
net outflow (divergence) around an anticyclone.
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How Winds Generate Vertical Air Flow
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First consider the situation around a surface low pressure system (cyclone) in which the
air is spiraling inward. Here the net inward transport of air causes a shrinking of the
area occupied by the air mass, a process called horizontal convergence. Whenever air
converges horizontally, it must pile up. That is, it must increase in height to allow for
the decreased area it now occupies.
This process generates a “taller” and therefore heavier air column. Thus low pressure
centres cause a net accumulation of air, which increases their pressure.
For a surface low to exist for very long, surface convergence must be maintained by
divergence aloft at a rate equal to the inflow below. This process is shown in Fig LT 619.
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Vertical Air Flow - continued...
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Divergence aloft can exceed surface convergence. If this happens, then the result is an
intensified surface inflow and accelerated vertical motion. Thus divergence aloft can
intensify storm centres as well as maintain them.
If surface convergence exceeds divergence aloft, then the result is as described above:
the surface flow “fills in” and weakens the cyclone.
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