CHAPTER 6
AIR PRESSURE
AND WINDS
Understanding Air Pressure
- Air pressure is a very abstract
term. We cannot actually see it
or touch it.
--- It is the force exerted against a
surface by the continuous collision
of gas molecules
- We can see its secondary effects
--- storms; surge; inversion & smog
Understanding Air Pressure, cont
- Pressure is important in the
mechanics of weather
- We are generally aware of it for its
results – particularly as the driving
force of the winds
--- simplified, but… it is the uneven
heating of the Earth’s surface that
produces pressure differences,
and therefore the winds
Understanding Air Pressure, cont
- Differences in pressure are generally
the result of changes in:
(1) density
(2) temperature
--- closely related concepts as temperature
change will produce pressure change
through increased molecular motion
… temperature up / pressure
down (real world)
… temperature up / pressure up
(closed container)
Understanding Air Pressure, cont
- Earth atmosphere adds:
(3) altitude… creates
difference in air pressure
- In the atmosphere, the pressure at
any altitude is equal to the “weight”
of the air directly above that point
- This numerical change in
pressure is not constant as
altitude increases (1/2n per 5.6k)
Understanding Air Pressure, cont
- Associated with the pressure-altitude
relationship is related Dalton’s Law
--- total pressure of a gas is equal to the
sum of the partial pressures of the
individual gases chemically
--- partial pressure of a gas is the pressure
it would exert at the same temperature
as the mixture if it alone occupied the
volume that the mixture does
--- partial pressure will equal ATA for an
altitude
Measuring Air Pressure
-
Atmospheric pressure is measured from sea
level and is expressed in terms of a
standard atmosphere or atmospheres
absolute (ATA)
[I have problems with concept of a standard
atmosphere]
- General public knows it as inches of
mercury
- Meteorologists generally measure it in
millibars
- Other measures include: atmospheres,
dynes, newtons, pascals
Measuring Air Pressure, cont
- 1 standard atmosphere:
29.92 in of mercury
1013.25 millibars
101,325 newtons
- Atmospheric pressure differs
throughout the world according to
temperature, elevation or altitude,
etc… to be compared these pressure
readings must be mathematically
“corrected”
Factors Affecting Wind
- Wind (advection)- horizontal
movement of air… nature’s
attempt to equalize air pressure
--- why:
difference in horizontal air
pressure between locations
… air flow from high to low
pressure
… unequal global heating
produces
Factors Affecting Wind, cont
-
Factors and causes of horizontal
motion:
(a) thermal – temperature
induced; “expansion” and
“subsidence” of air
… convection cell
(b) dynamic – motion related;
related to the motion of the
Earth
Forces Affecting Air flow and
Direction
(1)
Pressure Gradient Force – rate of
pressure change across spatial
distance
… pressure differences must
create a force in order to
drive the wind - PGF
… pressure patterns are illustrated
by isobar change
… magnitude: from isobar spacing
… direction: perpendicular to
isobars
Forces Affecting Air flow and
Direction, cont
- The closer the spacing of the isobars,
the stronger the PGF
- The stronger the PGF, the stronger
the wind
- Because of the unequal heating
between the Tropics and the Poles, a
PGF is created (thermal / global
convection cell)
- If the Earth did not rotate (dynamic)
PGF would be the only force driving
wind
Forces Affecting Air flow and
Direction, cont
(2) Friction – as soon as air flow
begins, it is affected by contact
with the surface of the Earth…
negligible above a few kms
… may not consider friction to be a
force, but is a significant
constraint to air flow
… the rougher the terrain, the
greater the frictional force
Forces Affecting Air flow and
Direction, cont
… direction: 180o to air flow
… magnitude: affected by
surface characteristics and
speed of air flow
… as frictional force decreases
wind speed it (a) decreases
Coriolis Effect, (b) does not
effect PGF
Forces Affecting Air flow and
Direction, cont
(3) Coriolis Effect- (CF)“apparent”
force deflecting items that
move latitudinally on the
Earth
… deflected to the right
(north) and to the left
(south) hemisphere
Forces Affecting Air flow and
Direction, cont
… direction: 90o to wind flow
… magnitude: directly
proportional to wind speed
and latitude
Forces Affecting Air flow and
Direction, cont
-
For high altitude winds, CF turns
airflow until winds are blowing
parallel to isobars, effectively
balancing PGF (Fig 6.15)
… termed gradient winds or
geostrophic winds
(“turned by the Earth”)
… lack of friction allows faster
winds
Forces Affecting Air flow and
Direction, cont
- For surface winds: Friction
weakens CF, and PGF is
strengthened, winds blow across
isobars at approx 30o
… atmosphere influenced by
friction is the planetary
boundary layer
Forces Affecting Air flow and
Direction, cont
Influence of CF, PGF and friction result in
patterns of air flow around high and low
pressure (Fig 6.17)
- Around a low, PGF attempts to pull air
inward. It is balanced by CF to curve in a
counter-clockwise flow in the northern
hemisphere
- Around a high, PGF attempts to pull air
outward. It is balanced by CF to curve in a
clockwise flow in the northern hemisphere
- Within the planetary boundary layer friction
reduces CF and PGF “spirals” air