Midterm #1 on Thursday!!
- Bring your catcard
~ 20 questions:- short answer, multi-choice, and problems
- covers all lectures through today
- covers chapters 1 – 4 from textbook
You also need:- calculator (non-programmable)
- grey-lead pencils and eraser
Closed exam – no textbook or notes
Midterm #1 on Thursday!!
Tools for Review:- review problem set and solutions available on d2l
- also solutions to homeworks 1 and 2
- also review problems on reading list
NATS 101
Lecture 7
Wind Balance
(Chapter 4)
Winds in the Upper Atmosphere:-
1. Pressure Gradient Force
2. Coriolis Force
3. Friction
Winds in the Upper Atmosphere:-
PGF
L
CF
H
PGF
CF
Winds in the Upper Atmosphere:1. Geostrophic Balance:- When the PGF and the Coriolis force are in balance, the wind flows
parallel to the height contours at a constant speed with no change of
direction (i.e., no net acceleration)
PGF
L
H
L
PGF
CF
CF
H
Winds in the Upper Atmosphere:-
1. Geostrophic Balance:NH
L
H
- This non-accelerating flow is called the Geostrophic flow (or
geostrophic wind), and occurs when the PGF and Coriolis force are
equal and opposite.
1. Geostrophic Balance:- In the Northern Hemisphere, the geostrophic wind blows with lower
pressure to its left and higher pressure to its right.
NH
L
H
SH
H
L
- In the Southern Hemisphere, the geostrophic wind blows with lower
pressure to its right and higher pressure to its left because the
coriolis force is in an opposite direction.
Geostrophic Balance
- The velocity (or speed) of the geostrophic wind is directly related to
the strength of the PGF.
- The weaker the PGF, the weaker the geostrophic wind.
- The stronger the PGF, the stronger the geostrophic wind.
- Geostrophic balance occurs
when isobars are straight and
parallel to each other.
Gradient Balance
- Recall that in order for an object to change speed OR change
direction it must have a “net force” actign on it.
In Geostrophic balance there is no “net force” the PGF exactly
balances the Coriolis force.
When the isobars are NOT
straight and parallel
i.e., when they are curved, in
order for the wind to still flow
parallel to the isobars, it must
change DIRECTION!
Gradient Balance:- In order for the flow to follow
curved contours, it must be
constantly changing direction.
L
H
- To be changing direction,
there must be a net
acceleration and a net force
acting on the flow.
- there must be a continual mismatch between the pressure
gradient and coriolis forces to cause a direction change.
- the flow cannot be in geostrophic balance in this situation.
- instead, it is in “gradient balance”.
Gradient Balance:- When this mismatch between
the PGF and CF occurs so that
the air can flow parallel to
curving isobars, it is known as
gradient flow or gradient
wind.
- Gradient flow develops only in the absence of friction i.e., at
upper-levels away from the surface.
- Geostrophic flow (balance) is a special case of gradient flow
(balance) arising if the contours happen to be straight and parallel.
Gradient Wind:1. Flow around high pressure - Supergeostrophic Flow
In a high pressure system, there is high pressure at the center of the
curved contour and low pressure on the outside.
The air motion at an instant in time is
tangential to the isobar
L
PGF L
L
L
PGF
CF
L
CF
L
H
L
L
The PGF is pointed away from the center
of the curved contour (toward the low
pressure).
The CF is pointing into the center of the
curved contour (at right angles to the air
motion.
Gradient Wind:1. Flow around high pressure - Supergeostrophic Flow
Now, if the air is following the curved isobar, it is constantly changing
direction away from the low pressure and toward the high pressure. Thus, it
must be continuously accelerating toward the High pressure.
There must be a net force on the air
toward High pressure.
L
L
PGF
PGF
L
Thus, the CF must be greater than the
PGF.
CF
CF
L
L
H
L
L
L
The net difference between the two is
known as the inward-directed
centripetal force - Ce
Gradient Wind:1. Flow around high pressure - Supergeostrophic Flow
Recall that the CF is proportional to the wind
speed.
L
L
PGF
PGF
CF
CF
L
L
L
H
L
L
For the CF to be greater than the PGF, the
wind speed must be greater than that under
the same geostrophic conditions (i.e., same
pressure gradient, but straight isobars).
L
Such flow is known as supergeostrophic flow.
Gradient Wind:2. Flow around low pressure - Subgeostrophic Flow
- In the case of a low pressure system there is low pressure at the
center of the curved contour and high pressure on the outside.
H
H
CF
CF
PGF
PGF
H
H
H
H
L
H
H
The PGF is pointed in to the center of the curved contour (toward the
low pressure), and the CF is pointing out from the center of the curved
contour.
Gradient Wind:2. Flow around low pressure - Subgeostrophic Flow
For the air to follow the curved contour, it must be continuously
accelerating toward the LOW pressure because it is constantly
changing direction.
CF
NH
CF
PGF
PGF
H
H
The pressure gradient force must be greater
than the Coriolis force.
Again, the net difference is known as the
inward pointing centripetal force, Ce
L
H
For the CF to be less than the PGF, the wind speed must be less than
that under the same geostrophic conditions. Such flow is known as
subgeostrophic flow.
 Two types of Balance for Upper-level Winds:Gradient Flow: three-way balance among PGF, CF, and Ce.
 Wind direction is parallel to curving isobars.
Geostrophic Flow (special case of gradient flow): isobars straight and parallel.
 PGF and CF exactly cancel each other.
 Wind direction is parallel to isobars.
Flow around low pressure
NORTHERN HEMISPHERE
Counterclockwise flow
SOUTHERN HEMISPHERE
Clockwise flow
(because Coriolis force reverses
with respect to wind direction)
There is another force balance possibility if the Coriolis
force is very small or zero,
so it’s negligible.
In that case, the pressure gradient force would
balance the centripetal force.
Cyclostrophic Balance
PGF + centripetal force = 0
OR
PGF = Centrifugal force
L
Pressure
gradient
force
Centrifugal
force
Pressure gradient balances
the centrifugal force.
Occurs where flow is on a
small enough scale where
the Coriolis force becomes
negligible.
Important in (the really cool) meteorological phenomena that have
really strong winds and tight pressure gradients!
TORNADOES
Examples of
Cyclostrophic Flow
Inner-core of HURRICANES
And the
flushing
toilet, too!!
The Great Mystery
of the
Flushing Toilet Solved!
PGF
Centrifugal
force
To Bart and Lisa: “A swirling, flushing toilet is in cyclostrophic
balance, so the way it flushes depends more on the shape of the
drain—and nothing to do with whether you’re in Australia or not!”
Near-surface winds:- the effects of friction
- Friction makes winds near the surface
slower than those in the middle or upper
atmosphere, given equal pressure
gradients.
The lower wind speed reduces the Coriolis force, thus the flow
cannot be gradient or geostrophic near the surface.
Friction
Because the CF is reduced, the winds do not flow parallel to the
isobars in the boundary layer, but cross them at an angle as they flow
from high to low pressure.
NH
The deflection is to the right in the NH, and to
the left in the SH.
i.e. the direction of deflection due to Coriolis
is unchanged – just the amount of deflection
is less than in the upper atmosphere …
SH
Friction
- For high pressure systems at the
surface, friction slows the wind down
so that CF does not act as strongly.
 cross-isobar flow outward from the
center of the high (divergence)
Boundary layer
Upper levels
- For low pressure systems at the
surface, friction slows the wind down
so that CF does not act as strongly.
 cross-isobar flow into the center of
the low (convergence)
Boundary layer
Upper levels
Cyclones, anticyclones, troughs, and ridges
1. High pressure systems:are areas of enclosed high pressure marked by “roughly” circular
isobars or height contours – called anticyclones.
- The wind rotates clockwise around
anticyclones in the NH as Coriolis
deflects the air to the right and the
PGF directs it outward.
Boundary layer
Upper levels
1. High Pressure Systems:-
- In the boundary layer, the air spirals out of anticyclones because of
frictional effects, while in the upper atmosphere it flows parallel to the
height contours (gradient balance).
- Because the air in the boundary layer is
diverging in anticyclones, this surface air
is being constantly replaced by air from
above it  sinking air.
Boundary layer
Upper levels
H
1. High Pressure Systems:-
Large areas of subsidence inhibits cloud formation and causes
warming by compression  Anticyclones typically exhibit clear skies
and fair weather because of this large-scale subsidence.
The divergence at the surface also causes the pressure isobars to
spread apart  as a general rule anticyclones are bigger than
cyclones, and have weaker pressure gradients (and weaker winds).
2. Low pressure systems:are areas of enclosed low pressure marked by “roughly” circular
isobars or height contours – call cyclones.
- The wind rotates anticlockwise
(counterclockwise) around cyclones
in the NH as Coriolis deflects the air to
the right and the PGF directs it inward.
Boundary layer
Upper levels
2. Low Pressure Systems:-
- In the boundary layer, the air spirals in to cyclones because of
frictional effects, while in the upper atmosphere it flows parallel to the
height contours (gradient balance).
- Because the air in the boundary layer is
converging in cyclones, this surface air is
being forced upwards  rising air.
Boundary layer
Upper levels
L
2. Low Pressure Systems:-
Areas of rising air encourages cloud formation  Cyclones typically
exhibit cloudy skies and often precipitation because of this organized
surface convergence.
The convergence at the surface also causes the pressure isobars to
contract in  as a general rule cyclones are smaller than
anticyclones, and have stronger pressure gradients (and stronger
winds).
L
H
Winds at the surface…
Low pressure
systems:Closed isobar around
low pressure
Winds turn
anticlockwise around
low pressure in the NH
Winds cross isobars
moving toward the low pressure (converging) because of the effects of
friction
Isobars are spaced more closely together and thus winds tend to be
stronger in a low compared to a high pressure system.
Lows have a tendency to be smaller than highs
Winds at the surface…
High pressure
systems:Closed isobar around
high pressure
Winds turn clockwise
around high pressure
in the NH
Winds cross isobars moving away from high pressure toward low
pressure (diverging) because of the effects of friction
Isobars are spaced more widely apart and thus winds tend to be weaker
in a high compared to a low pressure system.
Highs have a tendency to be bigger than lows
3. Troughs and Ridges:-
Many pressure systems occur not as closed cells, but as elongated
“waves” called troughs (low pressure) and ridges (high pressure).
3. Troughs and Ridges:Generally we see cyclones and anticyclones at the surface that
gradually give way to troughs and ridges in the upper atmosphere. This
is partly because of friction at the surface.
300 mb
500 mb
700 mb
850 mb
Winds in the upper atmosphere…
Troughs:- Winds turn
anticlockwise around
troughs in the NH
- Winds follow isobars
(gradient balance)
Ridges:- Winds turn clockwise around ridges in the NH
- Winds follow isobars (gradient balance)
Measuring Wind:-
Measure wind direction and speed
northerly wind is from the north (0°/360°)
easterly wind is from the east (90°)
southerly wind is from the south (180°)
westerly wind is from the west (270°)
Measuring Wind:At the surface:Wind direction is measured with a wind vane
Wind speed is measured with an anemometer –
rotating cups on a shaft that catch the wind and
generate an electric current – the strength of the
current is proportional to the strength of the wind.
The two can be combined to give both direction and speed – an
aerovane
Measuring Wind:At upper levels:- rawinsondes attached to balloons
launched twice daily are tracked by
GPS to give upper-level winds. They
also carry instrument packages
including temperature and humidity
sensors
Upper-level Wind:The balloons continue to rise until they burst. It is possible to
get a second profile as the instrument package descends
(rapidly!!) back to Earth,
Upper-level Wind:-
The same instrument package can
be dropped out of the belly of of
an airplane with a parachute
attached to limit fall-speed
dropwindsonde