Transcript Wind - UK Ag Weather Center

Chapter 8
Wind and Weather
Driving Question
• What forces control the wind?
Wind
• Wind
– The local motion of air relative to the rotating
Earth
• Wind is measured using 2 characteristics
– Direction (wind sock)
• N, NNE, NE, ENE, E, ESE, SE, SSE, S, etc…
• Degrees: N = 360o, E = 90o, S = 180o, W = 270o
– Speed (anemometer)
What Causes Wind?
• Newton’s 2nd Law of Motion
– F = m*a (force = mass * acceleration)
• Air is at rest, what forces cause it to accelerate?
–
–
–
–
–
Pressure gradient force
Centripetal force
Coriolis effect
Friction
Gravity
Pressure Gradient Force (PGF)
• Same concept as 2nd Law of Thermodynamics
(heat flows from hot to cold objects to
eliminate temperature gradient)
• Horizontal pressures are not equal – therefore
there is a gradient
• PGF
– The force that causes air parcels to move as a
consequence of an air pressure gradient
– Wind is greater where pressure gradient is larger
Centripetal Force
• Center seeking force
• The net force is directed inward  toward
the center of the orbit and perpendicular to
the direction of motion
• This force operates when an air parcel
follows a curved path
• It is NOT and independent force – it is the
result of the imbalance of other forces
• Causes a change in direction, but not speed
Coriolis Effect
• It is a net force responsible for curved motion
due to changing the coordinate system from nonrotating to rotating
• Deflects winds to the right (left) in the NH (SH)
• Coriolis is dependent on latitude
– No deflection at equator/Max deflection at poles
• Coriolis is dependent on wind speed and spatial
scale (size and distance)
– Coriolis does not affect the direction of toilet flushes
Friction
• Friction: the resistance that an object or
medium encounters as it moves in contact with
another object or medium
• Friction acts opposite to the wind direction
• Friction increases with increasing surface
roughness
– Greater over a forest than a soybean field
• Friction slows horizontal winds in the lowest
kilometer
Gravity
• Force of gravity is 9.8m/s2
• Always directed downward
– Does not modify horizontal winds
Joining Forces
• These 5 forces all interact to govern the
direction and speed of the wind
• These interactions result in 4 cases
–
–
–
–
Hydrostatic Equilibrium
Geostrophic Wind
Gradient Wind
Surface Winds
Hydrostatic Equilibrium
• Balance of the vertical
pressure gradient force
and gravity
• Since a balance is in
place the net
acceleration is zero
• Parcels that are
moving move at a
constant speed
Geostrophic Wind
• Unaccelerated
(constant speed)
horizontal wind
• Balance between
coriolis effect and
horizontal pressure
gradient
• Only develops in large
scale systems
• Frictionless
Gradient Wind
• Similar to geostrophic wind in that it is a large
scale, horizontal, frictionless wind that blows
parallel to the isobars
• The difference is that the path of the gradient wind
is curved
• Forces are not balanced
– There is a net centripetal force
• Develops around highs and lows
Surface Winds
• Friction at the surface affects speed and direction
of wind
• Friction acts directly opposite the wind direction
• Friction slows wind speed, which weakens the
Coriolis effect and affects balance with the
horizontal pressure gradient force causing winds
to blow towards low pressure
• Friction loses influence with height
Continuity of Wind
• There is a link
between the
horizontal and
vertical
components of
the wind
• Vertical motions
can be induced
by downwind
changes in
surface
roughness
Scales of Weather Systems
Circulation
Space Scale Time Scale
Example
Planetary
10,00040,000km
Weeks to
months
Trade winds
Synoptic
10010,000km
Days to
week
Hurricanes,
air masses
Mesoscale
1-100km
Hours to day T-storms
Microscale
1m-1km
Seconds to
hour
Weak
tornado