Chapter 6 Flashcards
Wind
air in motion that arises from a combination of forces
Wind speed is reported on U.S. weather maps in ____________
knots
Knots
One nautical mile per hour (equivalent to about 1.15 miles per hour, or 0.5 meters per second
If the wind speed is strong (greater than 15 knots) and highly variable, the weather report will include the __________
wind gust - the maximum observed wind speed
Wind direction
The direction from which the wind is blowing
Cyclones
Low-pressure centers
anticyclones
high-pressure systems
velocity
the magnitude and direction of its motion
speed
the distance traveled in a given amount of time
acceleration
a change in an object’s velocity - magnitude, direction, or both
Forces are characterized by:
direction and magnitude (or strength)
Net force or Resultant
The overall force that results from interacting forces can be expressed as a single force
Newton’s Second Law of Motion
A force exerted on an object (or a parcel of air) of a given mass causes the object to accelerate in the direction of the applied force
Force =
Mass x Acceleration
Acceleration =
Force / mass
5 different forces combine to move air:
the gravitational force, the pressure gradient force (PGF), the centrifugal force, the Coriolis force, and the frictional force
Gravitational force (GF)
Directed downward perpendicular to the ground and is approximately equal to the mass times the gravitational acceleration, 9.8 meters per second per second
pressure gradient
A change in presure over a distance
PGF always pushes from
higher pressure toward lower pressure
PGF =
(- 1 / Air density) x (change in pressure / distance)
When pressure changes rapidly over a small distance =>
PGF is large
The PGF always pushes directly from
higher pressure toward lower pressure
Isobaric maps are useful for portraying:
horizontal pressure gradients above the ground
The steeper the slope of the pressure surfaces =>
the greater the PGF because the pressure gradient is the change in pressure over distance
Centripetal acceleration
When an object changes its direction of motion, it is accelerating even if its speed does not change
CENTF =
V^2 / R
V = wind speed
R = radius of curvature of the curved path
The faster the speed and the tighter the curve of the path traveled (smaller R), the larger is the
centripetal acceleration
Coriolis force
Characteried by a turning or deflection of the direction of moving air
The Coriolis force deflects movement to +>
the right in the Northern Hemisphere
to the left in the Southern Hemisphere
Frictional force is caused by:
The flow of wind over the roughness of the Earth’s surface
What determine the magnitude of frictional force
The roughness of the surface and the speed of the wind
Sum of Forces =
Mass x Acceleration
When the vertical pressure gradient is equal to coriolis force (gravity) and push in opposite directions =>
hydrostatic balance exists
When the PGF is counterbalanced by the Coriolis force =>
geostrophic balance
Because the Coriolis force always pushes to the right of the wind in the Northern Hemisphere, this requries a wind in geostrophic balance =>
geostrophic wind
Buys Ballot’s Law
The “low pressure lies to the left of the wind” rule
Because air moves with low pressure on its left in the Northern Hemisphere =>
the wind must blow clockwise around highs and counterclockwise around lows
Winds on a weather map are strong where:
isobars or isoheights are close together
Winds on a weather map are weak where:
isobars or isoheights are far apart
Gradient balance
The three-way balance of horizontal pressure gradient, Coriolis force, and centrifugal force
The wind that results from gradient balance is called =>
gradient wind
Supergeostrophic flow
The wind speed, to which the Coriolis force is proportional - must be considerably higher than in the purely geostrophic case
For high and low pressure areas that have the same spacing of isobars or isoheights,, winds in gradient balance around a high will be:
stronger than winds around a low
