Download 11 October

Newton’s Laws of Motion
An object at rest will remain at rest and
an object in motion will remain in
motion as long as no force is applied to
the object.
The force exerted on an object equals
its mass times the acceleration
produced.
Acceleration: speeding up, slowing down,
change of direction of an object.
Newton’s Laws of Motion
Newton’s Law of Motion
Forces affecting speed and direction of
wind
Pressure gradient force
Coriolis effect
Friction
Forces that Influence Winds
Pressure Gradient Force
Difference in pressure over distance
• Horizontal gradient is smaller than vertical one
Directed perpendicular to isobars from high
to low
• Close isobars = strong gradient
Large (small) change in pressure over short
distance is a strong (weak) pressure
gradient
The force that causes the wind to blow
• Wind speed increase with gradient
Pressure gradient force
Pressure gradient force
Pressure gradient force
Forces that Influence Winds
Coriolis Force
Apparent deflection due to rotation of the
Earth
• Right turn in northern hemisphere and left turn
in southern hemisphere
Only influence direction, not speed
• Stronger wind = greater deflection
• Closer to pole = greater deflection
Only has significant impact over long
distances
Coriolis effect
Coriolis effect
Coriolis effect
Coriolis effect
Coriolis effect
Forces that Influence Winds
Winds in the Upper Atmosphere
No friction
Upper air motions undergo Coriolis deflection
Pressure gradient force balances with the
Coriolis force
• Geostrophic flow (wind) results
Temporary imbalance
• Gradient wind results
Winds in the upper atmosphere
Geostrophic Winds
Balance between PGF, Coriolis
• No other significant forces
“Earth turning” winds
• Caused by only Earth’s rotation and pressure
gradients
• Move in direction of rotation (W to E)
Travel parallel to isobars
• Spacing of isobars indicates speed
– Close = fast
– Spread out = slow
Geostrophic winds
Free Atmosphere Pressure Gradient
Geostrophic winds
Geostrophic wind
Fig. 6, p. 215
Geostrophic & Gradient Flow
Winds in the upper atmosphere
Gradient Winds Aloft
Gradient wind parallel to curved isobars
• PGF ≠ coriolis Æ curving wind
Cyclonic
• Counterclockwise
• Around Low
Anticyclonic
• Clockwise
• Around High
Gradient winds
Gradient wind
Gradient winds
Winds on Upper-level Charts
Winds parallel to contour lines and flow
west to east
Heights decrease from north to south
Forces that Influence Winds
Friction
Surface effect
Friction reduces the wind speed which in
turn decrease the Coriolis effect.
Winds cross the isobars at about 30° into
low pressure and out of high pressure
Friction
A force of opposition
Slows air (accelerates opposite motion)
Maximum near surface
• Planetary boundary layer
Minimum aloft
• Negligible above 1.5 km = the free
atmosphere
Affects Coriolis deflection
Friction
Friction
Friction
Fig. 8-30, p. 216
Friction
Down under
Fig. 8-31, p. 216
Winds and Vertical Motion
Air rises over low
Air sinks (subsides) over high
Ridges & troughs
Ridges and troughs in the northern hemisphere
Cyclones & anticyclones
High pressure areas (anticyclones)
Clockwise motion in northern hemisphere
Descending air
Clear skies
Low pressure areas (cyclones)
Counterclockwise motion in northern hemisphere
Ascending air
Clouds
Upper atmosphere
Ridges = surface anticyclones
Troughs = surface cyclones