Transcript Atmospheric Forces and Wind Chapter 6

AOSC 200
Lesson 6
p. 159
p. 159
Fig. 6.3
Newton’s Laws
• First Law (Law of Inertia): A body at rest
tends to stay at rest while a body in motion
tends to stay in motion, traveling at a
constant speed and in a straight line, until
acted upon by an outside force
Newton’s Laws
• Second Law (Law of Momentum): A force
exerted on an object (including a parcel of
water or gas) of a given mass causes the
object to accelerate in the direction of the
applied force and in proportion to the force
divided by the objects mass.
• Force equals Mass times Acceleration
F  ma
Fig. 6-2, p. 160
Fig. 6.2
Box 6.1
Box 6.1
Forces that Move the Air
• Gravitational force
• Is directed downward , normal to the
ground
• Approximately equal to the mass of the air
times the gravitational acceleration
• Pressure gradient force
Fig. 6-4, p. 161
Pressure Gradient Force
PRESSURE- GRADIENT FORCE
• IS A FUNCTION OF THE PRESSURE DIFFERENCE AND AIR
DENSITY.
•
FPC = ΔP / D.σ
• WHERE
•
FPC = PRESSURE GRADIENT FORCE PER UNIT MASS
•
σ = DENSITY OF AIR
•
ΔP = PRESSURE DIFFERENCE BETWEEN TWO
POINTS
•
D = DISTANCE BETWEEN THE TWO POINTS
• .NEWTON’S LAW STATES FORCE=MASS TIMES
ACCELERATION
• FPC IS A FORCE PER UNIT MASS, I.E. IS EQUIVALENT TO
ACCELERATION
Apparent Forces in the
Atmosphere
• Coriolis Force
• Centrifugal Force/Centripetal acceleration
Coriolis Effect
Fig. 6-9, p. 165
Fig. 6-10, p. 167
CORIOLIS EFFECT
• •WEATHER MAP SHOWS THAT WINDS
GO AROUND A LOW PRESSURE AREA,
NOT ACROSS THE ISOBARS.
• THIS DEVIATION IS A RESULT OF THE
EARTH'S ROTATION AND HAS BEEN
NAMED THE CORIOLIS EFFECT
(CORIOLIS FORCE)
• THIS FORCE ALWAYS ACTS AT RIGHT
ANGLES TO THE DIRECTION OF
MOTION.
A popular misconception
Centrifugal Force
Fig. 6.11
The Mechanism for Geostrophic Flow
GEOSTROPHIC FLOW
• EVENTUALLY A BALANCE IS REACHED
BETWEEN THE CORIOLIS FORCE AND THE
PRESSURE GRADIENT FORCE
• IF THERE IS NO FRICTION THEN THIS OCCURS
WHEN THE PARCEL OF AIR IS PARALLEL TO
THE ISOBARS.
• AT THIS POINT THERE IS NO NET FORCE ON
THE AIR PARCEL - NO ACCELERATION - IT
NOW MOVES WITH CONSTANT VELOCITY.
• UNDER THESE IDEALIZED CONDITIONS THE
AIRFLOW IS SAID TO BE IN GEOSTROPHIC
BALANCE.
• WIND GENERATED IS CALLED THE
GEOSTROPHIC WIND
• NORMALLY ONLY APPLIES TO WINDS ALOFT.
• SURFACE WINDS ARE SUBJECT TO FRICTION.
Fig. 6-15, p. 172
GRADIENT WINDS
• WINDS AROUND CENTERS OF HIGH OR LOW
PRESSURE FOLLOW CURVED PATHS IN ORDER TO
STAY PARALLEL WITH THE ISOBARS.
• THESE WINDS ARE CALLED GRADIENT WINDS.
• LOW PRESSURE CENTERS ARE CALLED
CYCLONES - ROTATION IS COUNTER-CLOCKWISE
- SAME AS THE EARTH
• CENTERS OF HIGH PRESSURE ARE CALLED ANTICYCLONES.
• IN SOUTHERN HEMISPHERE THE DIRECTIONS OF
FLOW ARE REVERSED
Fig. 6-12, p. 168
Fig. 6-18, p. 175
Fig. 6-24, p. 181
SURFACE WINDS
• FRICTION AFFECTS WINDS ONLY CLOSE TO THE
EARTH'S SURFACE.
• NOW WE MUST BALANCE THREE FORCES - CORIOLIS,
PRESSURE GRADIENT AND FRICTION.
• NET EFFECT IS TO INDUCE A NET INFLOW AROUND A
CYCLONE, AN EFFECT KNOWN AS CONVERGENCE.
• AROUND AN ANTICYCLONE WE GET A NET OUTFLOW,
DIVERGENCE
Aneroid Barometer
Aneroid Barometer
• On an aneroid barometer fair corresponds to
high surface pressure.
• Air is subsiding and is subject to adiabatic
heating. This lowers the relative humidity.
Hence possibility of clouds forming is low.
• Rain corresponds to low pressure.
• Air is rising and is subject to adiabatic
cooling. The raises the relative humidity.
Possibility of clouds is high.
Effect of temperature on pressure
• Initially the pressure above each city is the same. But if we
heat the air above one of them the column of air will
expand. If we cool the air above the other it will contract.
• If we now look at the pressure at the top pf the cool air it
will be lower than that in the heated column. This is
because pressure is defined as the weight of air above a
given altitude.
• The weight of air above the cold column is less than that at
the same altitude in the not column
• This will cause air to move from the hot column to the cold
column.
• Hence the pressure will increase in the cold column at the
ground, and the pressure will decrease in the hot column.
Fig. 6-22, p. 179
SEA BREEZES
• ARE THE RESULT OF DIFFEERENTIAL
HEATING OF THE OCEAN AND THE LAND
• DURING THE DAY THE LAND HEATS UP
QUICKLY WHILE THE OCEAN HEATS UP
SLOWLY
• HIGH TEMPERATURE OVER THE LAND, LOWER
TEMPERATURE OVER THE OCEAN
• AT THE SURFACE - HIGH PRESSURE OVER THE
OCEAN, LOW PRESSURE OVER THE LAND CAUSES WIND AT THE SURFACE TO FLOW
FROM THE OCEAN TO THE LAND (SEA BREEZE)
• AT NIGHT THE LAND COOLS RAPIDLY TO A
TEMPERAURE BELOW THAT OF THE OCEAN.
WIND REVERSES - FLOWS FROM THE LAND TO
THE OCEAN AT THE SURFACE (LAND BREEZE)
Stepped Art
Fig. 7-6, p. 175
Fig. 6.26
Fig. 6.26
Fig. 6.26
Fig. 6.26