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Lecture 12 Winds Continued. What is Wind?. Wind is nature’s attempt at balancing inequalities in pressure FACT: Unequal heating of the Earth’s surface generates these inequalities. FACT: Solar radiation is the ultimate source of energy for Wind. What is Wind?.

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what is wind
What is Wind?

Wind is nature’s attempt at balancing inequalities in pressure

FACT:

Unequal heating of the Earth’s surface generates these inequalities.

FACT:

Solar radiation is the ultimate source of energy for Wind

what is wind1
What is Wind?

Windis the result of horizontal differences in air pressure!

Air flows from areas of HIGH pressure to areas of LOW pressure

HIGH

LOW

factors affecting wind
Factors Affecting Wind

If the Earth did NOT rotate and if there was NO friction wind would flow in a straight line from High to Low pressure

Main forces that affect wind

YOU NEED TO MEMORIZE THESE!!!

Pressure Gradient Force

Coriolis Force

Centripetal force

Friction

slide5
HPD?
  • Unfortunately HPD doesn’t stand for “Honolulu Police Department”….
  • HPD = Horizontal Pressure Differences
pressure gradient force
Pressure Gradient Force

Horizontal Pressure Differences (HPD)

Winds flow from High pressure to Low pressure if only affected by HPD

Higher P at surface

Lower P at surface

500 mb

500 mb

700 mb

700 mb

Sea Breeze

1000 mb

1000 mb

WARM Nighttime

COOL

pgf change over horizontal difference
PGF – Change over Horizontal Difference

STRONGER when isobars are closer together

Same CHANGE in Pressure (ΔP)

When given Pressure Heights, the PGF points from regions of High Pressure to regions of Low Pressure

ΔP

ΔP

T = 20

T = 30

T = 20

T = 30

LARGE DISTANCE

SMALL DISTANCE

isobars pgf
ISOBARS & PGF

If all we had was the PGF, wind would act like a Ball rolling down a slope… rolling at 90 Degrees to the slope!

100 m

500 m

200 m

300 m

400 m

400 m

500 m

300 m

200 m

100 m

The STEAPER the SLOPE the FASTER the ball will roll!!!

100 m

500 m

300 m

isobars pgf more examples
ISOBARS & PGF - More Examples

Winds if we ONLY knew the PGF.

WIND

IS

SLOW

WIND

IS

FAST

If the isobars are further or closer together…

1004 mb

992 mb

996 mb

1008 mb

1000 mb

1012 mb

1004 mb

1008 mb

1016 mb

1012 mb

1016 mb

For a conical hill, the PGF points in all direction

1020 mb

1020 mb

PGF

PGF

Change in P over large distance:

SMALL PGF

Change in P over small distance:

LARGE PGF

pressure gradient force summary
Pressure Gradient Force Summary:

Change in P over large distance = small PGF

Change in P over small distance = large PGF

PGF is at right angles to isobars

Causes wind to START MOVING

However… two forces cause wind speed and direction to be different than predicted by the PGF

Coriolis (rotation of the Earth)

Friction

surface charts
Surface Charts

NOTE: Wind blows across the Isobars

WIND SPEED

coriolis force
Coriolis Force

Results from the rotation of the Earth

Causes the PGF to cross isobars NOT at right angles.

  • Winds curve to the RIGHT in the Northern Hemisphere
  • Winds curve to the LEFT in the Southern Hemisphere
coriolis force example1
On a non-rotating Earth, the rocket would travel straight to it’s target.Coriolis Force - Example
  • Earth rotates 15 deg per hour….
  • Even though the rock travels in STRAIGHT line, when we plot it’s path on the surface it follows a path that CURVES to the RIGHT!
coriolis force summary
Coriolis Force – Summary

Always Deflects a moving body (wind) to the right in the Northern Hemisphere

Only affects wind direction, not speed

Is affected by wind speed (the stronger the wind, the greater the deflecting force)

Is strongest at the poles and nonexistent at the equator… latitude dependent

These two determine the MAGNITUDE of the Coriolis Force

winds aloft and geostrophic flow
Winds Aloft and Geostrophic Flow

Where friction doesn’t play a role!!

When only the PGF and Coriolis Forces (Fc) affect an air parcel

1000 mb

1004 mb

1008mb

1012 mb

1016 mb

1020 mb

PGF

Fc

Fc

Fc

Fc

WIND

Direction of MOTION!

winds aloft and geostrophic flow1
Winds Aloft and Geostrophic Flow

An air parcel is at equilibrium only if PGF acts in the opposite direction to the Coriolis force (no net force).

Therefore in Geostrophic Flow, winds run parallel to isobars in a straight path

PGF

Direction of MOTION!

Coriolis, Fc

WIND

900 mb

904 mb

908 mb

912 mb

curved flow and gradient wind
Curved Flow and Gradient Wind

Gradient Wind – winds that follow curved paths around high and low pressure cells.

Speed of the wind depends on how close the isobars are

L

H

PGF

Coriolis

Wind

slide21

ISOBARS – PGF + Coriolis

L

L

H

H

Wind Flows PARALLEL to Isobars!!!

c

upper air charts
Upper Air Charts

NOTE: Wind blows PARALLEL to the Isobars

These aren’t Pressure lines. They are meters above sea level

friction
Friction

Applied to wind within ~1.5 km of the surface

Friction ALWAYSacts in the direction OPPOSITE the direction of motion!!!!

Friction affects air at the surface more than air aloft.

adding in friction to coriolis and pgf
Adding in Friction to Coriolis and PGF

Geostrophic Flow and Friction

Causes parcel to slow down

Coriolis decreases in strength

Friction cases wind to lean towards the direction of the PGF

PGF

Direction of MOTION!

Friction

Coriolis, Fc

adding in friction to coriolis and pgf1
Adding in Friction to Coriolis and PGF

The addition of friction causes the wind to lean toward the PGF force (or in the direction of the low pressure) in both hemispheres.

Because the Coriolis Force pulls wind to the right in the NH and to the left in the SH we see opposite wind directions when comparing the NH to the SH.

isobars pgf coriolis friction
ISOBARS – PGF + Coriolis + Friction!

L

L

H

H

Wind CROSSES the Isobars!!!

c

lows in different hemispheres
Lows in Different Hemispheres

Northern Hemisphere

Southern Hemisphere

remember airflow and pressure
Remember: Airflow and Pressure

Movement of air can cause variations in pressure

Net flow of air into a region = CONVERGENCE

Net flow of air out of a region = DIVERGENCE

factors that promote vertical airflow
Factors that Promote Vertical Airflow

Friction – can cause convergence and divergence

When air moved from the smooth ocean to the “rough” land, the wind slows down

Results convergence as air “pile up” upstream (like on a highway with construction).

When air goes from land to ocean you see divergence and subsidence

factors that promote vertical airflow1
Factors that Promote Vertical Airflow

Mountains – hinder the flow of air

As air passes over it is compressed vertically, causing divergence aloft

After going over, onto the lee side, air experiences vertical expansion… causing horizontal convergence.

observing wind
Observing Wind

Prevailing Winds

Name given to the wind direction most often observed during a give time period

Can affect the climate of a region

Helpful for city planning (think dumps)

observing wind1
Observing Wind

Wind Rose

Diagram that represents the percentage of time the wind blows from different directions.

Example: September in Honolulu

observing wind2
Observing Wind

Wind Vane

Wind Direction

  • Anemometer
    • Wind Speed
  • Wind Profiler
    • Vertical profile of speed and direction
  • Aerovane
    • Wind Direction and Speed