A. Basic Terms

• "Wind" - the horizontal movement of air relative to the Earth's surface

• "Zonal Flow" - a predominately W-E or E-W wind flow

• "Meridional Flow" - a predominately N-S or S-N wind flow

• Wind Direction: winds are named for the direction from which they originate (from which they blow)

• "Wind Compass", "Wind Vane" -- a wind vane always points into the wind

 
• "Anemometer" -- wind speed is commonly measured using a "cup annemometer"

• "Isobars" - lines connecting points of equal pressure

B. Scales of Atmospheric Motions

• circulations are arranged according to their size (both in terms of space and time)

• from smallest to largest:

length: ~ 1 centimeter - 100 meters

time: ~ seconds - minutes

e.g., eddies (small whirls of air) present in the rising air of smoke stacks act to disperse the smoke; eddies also move tree branches, swirl dust, blows loose papers, etc.

2) Mesoscale Circulations:

length: ~ 0.1 - 100 kilometers (km)

time: ~ hours - days

e.g., the circulation of city air, local winds (shoreline and mountain circulations), thunderstorms, tornadoes, and small tropical storms

3) Synoptic Scale:

length: ~ 100 - 10,000 kilometers

time: ~ days - weeks

- wx systems observed on a wx map:

extratropical cyclones, high pressure systems, and hurricanes

4) Planetary-Scale Circulations (global scale or "general circulation")

length: greater than 10,000 km (earth's circumference is ~ 40,00 km)

time: weeks - months

e.g., "prevailing westerlies" in the middle latitudes, or the "trade winds" in the low latitudes

C. Newton's Laws of Motion

1) 1st Law:

"In the absence of unbalanced force: a body at rest will remain at rest; and a body in motion will remain in "straight-line" motion at constant speed"

"To change the motion of air, a force needs to be applied"

2) 2nd Law:

Force = Mass x Acceleration (F = m x A)

• the force (push or pull) acting on an object (air parcel) is directly porportional to the acceleration produced

• or: A = F/m

thus, to determine the motion (speed & direction) of air, we must examine all the forces that affect the horizontal movement of air:

• Pressure Gradient Force
• Coriolis Force
• Friction Force

1) Pressure Gradient Force (P)

- can be separated into 2 components:

P = Pv + Ph      (Pv = vertical component)

                        (Ph = horizontal component)

a) Vertical Pressure Gradient Force:

- direction: upward (toward lower pressure)

- magnitude: same as G (gravitational force)

                    (Pv = -G)

                        
          Hydrostatic Equilibrium

b) Horizontal Pressure Gradient Force:

- direction: toward lower pressure

(perpendicular to the isobars on a sea-level surface map)

- magnitude: proportional to the "pressure gradient"

pressure gradient = change in pressure /horizontal distance

            
more closely spaced isobars on a surface map indicate where the horizontal pressure gradient force is strongest !!!

2) Coriolis Force: (an "apparent" force)

direction: acts perpendicular (to the right of) the wind in the Northern Hemisphere; perpendicular (to the left of) the wind in the Southern Hemisphere

magnitude: proportional to the wind speed; increases for all wind speeds from a value of zero at the equator to a maximum at the poles

• the coriolis force only influences wind direction, never wind speed

• its affect is minimal for smaller-scale air flows

3) Friction:

direction: acts in opposition to the wind direction

magnitude: depends on the roughness of the surface over which the air moves

• friction is important to consider for air motions near the earth's surface; it's relatively unimportant in the free atmosphere (above 1 km)

1). Geostrophic Balance

• it exists for straight-line flow in the "free" atmosphere (above the friction layer - above 1,000 meters)

• Ftotal = Ph + Cor

• At equilibrium (balance): Ph = - Cor

• The wind that results from this balance between the horizontal pressure gradient force and the coriolis force is called the geostrophic wind (Vg).

• Vg blows parallel to the height contours (lines that connect points of equal elevation above sea level)

• the speed of the geostrophic wind is directly related to the pressure gradient

2) Surface Winds (Ekman Balance)

- for flow in the planetary boudary layer (PBL) (within the friction layer - the lowest 1,000 m)

Ftotal = Ph + Cor + Fr

friction reduces wind speed which reduces the coriolis force; the weaker coriolis force no longer balances the pressure gradient force and the wind blows across the isobars toward lower pressure

• the resultant wind from this balance blows at an angle of 20 to 40 degrees across isobars toward lower pressure !!

• the greater the force of friction, the greater the cross isobar flow

F. Depictions of Surface and Upper-Level Winds

.
                        
  Air Flow Around a Surface High
 

                    
 Air Flow Around a Surface Low
 
 
 

                                
      Surface, Upper-Air,  and Vertical Circulations
 

The air rises above surface lows (cyclones); and sinks above surface highs (anticyclones) !!!!!

Highs: upper-level convergence and lower-level divergence

                                
Lows: upper-level divergence and lower-level convergence

G. Global Circulation

• if we average the planet's winds over a long period of time, the local winds (circulations) vanish, and we see the global-scale wind patterns: the "general circulation of the atmosphere"

• the differential heating (unequal heating) of the earth's surface is the driving force behind the general circulation

• recall that averaged over the whole earth, insolation is equal to outgoing long wave from the earth's surface

• however, the tropical areas experience a net gain in energy, while the polar areas experience a net loss in energy. How is this inequity balanced?

• the atmosphere and oceans transport warm air poleward and cool air equatorward

1) Single-Cell Model

• assume: (1) that the earth's surface is uniformly covered with water (no land vs. water differential heating effect); (2) that the sun is always directly overhead at the equator (the winds will not shift seasonally); and (3) the earth doesn't rotate (no coriolis force)

• the only force operating is the pressure gradient force, and we get a giant convection cell with warm air rising at the equator, heading north and south on either side of the equator aloft, and then sinking motion at the poles. The air then travels from the poles toward the equator at the surface of the earth

• we call this giant convection cell the "Hadley Cell"

• we keep the first two assumptions, but we do allow for the coriolis force

• here too, the tropics heat-up more than the poles, so we (as in the single-cell model) have a surface high at the poles and a broad trough of low pressure at the equator

• in this model though, there are three circulation cells in each hemisphere that act to redistribute heat energy

(a) Hadley Cell - a circulation in which air rises at the equator, moves poleward at upper levels, cools, converges, and sinks at ~ 30 degrees latitude; and returns to the equator at lower levels. The latitudes beneath the rising branch of the Hadley Cell are known as the doldrums (much rain and light winds)

(b) Ferrel Cell - a circulation in which air sinks at ~ 30 degrees latitude, moves poleward at low levels, rises at ~ 60 degrees latitude, and returns to ~ 30 degrees latitude at upper levels. The latitudes beneath the sinking branch of the Ferrel Cell (or Hadley Cell) are known as the "Horse Latitudes (little rain and light winds)

(c) Polar Cell - a circulation in which air rises at ~ 60 degrees latitude, moves poleward at upper levels, sinks over the pole, and returns to ~ 60 degrees latitude at lower levels

Surface Circulations:

(a) Trade Winds

• winds blowing form the NE in the Northern Hemisphere, & from the SE in the Southern Hemisphere

• occur between the equator and ~ 30 degrees N (S) latitude

• are very steady in speed and direction

• the trades are the equatorward branch of the subtropical highs centered in the horse latitudes

• trade winds from both hemispheres converge in the doldrums, often referred to as the Intertropical Convergence Zone (ITCZ)

• winds blowing generally form the west in both hemispheres between ~ 30 and 60 degrees

• they occur over a region where there is a strong N-S temperature difference

• storms (extratropical cyclones) moving through this region cause the prevailing westerlies to be much less consistent than the trade winds

• winds blowing generally from the east poleward of ~ 60 degrees

• the "polar easterlies" circulate around a weak polar high which develops under the sinking branch of the polar cell

• cold air moving southward via the polar easterlies encounters mild air moving poleward via the westerlies

• these two air masses of contrasting temperatures don't readily mix. They are separated by a boundary called the "Polar Front" (it is along this boundary (front) that cyclones form)

• Equatorial Low Pressure Trough (ITCZ)
• Subtropical Highs
• Subpolar Lows
• Polar Highs

            
Summary of Global Winds
 

H.  Local Winds

Thermal Circulations: circulations resulting from changes in air temperature (density), in which warm (less dense) air rises and cold (more dense) air sinks

Types:

1) Land and Sea Breezes

• Occur along coastlines

2) Monsoons

• regional wind systems that change direction seasonally - e.g. India's Monsoon

3) Mountain & Valley Breezes

• warm (less dense) air rises upslope from the valley during the day; and cool, dense air sinks down the mountain slopes at night

            
Mountain & Valley Breeze
 

4) Katabatic Winds

• larger scale (regional-scale) density flows (e.g. Antartica & Greenland katabatic flows; Santa Ana (warm) winds

        
  Katabatic Winds
 
 

I. Upper Level Westerlies
 
 
 

                            
Upper-Level Westerlies in Both Hemispheres
 
 

                        
Mid-Latitude Westerlies (Zonal Flow)

                            
  Mid-latitude Westerlies (Meridional Flow)