Geography 40
Global Environmental Change
Fall 2002



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Lec. 6: Atmospheric Circulation System
Reading: finish chap. 3, skip urban heat island section (last) and focus box.
Ch. 4,

Latitudinal differences in Energy: The tropics receive a surplus of radiant energy and the poles run a deficit.

Movement of Air:
Buoyancy is the tendency of an object to float in a fluid and is controlled by differences in density between the fluid and the object.
Density is the mass of a substance within a unit volume (the greater the mass, then, the greater the density).
Ultimately, all these horizontal and vertical movements (except mechanical forcing – mt. Range) can be attributed to differences in temperature across the globe.

Changes in density between air masses due to temperature – why hot air balloons float. And why parcels of air rise or sink due to temperature.
Horizontal movements of air result from essentially a vacuum left behind when a heated parcel rises, so that surrounding, denser air, flows in to replace that which rose.

Convection and subsidence.

DRIVING FORCE: GLOBAL ENERGY DISTRIBUTION

Convergence: rising:
The converging air masses meeting at the tropics make up the Intertropical Convergence Zone (ITCZ).

Divergence: movement of air outward. This poleward moving air subsides at about 30°N and S latitude, replacing the air that is moving equatorward at the surface.
As air sinks, it warms up, preventing condensation as a result these areas at 30 ° tend to be dry and clear. Subsiding air also leads to area of high pressure and divergence at the surface. Subtropical Highs.

Hadley Circulation: This pattern of air movement – one large convection cell

Midlatitude and High latitude circulation. Very low temperatures at the poles esp. in winter, lead to increased air density near the surface, and higher pressures than in tropics. The higher density and pressure lead to divergence and general movement of cold air outward at the surface, i.e., towards the equator.
The air masses (cold dense air moving towards equator and the warm air moving towards the poles) produce a zone of steep temperature gradients called the polar front zone at about 60°N and South

Coriolis Effect. The apparent tendency for a fluid (air or water) moving across the Earth’s surface to be deflected from its straight-line path, applies to any object, or air parcel, moving on a rotating body.

In Northern hemisphere, objects moving either north or south are deflected to the right; in Southern hemisphere, objects moving either north or south are deflected to the left.
The Coriolis effect increases as the speed of the object increases.
The effect is zero at the equator and increases with latitude. The only place where the Coriolis effect is zero is at the equator.

Distribution of Surface Winds
Winds are named in terms of the direction from which they blow.

Lecture 7: Global Pressure Patterns and winds

RECAP:
Last time we looked at some of the major components of General Circulation of the Atmosphere. The key Driving force is Global Energy Distribution.
- Unequal receipt of Energy leads to differences in pressure.
- Air tends to move from low pressure to high pressure
- large-scale circulation cells (the Hadley Cell from equator to Subtropics 30°C);
midlatitude and high latitude circulation
- The Coriolis Effect.
- Seasonality of solar receipt over the globe also affects the distribution of Energy.

Low pressure cell counter-clockwise, or cyclonic flow.
High pressure cell clockwise, anticyclonic flow.
*** Opposite in the Southern Hemisphere ***
Three physical forces integrate to produce surface wind patterns:
-Effect of pressure gradient and Coriolis effect produce a Geostrophic wind flow, seen in the upper atmosphere;
- Friction is added near the surface and this produces the wind patterns we see.
In the upper atmosphere, primarily westerly (from the west) flow.
Waving undulations called Rossby waves form in the upper atmosphere bringing cold dense air south into contact with warmer tropical air.
The Polar and subtropical jet streams form near the tropopause and flow at very high speeds.
Seasonal variation of High and Low pressure cell locations in Northern Hem. and Southern Hem.

Land-Ocean contrasts
Water has greater heat capacity – the heat energy required to raise the T. of a unit mass of a substance by 1K or 1°C without changing its volume.

Oceans also transfers heat downward by turbulent mixing.

Land-Sea Breeze: land heats up, pulls cooler air in from the sea. At night, the land cools and air drawn off shore.

Continentality: Pressure patterns due to temperature gradient between oceans and large land masses, change with seasons.
Monsoons

Lecture 8
Global Precipitation Patterns:

Point to Clarify:
Land - Sea Breezes: 1st, we see this phenomenon in Sacramento, as mean monthly temperatures in Sacto are moderated by the coastal location. The high day time temperatures bring in air from the Pacific. At night, the cold air over the land sinks and a "land" breeze forms (towards the sea)
BUT, as for our coastal fog, the story is slightly more complicated:
Because we have a cold water current that flows immediately off our coast, so that the relatively warm air, pulled in from further out in the Pacific ocean flows over this colder surface (the California Current) and is further cooled when it reaches the land in the early morning hours due to radiation (that has gone on through the night). This causes the moisture in that air parcel that came from further out at sea to condense and form fog.

Some short clips of these systems

In addition to energy, circulation of atmosphere also transport of material across the earth. Most importantly, water in form of vapor and clouds.

Global Hydrologic Cycle
Oceans cover 70% of Earth surface
Poles are encased in extensive ice sheets
Clouds (condensed water vapor) always cover about 50% of globe at any one time.
Water in gaseous form (vapor) varies across the globe – 7% at tropics, 0% over ice caps.
Water is unique as it is only naturally occurring substance that can exist in all 3 phases at temps found on Earth surface. B/c it changes readily from one to another, it cycles easily among the system components.
Latent heat transfer – when energy stored in water (heat absorbed) LH of vaporization; when energy released, LH of fusion So can transfer energy

Major reservoirs in the Earth System:
1. Oceans (97%)
2. land surface (3%)
(ice sheets (75%), glaciers, snow, lakes, rivers, groundwater
3. atmosphere as water vapor (< 0.001%).

Precipitation and saturation vapor pressure

saturation vapor pressure Point when the rate of condensation = rate of evaporation, or gas is at equilibrium. At this point, the SVP depends only on the rate at which molecules are transferred from liquid to gas and back again. This rate depends on the energy of the molecules, meaning it depends on Temperature. Therefore, as temperature increases, the saturation vapor pressure increases. See graph.

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