atmosphere i, esci 2721 atmosphere i review. atmosphere i, esci 2722 the atmosphere origin: ---...

Atmosphere I, ESCI 272 1

Atmosphere I

Review


The Atmosphere

Origin:

--- Outgassed volatile elements & compounds from the solid Earth Continuous and episodic outgassing by volcanism & plate tectonics

--- Cometary contributions?

These Gases have accumulated since Moon-forming collision.

Original composition is uncertain H2O, CO, CO2, HCl, CH4, NH3, N2, & Sulfur gases.

No free oxygen.


Oxygen was a WASTE PRODUCT of photosynthetic autotrophs Most popular theory due to early fossil record of cyanobacteria Stromatolites


Consequences of oxygen build-up

**** Development of ozone (O3) layer which absorbs harmful UV radiation and eventually allowed life on land.

Atmospheric composition has remained relatively stable since Earth's early history

E.g., oxygen concentration has remained between 15% - 30% Small or short-term fluctuations controlled by changes in size of C & N reservoirs Atmosphere Oceans Crust (sediments and lithosphere) Biosphere, life processes Tectonic recycling


Principal Constituents of Dry Air

% VOLUME % MASS

Nitrogen 78.09 75.51

Oxygen 20.45 23.15

Argon 0.93 1.23

Carbon Dioxide (CO2)0.03 0.05


Today's atmosphere

Present composition of atmosphere

By volume: 78% N2 21% O2

1% Ar 0.03% CO2

+ small, variable amounts of H2O.Originally very high concentrations of CO2

Drawn down by "weathering" of crust in presence of water

CaSiO3 + CO2 <> CaCO3 + SiO2

Drawn down by biological activity (organic carbon storage)

6CO2 + 12H2O + sunlight <> C6H12O6 + 6H2O + 6O2

Build-up of oxygen (& draw down of CO2 & break down of other gases)


Physical Properties of the Atmosphere

•Atmosphere: Is a fluid!!!•like the interior of the earth•like the oceans•Fluid follows the same laws of physics (nature)

•Stratified: That is, density differentiation is extremely important


Structure of the Atmosphere

• pressure varies with height

• temperature varies with height

• Density varies with height.


• Main layers of interest to us:

– Troposphere: surface to ~12 km

– Stratosphere : ~12 km to ~50 km (Ozone layer is located within)

Structure of the Atmosphere


Troposphere

----where we live and where normal weather processes dominate.

---strongly decreasing temperature with altitude.

---proximity to the oceans and earth strongly influences its behavior.

---strong temperature maximun at the ground is due to absorption of sunlight by the surface and the warmth of the oceans.

---we know most about this region and are interested in the day-to-day variation in the "weather".

---atmosphere within the troposphere is strongly influenced by anthropogenic activities.


Stratosphere

--- air in this region moves in large-scale ordered circulation systems that change much more slowly than in the troposphere.

---- much less "weather" in the stratosphere than in the troposphere. (Why?)

----- increase in temperature with altitude here is due to absorption of solar ultraviolet light by ozone.


Relative activity of troposphere, stratosphere

• Most “weather events” occur in troposphere since:

• most water vapour is found in this layer

• density effects are maximized by surface heating, roughness


Atmospheric cooling

Lift mechanisms:

-->Warm air is less dense than cold air (e.g. oil floats on water, continents float on mantle.. etc.) This is a buoyancy-driven lift mechanism. Convection.

---> Physical lift mechanism...air can be forced over a mountain.

--> Can have combination as when air is forced over a cold front.


Pressure-driven cooling/heating:

--- Pressure decreases with altitude, as a parcel of air rises it then expands. --- As the parcel of air expands its molecules do work which uses energy. This energy is supplied by the molecules of air in the parcel.

--- Energy loss decreases the kinetic energy of the molecules, thus temperature decreases.

NOTE that there is no heat transfer with the surrounding environment!!!!!Low

Pressure

High

Heat and buoyancy may haveinitiated movement, but parcelcools only as a result of expansion

5o

15o

25o

Assume 5o

surrounding our parcel


Such process is called adiabatic, defined as the internal changes within the gas during expansion and contraction when no energy is added or removed from the gas.

Note: the fist law of thermodynamics states that the temperature of a gas may be changed by addition (or subtraction) of heat, a change on pressure, or a combination of both.

Ideal gas lawPV=nRTP and T are proportionalwhen a parcel of air goes to a region of decreased pressure, its temperature will also decrease as a result of expansion

Adiabatic process:

behaviour follows ideal gas law – no environmental interaction


Water Vapour

• Prevents perfect adiabatic heating and cooling. In other words, water has the potential to add or remove energy from air parcels as they move.

• The capacity of air to “hold” water vapour varies with temperature.

• Air can hold less water vapour at low temperatures.

• Condensation


Capacity

Air capacity to “hold”water vapour varies with temperature.

Review concept of “relativehumidity” from theESCI 170 notes.


Humidity

Relative humidity:

Percent ratio between the actual amount of water vapour present and the vapour carrying capacity of an air mass. (%)

VP(present) / VP(carrying capacity) *100 = R.H.

~25% R.H.


Dew point:

The temperature at which an air mass becomes saturated.

(Think about what occurs when you take a cold drink from the fridge. condensation appears on the walls of the container.. Why?)

~25% R.H. ~100% R.H.

Same air parcel,different temperatures

cooling


Water Effects on Cooling/Heating

High capacity, extra capacity to absorb/reject heat during phase (state) changes.

Calorie: Amount of heat energy needed (at sea level pressure!!) to raise the temperature of 1 gram of water by 10 C


Latent Heat Processes

State changes are important transition points where cooling/heating does not happen smoothly.

Solid/liquid change of state

Melting: absorbs 80 cal. of heat per gram of water (80 cal/g) (latent heat of melting)

Freezing: releases 80 cal/g of water (latent heat of fusion)

Solid/Gas change of state

Sublimation: process by which ice turns into water vapour or vice-versa. (680 cal/g)


Gas/liquid change of state

Gas/liquid state changes occur frequently in troposphere

Evaporation: the heat -absorption process that turns liquid water into vapour. Water absorbs heat from the surroundings (cooling them down). 600 calories per gram!

Condensation : heat-release process that turns water vopour into liquid. Water releases heat from its surroundings (heating them).


So, when evaporated water is displaced by air currents and then condenses at a different location it is actually transferring heat from one place to another.

This is a heat and mass transfer process

Lift and cool to dew pointadiabatically

600 cal/g


Vertical mass-heat transfer

• This is a very important. It is the mechanism driving weather phenomena.

• Heat/pressure AND water vapour driven

• Physical AND Chemical


Life Cycle of an Air Parcel

• 1) warm air rises• 2) as air parcel expands ( adiabatically),

– it cools and its temperature decreases• 3) thus its relative humidity increases• 4) saturation occurs• 5) water vapour condenses into droplets or crystals• 6) latent heat is released• 7) as heat is released, air parcel warms up

– as parcel warms up --> density decreases– density decrease --> air mass rises

• This is the main mechanism driving storms and hurricanes. I.e. source of energy.


Important Determinants

How much water is there??

-Where is this water??

-How long does water stays in one place?


~3/4 of the surface of the planet is covered by oceans and considering the large and continuous exchanges of water among reservoirs, water plays an important role in the heat budget or heat transfer regime on Earth!!!!!


Hydrologic Cycle


Summary

• Atmosphere is layered or stratified

• Chemistry different in each layer

• Most important layer to earth’s climate is the troposphere

where the weather happens

• Weather is the result of buoyancy/pressure and water

effects

• Water vapour, because of its chemical nature, prevents air

from moving according exclusively to the laws of physics

• The result is heat AND mass transfer which is the basis of

atmospheric circulation and weather at the surface.

•HEAT is the overall driver, which comes next...


Radiation and Circulation

ESCI 272


Energy (Heat) Budget


uneven energy distribution at the Earth’s surface...


Due to angle of incidence


• excess energy at low latitudes must be transported to other areas.

• This transport takes place -in the large scale- by convection

Evening out the uneven energy distribution


tropics poles

Convection


Circulation in an idealized non-rotating earth


Pressure Systems:

Due to differences in temperature,

we have differences in pressure.

Common Names you might encounter:


-> A low is an area in the atmosphere where pressure is less than the surrounding area

-> A trough is an elongated area of low pressure

-> A high is an area of the atmosphere where pressure is higher than the surrounding area.

-> A ridge is an elongated area of high pressureIn all cases the pressures are relative and do not have fixed ( pre determined ) values or ranges

Pressure Lingo:


Pressure Gradients:

Differences in heating and internal motion in the atmosphere produce differences in atmospheric pressure.

This spatial difference in pressure is called a pressure gradient. Air moves from high to low pressure.

You can think of this in terms of energy, for example an inclined plane where you let a ball roll down!!

Ball moves from high to low...


Pressure Gradients:


The greater the incline, the faster it moves...

Pressure Gradients:


How does the gradient affect air motion??

-> The rate at which air moves depends on the steepness of the gradient.

When small differences of pressure occur over large areas, a weak gradient gives rise to weak winds.

A steep pressure gradient causes rapid motion or high winds.


In the same way that water vapour state changes can prevent air parcels from moving adiabatically (according to the ideal gas law), the

Coriolis “Force”

influences the direction of moving fluids across its surface. It is a force in name only...

Another force...


Coriolis “Force” or Effect

Lots of great youtube videos to help visualize this. For example:http://www.youtube.com/watch?v=49JwbrXcPjc



Geostrophic winds



Effect is different at the surface and aloft



In the case of air flow around a low pressure center, this implies a net air “ inflow” or convergence and in the case of air flow around a high pressure center a net air “outflow” or divergence


Coriolis “force” can be balanced by the pressure gradient to give rise to geostrophic flow.

Friction intervenes near the surface, weakening the effect of the Coriolis effect and in fact making the wind direction intersect the isobars.



Back to circulation...

•A low pressure area is created by rising air.

•Similarly, a high pressure area is created by falling air

•Convection causes divergence or convergence


Circulation Model:Cellular model proposed by Hadley.

-> In a non-rotating earth excess energy at low latitudes would theoretically produce a single 3-d convection cell carrying energy to the poles


Atmosphere II, Geology 372-2000 8


Problem.. Earth rotates..

So, warm air at low latitudes (low pressure area) rises by buoyancy to form a convection cell. Rising warm air moves poleward.. This air mass cools AND deflects to the right (N.H.) or to the left (S.H.), that is, it moves clockwise or anticlockwise respectively.

Circulation Model


-> In it journey poleward, this mass of air cools, dries (precipitation) and by 20 or 30 degrees north it sinks again. (deserts of the Earth exist at these latitudes).

->Some of the mass of air returns over the surface equatorward and it is deflected , once again, by the Coriolis “force” giving rise to trade winds.

Circulation Model


The rest of the air mass travels poleward giving rise to westerly winds (at all altitudes).

Circulation Model


Idealized general circulation in a rotating Earth


These processes we’ve talked about in the last two classes are simulated by computer models called:

General (or Global) Circulation Models

which are used for weather or longer term climate prediction.

atmosphere i, esci 2721 atmosphere i review. atmosphere i, esci 2722 the atmosphere origin: ---...

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