The Atmosphere summary and notes




The Atmosphere summary and notes


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The Atmosphere summary and notes


Earth Science 300

Summary notes

The Atmosphere



important gases in Earth’s atmosphere – N2 = 78%, O2 = 21%, Ar = 0.9% +

            H2O, CO2, O3

CO2 increase as a result of burning fossil fuels, shows seasonal variation

O3 produced and destroyed in stratosphere

            air pressure

falls off with altitude (halves about every 5.5 km)

            caused by weight of air above

            temperature structure of atmosphere

                        names of layers (troposphere thru’ thermosphere) and boundaries separating them

                        approximate altitude of tropopause

                        why does temperature fall off with altitude in troposphere

                        why does temperature increase in stratosphere and thermosphere

                        in which layer does all weather, winds, rain etc. happen


Energy and Radiation

            mechanisms of heat transfer

conduction = transfer of heat through matter via collisions between molecules (requires contact. air is poor conductor. metals are good conductors.)

convection = transfer of heat by mass movement (can only take place in fluids, gases) – responsible for rising parcels of warm air, descending parcels of cool air; and for global circulation

                        radiation = heat transfer via electromagnetic waves (light)

                                   can travel through a vacuum

            electromagnetic spectrum

radiation = light (some is visible to us, some is not)

photon = packet of light energy, travels at speed of light

the electromagnetic spectrum – in order of increasing wavelength (= decreasing energy):

X-rays, Ultraviolet light, visible light, infrared, microwave, radio waves

everything gives off radiation.  The hotter the object the higher the energy of the radiation given off, and the longer the wavelength.  The Sun gives off most energy in the visible part of the spectrum.

the wavelengths of radiation emitted by an object depend on the object’s temperature - the hotter the object, the shorter the wavelength of radiation emitted

The earth absorbs nearly all the solar radiation which arrives at its surface and emits the energy at infrared wavelengths

The gases that absorb in the infrared spectrum block outgoing radiation from the earth and re-radiate it back to the earth’s surface, warming the surface. These are Greenhouse gases.

Energy balance:

albedo of earth-atmosphere ~ 30% (20% reflected by clouds, 10% by earth’s surface and scattered back by gases)

~19% of incoming solar radiation is absorbed by gases, dust and clouds in the atmosphere

                        ~51% absorbed at the earth’s surface

Most of the radiation emitted by the earth is absorbed in the atmosphere by greenhouse gases and clouds. Some of this energy will be re-radiated back to the surface of the earth. This raises the temperature of the earth to an equilibrium value of ~ 15 ºC.

The greenhouse effect is a natural phenomenon which is necessary to sustain life on earth (otherwise it would be too cold). The reason that we are concerned with rising CO2 levels is that we are enhancing the natural greenhouse effect and thus warming up the earth. If the earth warms up by even 1 or 2 degrees our climate will change dramatically and ice caps could melt, sea levels rise, and coastal cities flood.



Effect of land and water on temperature

Land heats more rapidly and to higher temperatures than water, and cools more rapidly and to lower temperatures than water, so temperature variations over land are very much greater than over water. Winds are from the west, so west coast has strong ocean influence. The middle of continents and E coast are not influenced much by the ocean.

                   Why are temperature changes much smaller over ocean?

                       1. specific heat of water compared to land

                       2. water is highly mobile

                       3. water is more transparent

                       4. evaporation greater over water (some heat is used as latent heat)

Compare annual temperature variations of coastal and inland cities


Humidity and Saturation

vapor pressure (v.p.) = pressure resulting from water vapor molecules in air (mb)

dew point = temperature at which the water vapor in the air is saturated

If air is unsaturated there are 2 ways of bringing it to saturation:

1.  cool it (keeping the amount of water vapor the same)

2.  add water vapor to it (keeping the temperature the same)

Low dew point = dry air

High dew point = moist air


Condensation and Clouds

air temperature must be at or below dew point for condensation to occur

when air rises it passes through regions of successively lower pressure, expands and cools. It cools at a rate of about 10°C/km.

If air rises enough it will cool to the dew point and condensation can occur. The height at which this occurs is the lifting condensation level.

Processes that force air to rise:

convective lifting: local surface heating, rising warm air, cool air sinks forming convective cell circulation

orographic lifting: horizontally moving air forced to rise over a mountain (topographic barrier)

frontal wedging: formed when masses of warm and cold air meet. warmer, less dense air forced to rise over colder denser air.

fog = cloud on the ground

atmosphere is stable if it inhibits rising (ie. air parcel is colder than the environment)

atmosphere is unstable if it allows rising (ie. air parcel is warmer than the environment)



typical cloud droplets are too small to produce rain.

droplet growth by vapor condensation is too slow to produce a raindrop (would take a day or more)

cloud droplets grow to form raindrops by the process of collision and coalescence (or coagulation):

ice can form in clouds if the air is cold enough.  Ice crystals can fall as snow or rain – it depends on the temperature at which the crystal forms and the temperature below the cloud (i.e. whether the ice melts on its way to the ground)

cloud classification:

cirrus – high, white, thin

cumulus – globular individual cloud masses

stratus – sheets or layers. no distinct cloud units.


Air Pressure

pressure is exerted by weight of air above

halves about every 5.5 km

lines of constant pressure are isobars

surface pressure at earth is around 1000 mb

‘H’ = center of high pressure = anticyclone

‘L’ = center of low pressure = cyclone



wind is caused by horizontal differences in pressure – which are caused by unequal heating

if earth did not rotate and there was no friction, air would go directly from high -> low pressure

pressure gradient force:

spacing of isobars shows pressure gradient = difference in pressure/distance. The closer the isobars the greater the pressure gradient, and the greater the pressure gradient force, and the stronger the wind. pressure gradient force is perpendicular to isobars –  in a direction from high to low pressure.

Coriolis force:

apparent force due to rotation of earth (if we were fixed out in space we would not see this force. It is ‘apparent’ because it exists only because we are on a rotating earth and so to us free-moving objects are deflected from straight-line travel.)

in NH objects (winds) are deflected to the right

in SH objects (winds) are deflected to the left

CF –

is at right angles to direction of movement

strongest at poles, 0 at equator

Geostrophic wind = wind generated by balance between PGF and CF.

flows parallel along straight isobars

stronger for closely-spaced isobars

For a Low, flow is anticlockwise in NH, clockwise in SH

For a High, flow is clockwise in NH, anticlockwise in SH

Surface wind = wind at/close to earth’s surface where friction is important

friction caused by trees, buildings, topography

PGF directed from H to L

CF at right angles to wind

friction directly opposes wind

causes wind to blow at an angle to isobars (in towards L, out away from H)

Vertical air motion

surface lows (cyclones) cause air to blow inwards = convergence -> air must rise -> divergence aloft. Therefore surface lows are associated with rising air, clouds, precipitation.

surface highs (anticyclones) cause air to blow outwards = divergence at surface -> convergence aloft -> subsidence of air. Therefore surface highs are associated with sinking (and warming) air and fair weather.


Global circulation

3-cell model

Hadley cell: equator - 30° - rising air at equator

Equatorial low, 30° high = Subtropical High, 60° low, 90° polar high

Surface winds –Trade winds, Westerlies, Polar front, Easterlies

Zones of precipitation

wet at equatorial low

dry in subtropical high

wet at subpolar low

dry at polar high

zones shift with season

actual precipitation patterns affected by distribution of land and water

ITCZ = Intertropical convergence zone (where winds come together from 2 hemispheres)



boundary between air of two different temperatures (and density)

get cold fronts, warm fronts, (and occluded fronts)

Learn weather map symbols (wind directions and speed, symbols for fronts, pressures)


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