What have we learned?• What is an atmosphere?

– A layer of gas that surrounds a world

• How do you obtain an atmosphere?– comet impacts.– outgassing by differentiation, volcanoes,

• Why do atmospheric properties vary with altitude?– They depend on how atmospheric gases interact with

sunlight at different altitudes.

The Atmosphere• The atmosphere is the envelope of gases that surround our

planet• It is a very thin layer compared to the radius of the Earth• We refer to this layer informally as “air”• The majority of the energy that drives the atmosphere is from

the Sun.• The atmosphere receives most of this energy from the

surface.• Imbalances of energy drive the winds. Eventually, all energy

degrades down to friction.

State of the Atmosphere

• We commonly express the state of the atmosphere by measuring it using the following variables:– Pressure– Temperature– Wind– Humidity– Cloud cover– Precipitation type and amount– Visibility (distance one can see horizontally)

Atmospheric Composition

Dry air (neglecting water vapor) is composed of the following gases:

Nitrogen (N2) = 78%Oxygen (O2) = 21%Argon (Ar) = 1%Trace GasesDust, bacteria, and other particulates

Atmospheric Composition

• Trace Gases– Gases that are only found by examining a million

or billion air molecules– Examples include:

• Carbon Dioxide (CO2) = 340 ppmv• Neon (Ne) = 18 ppmv• Helium (He) = 5 ppmv• Methane (CH4) = 2 ppmv

• Hydrogen (H2) = 0.5 ppmv

troposphere

stratosphere

Mesoshpere/Thermosphere

Atmospheric Circulation (convection)

• Heated air rises at equator

• Cooler air descends at poles

Maximum Sun warming

Coriolis Effect

Coriolis EffectCoriolis effect deflects north-south

winds into east-west winds

Coriolis Effect breaks upGlobal Circulation

• On Earth the large circulation cell breaks up into 3 smaller ones, moving diagonally

• Other worlds have more or fewer circulation cells depending on their rotation rate

Coriolis Effect

Winds blow N or S Winds blow W or EWinds are diagonal

Venus EarthMars

Jupiter, Saturn Neptune, Uranus(?)

TotalAtmosphere Circulation

Hadley Cell• Air rises at the Equator due to high insolation at the surface

(convection)• The belt of rising air and clouds is called the Intertropical

Convergence Zone (ITCZ)• Large amounts of latent heat are released in the ITCZ• Air blows in at the surface to replace the rising air; these

winds are the trade winds• Wind speeds beneath the ITCZ are low, and this zone is also

called the doldrums• Air sinks in the subtropics as part of the Hadley Cell and forms

the subtropical highs• The winds in the subtropical highs are also very slow, and this

zone is called the “horse latitudes”

Polar Cell• Thermally-direct cell at high latitudes• Rising air occurs at a boundary between cold, polar

air and warmer air in the mid-latitudes, called the polar front

• Extremely cold air at the poles lead to surface high pressure

• Winds blowing from the north to the south turn to blowing from the east due to the Coriolis force, so the surface winds are called the polar easterlies

Ferrel Cell• This cell is thermally-indirect (hot air sinks and cold air rises)• The upper branch of the Ferrel Cell predicts easterly winds

aloft, but they are observed to westerly• Even though the complete cell doesn’t exist, the rising air at

the polar front, the sinking air at the subtropical high pressures, and the correct surface winds exist

• Surface flow from the south turns to the blowing from the west, resulting in our wind zone of the “prevailing westerlies”

• The mid-latitudes is a very complex region, with many secondary circulation features (storms) present