Atmosphere

Atmosphere structure

Tropopause

Troposphere

20 km

40 km

10 mi

20 mi

30 mi

Weather zone Water Vapor

Dry

Ozo

ne

Stratosphere

Stratopause

Mesosphere

Tem

pera

ture

0oC

20oC

-55oC

Structure of the Atmosphere

• Troposphere is densest and is where our weather occurs

• Substances in the stratosphere persist for long periods because there are few removal processes

• In troposphere, temperature decreases with altitude

• In stratosphere, temperature increases with altitude due to interactions with particles and radiation from the sun

• The ozone layer is within the stratosphere

Atmosphere & Ocean

• Gases and water freely exchange at the ocean-atmosphere interface

• Movement of air (and water) by wind help minimize worldwide temperature extremes.

• Weather is influenced by the movement of water in air (state of the atmosphere at a specific time and place)

• Climate is the long-term average of the weather in an area

Composition of the atmosphere

• 78% nitrogen and 21% oxygen

• Other elements make up < 1%

• Air is never completely dry and water can be up to 4% of its volume.

• Residence time of water vapor in the atmosphere is ~10 days.

Properties of the atmosphere

• Air has mass (and density)• Molecular movement associated with heat

causes the same mass of warm air to occupy more space than cool air. So, warm air is less dense.

• Humid air is less dense than dry air at the same temperature because molecules of water vapor (H2O) weigh less than N2 and O2 molecules displaced.

Density structure of troposphere

• Influenced by temperature and water content• Water vapor is less dense than dry air so causes

density of air to decrease and air to rise• Warming air makes it less dense so it rises• Condensation of water vapor releases heat

which warms the air• Warm air can hold more water vapor than cold

air

Air density affected by pressure

• Air lifted to altitude experiences less pressure so expands and cools

• Air compressed as it descends from altitude warms

Air movement

• Water vapor rises, expands and cools• Condenses into clouds or precipitation

(cooler air can’t hold as much water)• Atmosphere can lose water by

precipitation• As air loses water vapor it becomes more

dense and air will then fall, compress and heat

Atmospheric circulation

• Powered by sunlight

• About 51% of incoming energy is absorbed by Earth’s land and water

• Light penetration varies depending on the angle of approach, the sea state and the presence of ice or other covering (e.g., foam)

Heat budget• Energy imbalance – more energy comes in at the

equator than at the poles• 51% of the short-wave radiation (light) striking land is

converted to longer-wave radiation (heat) and transferred into the atmosphere by conduction, radiation and evaporation.

• Eventually, atmosphere, land and ocean radiate heat back to space as long-wave radiation (heat)

• Input and outflow of heat comprise the earth’s heat budget

• We assume thermal equilibrium (Earth is not getting warmer or cooler) or the overall heat budget of the earth is balanced

Incoming radiation

• 16% of incoming solar radiation absorbed by dust

• 3% absorbed by clouds

• 51% absorbed by earth

• 6% backscattered by air (leaves atm)

• 20% reflected by clouds (leaves atm)

• 4% reflected by earth’s surface (leaves atm)

Outgoing radiation

• 38% emission by water and CO2 (leaves atm)

• 26% emission by clouds (leaves atm)

• 6% surface emission

• 30% that was reflected or scattered

Of that absorbed by earth

• 21% radiated– 15% is absorbed by water and CO2

(greenhouse effect)– 6% leaves the atmosphere

• 7% conductive transfer from ground to air

• 23% evaporation

Uneven solar heating

• Heat budget for particular latitudes is NOT balanced

• Sunlight reaching polar latitudes is spread over a greater area (less radiation per unit area)

• At poles, light goes through more atmosphere so approaches surface at a low angle favoring reflection

• Tropical latitudes get greater radiation per unit area and light passes through less atmosphere so they get more solar energy than polar areas

Solar radiation•Radiation hits the earth in parallel rays•Incident angle varies with latitude•Energy is spread out over more area

–Less heat per area

•Passes through more atmosphere

–Which absorbs radiation

•Poles are cooler because they receive lower intensity solar radiation do to angle of incident radiation.

S

N

Solar radiation•Second reason the poles are cooler is the tilt of the earth on its axis

–Variation in daylength–Even when poles have long daylength, the incident angle is long.

•Third reason is that poles are farther from the sun S

N

23.5o

Seasons & solar heating

• Mid-latitudes – N Hemisphere receives 3x the amount of solar energy per day in June than in December

• Due to the 23.5o tilt of Earth’s rotational axis

• N Hemisphere tilts toward the sun in June and away in December

• Tilt causes seasons

Circulation

• Atmospheric and oceanic circulation are governed by the redistribution of this energy

• Water moves heat between tropics to poles• Ocean currents and water vapor move heat. • Higher latent heat of vaporization means vapor

transfers more heat per unit mass than liquid water.

Atmospheric circulation

• Warm air rises and cool air sinks

• Warm air expands and rises

• Expansion causes cooling and contraction causing increasing density and sinking

• Air will rise where its warmer and sink where its cooler

Convection

Convection Current

Logically on the earth, one can imagine this

Atmospheric circulation

• But, this is NOT what happens

• Atmospheric circulation is governed not only by uneven solar heating but,

• The Earth’s rotation

• Eastward (CCW) rotation of the Earth on its axis deflects moving air or water (or any object with mass).

• CORIOLIS effect (1835)

Coriolis Effect

• Rotation of the Earth CCW • Relative speeds of sphere at different

latitudes• Caused by an observer’s moving frame of

reference on a spinning Earth• Curve is slightly to the right of initial path in

the northern hemisphere• Curve is slightly to the left of initial path in

the southern hemisphere

Relative speeds of objects at different radii moving at the same angular speed

AirplaneCoriolis

Coriolis effect and atmospheric circulation

• Coriolis effect influences wind direction• End up with 3 sets of cells• Air is deflected before getting all the way from

equator to poles• Air only makes it about 1/3 of the way to the poles

before it becomes dense enough to sink• Descending air turns back toward equator when it

reaches the surface because it is again deflected to the right

• Heats up when it gets back to equator and rises again.

Air movement

• Air is warmed at equator so rises

• As it rises, it dumps its moisture because its expanded and cooled

• Air moves south to replace air that’s risen

• Creates zone of low pressure (sinking air creates high pressure and rising air creates low pressure.

Take home points

• Water vapor and the atmosphere• Density structure of air• Seasonality• Uneven solar heating• Earth’s heat budget• Density, temperature, pressure and air

circulation• Coriolis effect• Atmospheric circulation cells