FLUID EARTH: THE ATMOSPHERE

 

In the “atmosphere” component of “Fluid Earth”  we will examine the processes within the Earth’s atmosphere, and how these processes produce the Earth’s weather and climate.

 

We will address the following questions:

 

¨      What is the atmosphere made of? How is its composition changing (due to natural and human activities)?

 

¨      What causes the observed variations in temperature, both spatially (regional and global scale) and temporally (days to centuries)?

 

¨      What cause the movement of air (winds) in the atmosphere, on small (e.g., wind gusts) to large (e.g., `global circulation’) scale?

 

¨      How do weather systems (e.g., fronts, cyclones, tornadoes, and hurricanes) `work’?

 

 

THE ATMOSPHERE

 

The atmosphere is on of 4 inter-related spheres that comprise the EARTH SYSTEM:

 

¨      SOLID EARTH (Earth’s surface and interior)

¨      HYDROSPHERE (Water portion: 97.2% ocean, 2.15% Glaciers)

 

¨      ATMOSPHERE (Gaseous envelope surrounding Earth)

 

¨      BIOSPHERE (All life)

 

 

The atmosphere is a thin blanket of air that surrounds the planet Earth.

 

No definite top to the atmosphere --- atmosphere becomes sparser with height.

 

But if we take 100km as the top (contains 99.9997% of the atmosphere) then atmosphere is thin compared to Earth’s radius of 6371km (i.e. < 2% radius).

 

Note that weather (clouds, thunderstorms, etc.) within bottom 10km

 

 

 


 

COMPOSITION OF THE ATMOSPHERE

 

Air within the atmosphere is a mixture of many discrete gases.

 

HOMOSPHERE (< 80km): composition relatively constant.

 

HETEROSPHERE (>80km): composition varies with height.

 

We focus on the Homosphere.  Within this region air is comprised mainly of

Nitrogen (N2)    78.08%

Oxygen (O2)     20.95%

Argon (Ar)          0.93%

 

Although above make up 99.93% of atmosphere there are generally not significant for weather or life on Earth.

 

More important are the trace gases (“trace” as only found in small amounts) such as:

Water Vapor (H2O)                 < 0.25%

Carbon Dioxide (CO2) 0.036%                        360 ppm

Methane (CH4)                        0.00015%                    1.5 ppm

Ozone (O3)                              0.01%                          100 ppm

(ppm = parts per million)

and

              Aerosols

 

 

Why is air well-mixed?

 

As air is composed of particles with different densities why don’t these particles “settle out”, with heaviest (CO2) near the ground?, i.e.

 

---------------------------------

Oxygen 2000m

---------------------------------

Argon     100m

---------------------------------

Carbon Dioxide        3m

/////////////////////////////////////////

 

Answer: air particles are moving extremely fast (often faster than speed of sound, 350m/s) and never settle out.  [Just like vigorously shaking a container with different density particles.]

 

In other words, air is an assembly of innumerable tiny particles in constant and rapid collisional motion.

 

 

CARBON DIOXIDE  CO2

 

CO2 is an efficient absorber of energy emitted from Earth and thus influences the heating of the atmosphere (i.e. a “greenhouse gas”).

 

Supplied to atmosphere by plant & animal respiration, decay of organic material and combustion of fossil fuels.

 

Removed from atmosphere by photosynthesis (green plants) and ocean uptake.

 

Large seasonal variations because of variations in plant growth (less removal in winter because of less growth/photosynthesis).

 

But there is also a steady growth over last half century due to human activities (e.g., burning of fossil fuels).

 

Other greenhouse gases are also increasing because of human activities, e.g.,  Methane  CH4  (rice fields, cattle).

 

đ        impact on climate ?

 

 

WATER VAPOR  H2O

 

The amount of water vapor is small and variable, from 0 to 4 %.

But very important as:

 

č    Source of clouds and precipitation.

 

č    Absorbs heat energy from Earth (i.e. greenhouse gas).

 

Also water can release or absorb energy as it changes state, and plays a very important role in atmospheric transport of heat and helps drive many storms (e.g., hurricanes and thunderstorms).

 

Atmospheric component of “hydrologic cycle” discuss later in course.


 

OZONE  O3

 

Majority of ozone is in the stratosphere (10-50 km), and then only in small amounts (<10ppm).

 

But crucial for life on Earth, as absorbs lethal UV radiation from the sun which is harmful to life forms.  Without the “ozone layer” there would be very little life on Earth.

 

Therefore anything that destroys stratospheric ozone could affect the well being of life on Earth.

đ        Concern about observed ozone depletion

 

Note that ozone is a “good-guy” – “bad-guy” gas. It is a toxic gas and is harmful to life when near the Earth’s surface.

 

 

AEROSOLS

 

Movements in the atmosphere are such that a large quantity of tiny liquid and solid particles are suspended in the atmosphere. These are called aerosols.

 

Large quantity = concentration of 1000 / cm3; 1 breath = 1000cm3 = 1 million aerosols).

 

Tiny = micrometers = 1 millionth of a meter.

 

Aerosols are formed by human and natural causes (e.g., sea salt from ocean waves; fine soil; smoke and soot from fires, vehicles, and aircraft; volcanic eruptions).

 

Important because

 

1.      act as surfaces for condensation of water (clouds and fog)

 

2.      absorb/reflect incoming solar energy (e.g., temperature change following large volcanic eruptions).

 

3.      act as surfaces for chemical reactions involved with ozone destruction.

 

 


 

EVOLUTION OF EARTH’S ATMOSPHERE

 

>4.5 billion years ago

Gravitational field  too weak and atmosphere lost to space

 

~4.5 billion years ago

A thin atmosphere formed by outgasing (volcanoes, meteorites), and atmosphere has similar content to current eruptions (CO2, H2O,  SO2, N2 etc.). No free oxygen (O or O2). CO2-rich atmosphere

 

[H2O may also have come from “cosmic snow balls”]

 

This CO2-rich atmosphere was more dense and warmer than current atmosphere (even if sunlight weaker).

 

~4 billion years ago

Cooling  of the planet lead to  water condensing, and formation of  clouds and rain, and eventually  oceans. This resulted in a reduction of atmospheric H2O and CO2 (dissolved). N2-rich atmosphere

 

~3.5-2.5 billion years ago

Existence of life forms that photosynthesis: removal of  CO2 and release of O2. Fromation of  current N2- and O2- rich atmosphere.

 

Note that only small amount of N2 produced from outgasing, but has very long lifetime.

 

Ozone layer (and protection from UV radiation) developed naturally from interaction of UV light and O2 molecules.

 


 

VERTICAL STRUCTURE

 

The atmosphere thins as you move away from Earth, until too few molecules to detect.

 

To understand the vertical extent we consider the variations of atmosphere pressure (p) with height.

p = weight of air above    ~ 1000 millibars (1mb = 1hPa) at surface.

 

p decays rapidly with height in lower atmosphere, and more slowly in upper atmosphere: halves around every 5.5 km. In other words, 50% of atmosphere is in bottom 5.5km.

 

Alternatively, we can consider the density (=mass/volume) variation.

 

Pressure and density are closely related as the atmosphere is compressible: greater pressure produces greater density. So, there is high density at the surface, which decays (rapidly in lower atmosphere) with height.

 

TEMPERATURE VARIATIONS

 

The atmosphere can be divided into 4 layers depending on the temperature variation with height.

 

Troposphere (0 – 10 km)        T decreases with height.

-------------------------------------  boundary is the tropopause

 

Stratosphere (10 – 50 km)     T increases with height.

-------------------------------------  boundary is the stratopause

 

Mesosphere (50 – 80 km)      T decreases with height.

-------------------------------------  boundary is the mesopause

 

Thermosphere ( >80  km)      T increases with height.

 

Nearly all meteorological phenomena (e.g., clouds, rain, storms) occurs within the troposphere, and we concentrate on this part of the atmosphere in this course. The one exception is stratosphere ozone.

 

Note that tropopause (and other boundaries) vary with latitude and season. Higher at equator (16km) than poles (10km), and higher in summer then winter. Both changes due to T variations.

 

The reasons for the decrease/increase of T with height is related  to vertical variations in the concentrations of gases and aerosols, and their different absorption properties. Whether T decreases or increases plays a major role in types of motions that occur (i.e. overturning).


 

IONOSPHERE

 

The region between 80 and 400 km is known as the ionosphere. Unlike previous regions this region is characterized by chemistry rather than temperature (note ionosphere includes mesosphere and thermosphere).

 

Ionosphere contains large number of charged particles (ions), produced by ionization of N2 and O2 by intense solar  energy.

 

No impact on weather but other significance:

 

1.      AM radio waves: inner region of ionosphere absorbs AM waves but outer regions reflect. The inner region exists only during day, so at night ionosphere reflects waves -> at night more distant radio stations can be picked up.

 

2.      Auroras: interaction of solar storms (flares) with the Earth’s magnetic field energizes gases in ionosphere and light is emitted. (Auroras emit light whereas clouds reflect light).

 

 

 

OTHER PLANETS

 

 

4 inner planets (Mercury, Venus, Earth and Mars: terrestrial planets) have well-defined solid surfaces.

 

Mercury: High temperatures and low gravity

đ        no atmosphere (escaped if there was one).

 

 

Venus: 90 times mass of Earth and 96% CO2 -> absorbs nearly all thermal radiation [“run-away greenhouse”]

đ        very hot (475C/890F at surface).

 

Mars: similar composition to Venus but much smaller mass (1/150th Earth) -> very little radiation from surface absorbed

đ        very cold.

 

 

5 outer planets (Jupiter, Saturn, Uranus, Neptune & Pluto: Jovian planets) have solid or liquid interiors that gradually merge with their atmospheres.


 

 

SOLAR ENERGY

 

Energy from the sun provides virtually all (99.9%) the energy that heats the Earth’s surface.

 

Furthermore, spatial variations in the solar heating drives the winds in the atmosphere and currents in the oceans.

 

Therefore, need to first understand how the sun heats the Earth and how this heating varies with location and time.

 

First must understand heat, temperature, and mechanisms for their transfer.

 

HEAT & TEMPERATURE

 

Heat is a measure of energy.

 

Matter is composed of atoms & molecules that are constantly vibrating and

 

Heat = total kinetic energy of atoms & molecules

 

By contrast, Temperature is a measure of intensity.

 

Temperature = average kinetic energy of individual atoms and molecules.

 

Heat and Temperature are of course closely related:

 

Add heat -> molecules move faster -> temperature rises

 

Remove heat -> molecules move slower -> temperature falls.

 

Quantity of heat depends on mass of material (as total energy) but temperature does not.

 

For example, compare a cup of boiling water with a hot bath.

 

                Cup                                       Bath

 

    Higher Temperature                       More Heat

                                                    (as larger volume)

 

đ        More ice can be melted in the bath than the cup

 

Note: Thermosphere has high temperatures but little heat (as little mass).

 


 

HEAT TRANSFER

 

2nd law of thermodynamics: all systems tend towards disorder.

 

Where there is a temperature gradient (change in T with distance) heat will flow in the direction to erase the gradient (and speed of flow will increase with the gradient), i.e.,

 

Heat always moves from a higher-temperature

body to a lower-temperature body.

 

e.g., Touch hot stove: heat enters hand and it warms.

       Hold an ice cube: heat enters ice and it melts.

 

There are 3 mechanisms of heat transfer:

 

đ        Conduction

 

đ        Convection

 

đ        Radiation

 

1.      Conduction

 

Transfer of heat through matter by molecular activity

e.g., metal spoon in hot pan.

 

The ability to transfer heat by conduction varies dramatically between substances, and is measured by the

 

Heat conductivity = rate of transfer / temperature gradient

 

e.g.,      copper                         0.92

            water (20C)                 0.0014

            air (20C)                                  0.00006

 

č      air is a very poor conductor of heat, and conduction in not important in the atmosphere.

 

2. Convection

 

Transfer of heat by movement within fluids (liquids and gases)

e.g., heated pan of water

 

As a fluid is heated it expands and becomes less dense, more buoyant, and rises. At the same time cooled fluids are more dense and sink. i.e., Warm air rises and cold air sinks

 

Convection is very important in the lower atmosphere, playing a crucial role in small scale (e.g., thermals) and large scale (e.g., global circulation) flows.

 

3. Radiation

 

Heat transfer that does not require a medium.

e.g., heat from open fire

 

Radiation is the mechanism that heat is transferred (through the vacuum of space) from the sun to the Earth.

 

Radiation is the transfer of electromagnetic (EM) energy via an electrical wave and a magnetic wave. When this energy is absorbed by an object there is an increase in molecular motion and hence temperature.

 

An important characteristic of all waves is the wavelength (crest-to-crest distance).  There is a spectrum of EM waves which can be characterized by their wavelength: 

from  radio waves (103 m) to gamma rays (10-14 m).

 

The Sun emits all forms of radiation but in varying quantities. The majority is in the UV to infrared range.

 


 

RADIATION LAWS

 

đ        All objects, at whatever temperature, emit radiant energy.

 

 

đ        Hotter objects radiate more total energy per unit area than colder objects.

 

E = s T4   (Stefan-Boltzman Law)

 

            Sun: T~6000K ->   E~74,000,000 W/m2

                        Earth: T~300K ->   E~           460 W/m2

 

đ        The hotter the body the shorter wavelength of maximum

 radiation

 

lmax   = c / T    (Wein’s Law)

 

Sun:  lmax = 0.44 mm , Earth:  lmax = 9.66 mm   (mm=10-6m)

 

     This is why solar radiation is called short-wave radiation,

            and terrestial radiation is called long-wave radiation.

 

đ        Objects that are good absorbers of radiation are also good

     emitters. A perfect absorber/emitter is called a blackbody.

 

The Sun and Earth absorb/radiate at nearly 100%, and are nearly   blackbodies.

 

However, gases are selective absorbers/radiators, i.e. they only      absorb or radiate at selected wavelengths. So the atmosphere is transparent to some wavelengths but opaque to others.

 


 

INSOLATION

 

Insolation= incoming solar radiation

 

 

What happens to insolation?

 

Answer: It is  absorbed, reflected, and scattered.

 

                        51% absorbed at surface (direct & scattered radiation)

 

                        19% absorbed in atmosphere/clouds

 

                        30% lost to space (reflection & scattering)

 

Most of the energy that is absorbed by surface is then re-radiated skyward … this is called terrestrial radiation

 

Absorption.

 

Absorption is the process by which atmospheric gases and particles reduce the intensity of insolation. Occurs via a transfer of energy into increase of molecular motion. Results in a warming of the absorber (atmosphere).

 

The absorbtivity of gases varies significantly, e.g., N2 is a poor absorber but O2, O3, and H2O are efficient absorbers (and are responsible for most of absorption in the atmosphere).

 

Note O2 and O3 absorb UV radiation -> important of O3.

 

Reflection:  Redirection of radiation away from the surface.

 

Sunlight reflected = albedo * incoming sunlight

 

Where albedo is the measure of ability of surface as a reflector, with albedo=1 for a perfect reflector.

 

            Fresh snow                   75-95  

            Old snow                                 40-60   (higher albedo for

            Sand                                        20-30     lighter colored

            Soil                                          15-25     surfaces)

          Black Road                    5-10

            Thick Cloud                 70-80

            Thin  Cloud                  25-30

 

Albedo also varies with angle of the sun: albedo of water varies from 5 (high sun) to 80 (low sun).

 

Total overall albedo of Earth and atmosphere is ~ 30 (most from clouds and not land-sea surface).

 

Scattering: Redirection of insolation in all directions (and not just back to space).

 

Solar radiation travels in straight line by can be redirected (scattered) by gases (Rayleigh) or aerosols (Mie). Scattering changes the direction but not the wavelength of light.

 

Gases and aerosols are more effective scattering different wavelengths -> why the sky is blue.

 

Why is the Sky Blue?

 

Gas molecules are most effective scattering shorter wavelengths of visual light (i.e. blue and violet), therefore

During the day blue light is readily scattered

 

đ        sky is blue

 

At dusk the Sun is at the horizon and light travels through more of the atmosphere, and blue light scattered out

 

đ        sky is orange-red

 

Aerosols scatter (Mie scattering) all wavelengths, therefore on polluted days

đ        sky is gray

 


 

TERRESTRIAL RADIATION

 

As TEARTH  < TSUN terrestrial radiation (TR) is a longer wavelengths than solar radiation (Wein’s Law).

 

Whereas the atmosphere is only a weak absorber of insolation it is an effect absorber on terrestrial radiation (i.e., gases within the atmosphere absorb wavelengths within the range of TR but not of solar radiation).

 

H2O and CO2 are the principal absorbers (but O3, CH4, and other gases also play a role). H2O absorbs 5 times more than other gases.

 

The fact the atmosphere is transparent to insolation by absorbs TR is the reason why there are high T in the lower troposphere and the decrease in T with height, i.e. the atmosphere is warmed from the bottom up.

 

When gases absorb TR  they warm and emit (radiate) this energy in all directions. Some of this travels back to the Earth’s surface, and heats the Earth’s surface. I.e., surface is heated by solar and atmosphere energy. This is the so called Greenhouse Effect (and the gases that absorb TR are called greenhouse gases).

 

[Glass in a greenhouse transparent to incoming but opaque to longer wavelengths of outgoing energy.]

 

Without this absorption by the greenhouse gases the Earth would not be as warm as it is. However, as the concentration of these gases are increasing there is concern the Earth’s T may increase.

 

Note Venus’ atmosphere is 97% CO2, and surface T = 475 OC/  890OF !   [“Runaway greenhouse effect”]


 

EARTH’S HEAT BUDGET

 

Earth’s T ~ constant -> balance must exist between incoming

  and outgoing radiation.

 

 

HEAT BUDGET AT EARTH’S SURFACE

INCOMING

 

OUTGOING

 

Solar Radiation

  51

Earth’s radiation

116

Atmos. Radiation

  95

Evaporation

  23

 

 

Convection

    7

 

146

 

146

 

 

HEAT BUDGET OF THE ATMOSPHERE

INCOMING

 

OUTGOING

 

Solar Radiation

  19

Radiation to space

  64

Condensation

  23

Radiation to surface

  95

Convection

    7

 

 

Earth’s radiation

110

 

 

 

159

 

159



HEAT BUDGET OF WHOLE PLANET

INCOMING

 

OUTGOING

 

Solar Radiation

100

Reflected/Scattered

  30

 

 

Atmos. Rad. To Space

  64

 

 

Earth Rad. To Space

    6

 

100

 

100



 


 

TEMPORAL AND SPATIAL VARIATIONS IN INSOLATION

 

Above balance is for an arbitrary 100 units of insolation, but amount of insolation is not constant with time or space

 

đ        insolation varies with season and latitude

 

We experience this as seasonal and latitudinal variations in T.

 

To understand these variations we need to understand the Earth-Sun relationships.

 

Earth’s Motions

 

¨      Rotation  (spin around own axis) – effects daily variations.

 

¨      Revolution (movement around Sun)

Distance between Earth and Sun varies between 147 (Jan 3) and 152 (July 4) million km. But only minor role in seasonal variations, e.g., Earth closest to Sun in NH winter.

 

ALTITUDE OF SUN

 

More important for seasonal & latitudinal variations of insolation is the altitude of the sun (angle to horizon).

 

In summer the noon sun is high in the sky but in winter it is lower (also earlier sunset in winter).

 

Altitude of sun affects amount of energy received at surface because

 

č    lower angle -> more spread out and less intense radiation

(as for flashlight beam).

 

č    lower angle -> more of atmosphere to pass through, and hence      more chance to be absorbed or reflected

(can look at sun at sunset).

 

(1st  more important than 2nd )

 

Why does the altitude vary with latitude?

 

Earth has spherical shape -> only places at a given latitude will receive vertical rays (i.e. sun at 90O = zenith), and as you move north or south the sun angle (& length of day) decreases.  Hence sun altitude, and therefore solar energy at surface, varies with latitude.

 

 

Why does the altitude vary with seasonal ?

 

Earth’s axis is at an angle of 23.5O to plane of orbit around the Sun (so called inclination angle) -> Earth’s orientation to the Sun changes with time, and hence so does latitude where rays are vertical.

 

For example, in June NH leans away and SH towards the Sun

      But in December NH lean towards and SH away from Sun

 

4 important dates:

                                           NH (SH)                         angle of vertical rays

Mar 21 or 22   Spring (Autumn) Equinox                      0

 

June 21 or 22   Winter (Summer) Solstice                       23.5N

 

Sept 22 or 23   Autumn (Spring) Equinox                      0

 

Dec 21 or 22    Summer (Winter) Solstice                       23.5S

 

23.5N and 23.5S are known as tropics of Cancer and Capricorn.

 

During NH summer solstice latitudes in NH have longer days than in SH, and above 66.5N (Arctic Circle) there continuous daylight while below 66.5S (Antarctic Circle) there is darkness. Opposite during winter solstice.

 

Above results in warmer temperatures in summer than winter.

 

Seasonal variations in the amount of solar energy is caused by the migration of vertical rays from the Sun, and resulting variations in Sun angle and length of day.

 

LATITUDINAL HEAT BUDGET

 

The migration of vertical rays means that, in annual mean, tropical latitudes receive more insolation than polar latitudes.

 

At same time, terrestrial radiation also varies with latitude (because of T variations, and fact that radiation emission is T-dependent).

 

đ        although globally balanced, incoming and outgoing radiation are not in balance at individual latitudes.

 

Incoming > outgoing at low, and outgoing > incoming at high latitudes.

 

This should mean that tropics continue to warm while the polar cool. But this doesn’t happen. Why?

 

The atmosphere and oceans are giant thermal engines that transfer heat from the tropics to the poles.

 

Alternatively, the latitudinal heat balance drives the atmospheric and oceanic circulations.

 

TEMPERATURE

 

The seasonal and latitudinal variations in altitude of the Sun explain a lot of the variations in temperature at the surface, but not all.

For example, if this was the only reason there would be no T variations around a latitude (which is not the case).

 

Several other factors that control the temperature.

 

Land-Water contrast

 

Land heats more rapidly and to higher temperatures, and cools more rapidly and to lower temperatures, than water

 

đ        greater T variations over land than over (near) water.

 

[Why? Heat penetrates deeper into water than soil/rock (convection in fluids), and so thicker layer of water warmed which can maintain temperatures. Also specific heat of water higher (more heat required) and evaporation leads to cooling.]

 

Ocean Currents

 

Movement of upper layers of ocean are closely coupled to atmospheric circulation (drag exerted by winds over ocean).

 

Ocean currents account for 1/4 of latitudinal heat transport.

 

Poleward moving warm and Equatorward moving cold ocean currents affect temperature of nearby locations. E.g.,

đ        London warmer than NYC  [Gulf stream + N. Atlantic drift].

đ        Walrus Bay (23S) cooler than Durban (29S) [Benguela Current]

 

Altitude

 

T drops 6.5 C/km -> colder T with height (but not whole story, as decay at surface is slower).

 

Daily variations also change with altitude: larger variations with increasing altitude (density decreases -> absorption decreases -> intensity of insolation increases -> rapid heating during day and cooling during night -> larger daily variations).

 

Geographic Position

 

Coastal locations:  T differs depending on whether prevailing wind onshore (ocean moderated) or offshore, e.g., Eureka vrs NYC.

 

Mountain barriers: mountains can ‘shield’ locations, e.g., Spokane vrs Seattle.

 

Cloud Cover

 

Clouds can reduce daily T variations: lower maximum (as clouds reflect insolation) and higher minimum (trap terrestrial radiation).

 


 

GLOBAL TEMPERARTURE DISTRIBUTIONS

 

Examination of global maps of January and July mean sea-level T shows:

 

¨      Decrease from equator to poles.

 

¨      Maximum south of equator in January, and north in July.

 

¨      Both warmest and coldest regions are over land.

 

¨      Less longitudinal variation in SH.

 

¨      Isotherms reflect ocean currents.

 

Also taking difference between two months shows:

 

č    Smaller seasonal variation near equator

 

č    Outside tropics there is greater seasonal variation over land than over ocean.

 

 

DAILY VARIATIONS

 

Maximum T after noon (~3pm), and minimum after midnight (~6am)

 

The maximum insolation is @ noon but maximum Earth re-radiation occurs later, around 3pm.