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 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%
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:
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.
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).
ð
impact on
climate ?
The amount of water vapor is small and variable, from 0 to 4 %.
But very important as:
è Source of clouds and precipitation.
è 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.
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.
ð
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.
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.
>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.
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.
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).
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).
4 inner planets (Mercury, Venus, Earth and Mars: terrestrial planets) have well-defined solid surfaces.
Mercury: High temperatures and low gravity
ð no atmosphere (escaped if there was one).
Venus: 90 times mass of Earth and
96% CO2 -> absorbs nearly all thermal radiation [“run-away greenhouse”]
ð
very
hot (475C/890F at surface).
Mars: similar composition to Venus but much smaller mass (1/150th Earth) -> very little radiation from surface absorbed
ð very cold.
5 outer planets (Jupiter, Saturn, Uranus, Neptune & Pluto: Jovian planets) have solid or liquid interiors that gradually merge with their atmospheres.
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 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)
ð More ice can be melted in the bath than the cup
Note:
Thermosphere has high temperatures but little heat (as little mass).
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.,
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:
ð Conduction
ð Convection
ð 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
è 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
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.
ð All objects, at whatever temperature, emit radiant energy.
ð 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
ð 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.
ð 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.
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.
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)
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.
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
ð sky is blue
At
dusk the Sun is at the horizon and light travels through more of the
atmosphere, and blue light scattered out
ð sky is orange-red
Aerosols scatter (Mie scattering) all wavelengths, therefore on polluted days
ð 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
ð
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
è
lower
angle -> more spread out and less intense radiation
(as for flashlight beam).
è
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).
ð
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
ð
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.,
ð
London
warmer than NYC [Gulf stream + N.
Atlantic drift].
ð
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:
è
Smaller
seasonal variation near equator
è
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.