Atmospheric Structure
and Processes
Spring 2012, Lecture 6
1
Tropospheric
Properties
• As altitude increases
within the troposphere,
temperature decreases
• Heating is from the
ground up
• Mountain climbers
experience cooling at
altitude
• At the level of the tropopause, a temperature
minimum occurs – about -70º C
2
Stratospheric
Properties
• Above the tropopause, the
temperature begins to climb
again
• The ozone layer within the
stratosphere absorbs
ultraviolet (UV) radiation,
and reradiates it in the
infrared
• This produces in-situ heating
• Since the UV radiation comes from the sun, heating
is strongest at the top of the stratosphere
3
Pressure
• Pressure is the force per unit area applied
perpendicular to the surface of an object
4
Response to Pressure
• Compressibility is a measure of the relative
volume change of a fluid or solid as a response
to a pressure change
• Objects may be said to be compressible or
incompressible, depending on the degree of
volume change they experience per unit of
pressure
5
Compression of Water
• Water is often said to be incompressible
• At a depth of 4 km, with pressures are around
40 megapascals, water has a volume decrease
of 1.8%
• At 0º C, the compressibility is less than one
part in a billion per Pascal
• (One atmosphere is 101,000 Pascals)
6
Linear Pressure Response
• As the figure shows,
this means that water
shows a linear response
to an increase in
pressure
7
Non-Linear Pressure Response
• The figure is a graph of the
actual change in pressure
with increasing altitude, and
is clearly non-linear
• At an altitude of 8
kilometers, pressure is half
as much as at sea-level
• This is because the
atmosphere is compressible
Vertical scale is km
8
Compressible vs. Incompressible
• The figure shows a response to
pressure by a compressible
substance (air), and an
incompressible substance, water
• There is more air per meter at low
altitude than at higher altitude
• The amount of water per meter
does not depend on the depth to a
significant extent
9
Exponential Function
• The change in pressure with altitude is an
example of an exponential function
• Q = ekx, where:
o Q = quantity in question
o k is a constant, which may be positive or negative
o x is a variable
o e is an irrational number equal to 2.718281828….
10
Exponential Change
• Exponential change can
be positive, like ex in the
diagram
o Population growth is an
example
• Exponential change can
be negative, often called
decay, like e-x in the
diagram
o Radioactive decay is an
example
11
Change of Pressure with Altitude
• Pressure clearly decays (grows smaller) with
altitude
• We can calculate the change in pressure as
follows
o P(z) = 1 atm • e-z[km]/8 km
• z is the height above the ground, measured in kms
12
Temperature
• Temperature is related to the
average kinetic energy of the
molecules in the volume under
consideration
• The faster molecules move,
the higher the temperature
• It does not matter how many
molecules there are per unit
volume
13
Heat Content (Enthalpy)
• The heat content is equal to the energy
required to create a system, plus the energy
required to displace the surroundings, creating
room for the system
• If a gas is compressed, it warms up – we did
work on the system to compress it, which
added energy
• If a gas expands, it cools down – the gas
expanded, doing work on the universe
14
Adiabatic Change
• Adiabatic change refers to change with no
change in heat content
• Adiabatic expansion – a gas occupies a bigger
volume, but the molecules move slower
• Adiabatic compression - a gas occupies a
smaller volume, but the molecules move faster
15
Lapse Rate
• As gas rises in the
atmosphere, it expands,
because pressure is less
• If conditions are adiabatic, the gas will behave as
shown in the diagram, depending on how much
water it holds
16
Lapse Rate Definition
• The lapse rate is defined as the change with
height of an atmospheric variable
• The variable is usually temperature
• The adiabatic lapse rate is the change with
constant heat content
17
Phase Changes
• Substances, such as water, can exist in any of
three phases
o Gas (Water vapor)
o Liquid
o Solid (Water ice)
• A change in phase involves heat
o Water vapor → Water + heat
o Ice + heat → Water
18
Latent Heat
• If you stick your hand in an oven at 100º C for
a short time, you will not be burned
• If steam from a kettle contacts your hand, you
probably will be
• Steam has extra energy, called latent heat
• When the steam hits your hand, some of it
condenses, transferring energy to your hand,
and burns you
19
Vapor Pressure
• Water molecules in the
air contribute to the total
pressure within a system
• The pressure is known
as the vapor pressure
• Vapor pressure is primarily a function of the
temperature
• The higher the temperature, the higher the
vapor pressure
20
Saturation
• At any given temperature, air can hold a
certain amount of water vapor at equilibrium
• Equilibrium means if one water molecule
evaporates, another will condense
• If the water vapor content is below the
equilibrium value, the air is undersaturated –
water will tend to evaporate
• If it is above the equilibrium value, it is
supersaturated – water will tend to condense
21
Humidity
• Relative humidity is the water vapor pressure
divided by the saturation pressure
• As relative humidity increases, it is harder to
evaporate water – sweating as a means of
cooling becomes less and less efficient
• Absolute humidity is the amount of water the
air holds, per unit volume
o Usually expressed as grams per m3
22
Convection
• Convection is a movement of molecules within
a fluid, either liquid or gas
• It is sometimes used to mean the heat transfer
produced by such motion
o As such, it is a third means of heat transfer, along
with radiation and conduction
23
Producing Convection
• Convection may occur when a fluid is heated from
below, which causes the bottom fluid to expand,
becoming less dense, and thus rising
• Or it may be produced by cooling from above, which
causes the top fluid to contract, becoming more
dense, and thus falling
• Convection is a common process in thunderstorms
and hurricanes
24
Convection
Diagram - 1
• In A, a fluid has a uniform temperature, and is wellmixed
o In this situation, the fluid is stable
• In B, the fluid is heated from below, increasing the
temperature and decreasing the density
o The fluid is now convectively unstable
25
Convection
Diagram - 2
• If the fluid consists of two immiscible components, the
heated portion will rise to the top, float until it cools,
and then sink – the principle of a lava lamp, as shown
in C
• If the fluid is a single component, it will mix, and the
entire fluid will become warmer, as shown in D
26
Lava Lamps
• Slow heating
• Rapid heating
27
Convection in Compressible Fluids
• Figure a represents a stable situation in the troposphere, with
temperature decreasing with altitude
• Figure b shows heating from below – the heated air is less
dense, so it rises, but along its own adiabat – it can rise to the
top of the gas column if mixing does not occur
• If mixing occurs, the temperature profile of the whole column
is increased
28
Dry vs. Wet
Adiabats
• If air with relative humidity = 100% rises in the
atmosphere, it will expand and cool
• Cool air holds less moisture, so the water vapor will
start to condense to form droplets
• Condensing water releases latent heat, helping to
offset the cooling due to expansion
• This accounts for the dry and wet adiabats in the
diagram
29
Radiative vs. Convective
Equilibrium
• In the layer model we examined, there is no
convection, only blackbody radiation
• In reality, convection is important
• The radiative equilibrium lapse rate is about
16K/km
• The convective lapse rate for a dry adiabat is
around 10K/km, and for a wet adiabat around
6K/km
• This is called radiative-convective equilibrium
30
Radiation Altitude
• Some IR radiation goes directly into space,
through IR windows
• Other IR wavelengths are absorbed and
reradiated from the coldest part of the
atmosphere, the tropopause
• We can imagine an equilibrium altitude that
averages the different wavelengths, and this
was the skin altitude encountered earlier
• Skin temperature is commonly defined as the
temperature of the interface between the
earth's surface and its atmosphere
31
Increasing Skin Altitude
• As GHG concentration
goes up, more radiation
is trapped, and more
radiation to space comes
from the tropopause
• This raises the skin
altitude, which we can
denote as zskin
32
Calculating Ground Temperature
• We can calculate the worldwide average
ground temperature if we know the skin
temperature altitude and the lapse rate
• If the lapse rate is 6K/km, and the skin altitude
is 5 km, the calculation is as follows:
o Tground = Tskin + 6K/km • 5 km , or
Tground = Tskin + 30K
33
Changing Skin
Altitude
• If GHG concentration goes up, so does skin altitude
• This shifts the point at which the moist adiabat
intercepts the ground to a higher temperature
• Thus, greenhouse warming
34
Changing Skin Altitude
• We can rewrite the equation for changing
ground temperature with changing skin
altitude, as follows:
o ΔT= Δzskin • 6K/km , where
• ΔT is the change in temperature
• Δzskin is the change in skin altitude
35
Incompressible Atmosphere
• If the atmosphere were incompressible,
convection would keep the temperature equal
at all altitudes, thus making the lapse rate zero
o ΔT= Δzskin • 0K/km = 0
• There would be no greenhouse effect
36
Ground Temperature Sensitivity
• The lapse rate determines the sensitivity of the
ground temperature to increasing GHG
concentration
• Thus, this is a critical parameter for model
calculations
37