Composition, Structure and Energy

ATS 351 Lecture 2September 14, 2009

Composition of the Atmosphere

Atmospheric Properties

• Temperature• Pressure• Wind• Moisture (i.e. water vapor)• Density

Temperature

• A measure of the average kinetic energy due to the random movement of atoms

• Faster motion of molecules = higher temperature

• No motion = absolute zero

Pressure

• A force exerted per unit area. In the atmosphere it • is a measure of the weight of the air above you.• Units

• Pa [N m-2]• 1 mb = 100 Pa• in Hg• atm

• Standard Pressure • (Sea Level)

• 1013.25 mb• 29.92 in Hg• 1 atm

• Typical Values• Fort Collins

• 850 mb• 25.10 inHg• 0.839 atm

• Long’s Peak: 600 mb• Mt. Everest: 300 mb Pressure = Force/Area

Force = Mass*Acceleration

Wind

• The movement of air due to pressure differences• Named for the direction from which it comes• Wind is measured in either miles per hour, meters per second,

knots (nautical miles per hour)• Expressed in either cardinal directions

(N,NE,E,SE,S,SW,W,NW) or in degrees from North

1 mph = 0.8689 kts1 mph = 0.4470 m s-1

180

270 90

0

Water Vapor

• Specific Humidity (q)• The actual amount of water vapor in the

air. Vapor compared to ALL air [g kg-1].• Mixing Ratio (r)• The actual amount of water vapor in the

air. Vapor compared to DRY air [g kg-1].• Relative Humidity (RH)• Ratio of the amount of water vapor that

exists to the amount of water vapor required for saturation. Expressed as a percentage. (T and P dependant)

• Dew point Temperature (Td)• The temperature at which the atmosphere

will become saturated (100% RH)

Density

• The ratio of the mass of any substance to the volume occupied by it

• Usually expressed in kilograms per cubic meter

Ideal Gas Law/Equation of State

• Relates the temperature, pressure, and volume of an ideal gas

• The Universal Gas Constant • R* = 8.314 [J mol-1 K-1]

• Gas Constant for Dry Air• R = 287.04 [J kg-1 K-1]

• The atmosphere is close to being an ideal gas.

n=mM

m=ρVnV=

ρM

R=RM

PV=nR T

P=ρRTn – number of molesm – mass [kg]M – molecular mass [kg

mol-1]

Hydrostatic Balance

• We tend to make the assumption that the atmosphere is in Hydrostatic Balance.

• Hydrostatic Balance is when the net upward force on a slab of air equals the net downward force.

dPdz

=−ρg

Layers of the Atmosphere

• Temperature and Pressure Profiles

Layers of the Atmosphere

• Defined by changes in temperature with height• Troposphere– Sun warms surface, surface radiates

• Stratosphere– Ozone absorbs solar radiation, warming results

• Mesosphere– No ozone, molecules lose more energy than

they absorb• Thermosphere– O2 absorbs solar radiation

Energy

• The ability to do work• Energy is always conserved• Potential Energy

• Represents the potential to do work (stored)• PE = mgh

• Kinetic Energy• Energy associated with motion• KE = 1/2 mv2

• The temperature of the air is a measure of its average kinetic energy or it is a measure of the average speed of the atoms and molecules.

• Internal Energy• Sum of all stored energy in molecules

Potential vs. Kinetic Energy

At any moment in its flight, the ball has exactly the same energy it had at the start (energy is conserved). The energy isdivided between potential and kinetic, but the total energy stays the same.What kind of energy does

the ball have as it leaves your hand?

As the ball goes higher, does it gain or lose potential energy?

All potential energy

Mostly kinetic

energy

Half potential,Half kinetic

Transfer of Energy• Conduction

• Molecules transfer energy to other molecules they come in contact with• Ex: The sun warms the ground, and this heats a

thin layer of air above the surface

• Convection• Energy transfer by the motion of matter

from one location to another• Ex: Warm, less dense parcel of air rising

• Radiation• Energy transfer not requiring contact

between bodies or a fluid between them• Ex: The sun warms the earth from 91 million miles

away

Radiation

• Radiation travels in the form of electromagnetic waves that release energy when they are absorbed by an object.

• All things, no matter how big or small, emit radiation.• The wavelengths emitted depend primarily on

the object’s temperature• Higher temperature → faster vibration of electrons

→ shorter wavelengths of emitted radiation• As the temperature of an object increases, more total

radiation is emitted each second (Stefan-Boltzmann Law):

E=sT 4

Radiation

• Radiation consists of waves propagating at the speed of light (c* = 3.0 x 108 m/s).

• Wavelength: λ

• Wavenumber (waves/length): ν = 1/ λ

• Frequency: ν ̃ = c*ν = c*/λ

Electromagnetic Spectrum

Shortwave (solar) radiation: Shortwave (solar) radiation: λλ < 4 < 4 μμmm

Longwave (terrestial) radiation: Longwave (terrestial) radiation: λλ > 4 > 4 μμmm

Wien’s Displacement Law

• The wavelength of maximum emission from an object is related to the temperature by a simple expression:

Sun: λmax= 0.5 μm

Earth: λmax= 10 μm

λmax=2897 [ μm·K ]

T

Solar vs. Terrestrial Radiation

Solar vs. Terrestrial Radiation

• Note the convenient “atmospheric window” directly under the peak solar emission, as well as the CO2 absorption over the peak terrestrial radiation

What happens to radiation in the atmosphere?

• Reflection• Albedo is a percentage of incident radiation that is

immediately reflected back• Absorption

• Everything that emits radiation also absorbs radiation• Some things are better at absorbing than others

• Scattering• EM waves can be scattered off in all directions when

they come in contact with particles in the atmosphere• The reason why the sky is blue and sunsets are red

• Transmission• Waves also may simply pass directly through an object

The Energy Budget

• To be in equilibrium > Energy in = Energy out• In our case,

– we receive shortwave solar radiation– and we emit longwave (infrared) radiation out to space

• How the energymoves around in the atmosphere is much more complicated

Greenhouse Effect

• Greenhouse gas molecules (and clouds) absorb outgoing infrared radiation, keeping the Earth from cooling without end.• Greenhouse gases are carbon dioxide,

water vapor, methane• These same absorbers radiate as well, slightly less

though since they are a lower temperature than the surface.

• Water vapor and CO2 absorb and radiate IR energy and act as an insulating layer around the earth ⇒ net effect is warming of the earth.

Greenhouse Effect