Composition, Structure and Energy. ATS 351 Lecture 2 September 14, 2009

Transcription

1 Composition, Structure and Energy ATS 351 Lecture 2 September 14, 2009

2 Composition of the Atmosphere

3 Atmospheric Properties Temperature Pressure Wind Moisture (i.e. water vapor) Density

4 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

5 Pressure A force exerted per unit area. In the atmosphere it is a measure of the weight of the air above you. Units Standard Pressure (Sea Level) Typical Values Pa [N m-2] 1 mb = 100 Pa in Hg atm mb in Hg 1 atm Fort Collins 850 mb inhg atm Long s Peak: 600 mb Mt. Everest: 300 mb Pressure = Force/Area Force = Mass*Acceleration

6 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 0 1 mph = kts 1 mph = m s

7 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)

8 Density The ratio of the mass of any substance to the volume occupied by it Usually expressed in kilograms per cubic meter

9 Ideal Gas Law/Equation of State PV=nR T m n= M m=ρv n ρ = V M R R= M P=ρRT Relates the temperature, pressure, and volume of an ideal gas The Universal Gas Constant R* = [J mol-1 K-1] Gas Constant for Dry Air R = [J kg-1 K-1] The atmosphere is close to being an ideal gas. n number of moles m mass [kg] M molecular mass [kg mol-1]

10 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. dp = ρg dz

11 Layers of the Atmosphere Temperature and Pressure Profiles

12 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

13 Energy The ability to do work Energy is always conserved Potential Energy Kinetic Energy Represents the potential to do work (stored) PE = mgh 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

14 Potential vs. Kinetic Energy All potential energy As the ball goes higher, does it gain or lose potential energy? Half potential, Half kinetic What kind of energy does the ball have as it leaves your hand? Mostly 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 is divided between potential and kinetic, but the total energy stays the same.

15 Transfer of Energy Conduction Molecules transfer energy to other molecules they come in contact with Convection Energy transfer by the motion of matter from one location to another Ex: The sun warms the ground, and this heats a thin layer of air above the surface 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

16 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

17 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

18 Electromagnetic Spectrum Shortwave (solar) radiation: λ < 4 μm Longwave (terrestial) radiation: λ > 4 μm

19 Wien s Displacement Law The wavelength of maximum emission from an object is related to the temperature by a simple expression: 2897 [ μm K ] λ max = T Sun: λmax= 0.5 μm Earth: λmax= 10 μm

20 Solar vs. Terrestrial Radiation

21 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

22 What happens to radiation in the atmosphere? Reflection Absorption Everything that emits radiation also absorbs radiation Some things are better at absorbing than others Scattering Albedo is a percentage of incident radiation that is immediately reflected back 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

23 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 energy moves around in the atmosphere is much more complicated

24 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.

25 Greenhouse Effect