1. 1 Microwave Interactions with the Atmosphere S. L. Cruz Pol Electrical and Computer Engineering University of Puerto Rico

2. 2 Atmospheric Constituents

3. 3 Introduction Up to now, we have assumed lossless atmosphere For 1 GHz< f< 15 GHz ~lossless For higher freqs., =>absorption bands H2OH2O O2O2 22.235 GHz 183.3 GHz IR & visible 50-70GHz 118.7GHz IR & visible

4. 4 Outline I. The atmosphere: composition, profile II. Gases: many molecules 1. Shapes( G, VVW, L ): below 100GHz, up to 300GHz e.g. H 2 O, O 2 2. Total Atmospheric Absorption  g, opacity  , and atm-losses L  3. T B : Downwelling Emission by Atmosphere

5. 5 U.S. Standard Atmosphere Troposphere – clouds, weather Stratosphere- no H 2 O or dust ozone absorption of UV warms air to ~40 o F Mesosphere no aircrafts here too cold ~-90 o F Thermosphere (or Ionosphere) 1000-3000 o F! Tropopause Stratopause Mesopause P= 1013 HPa T= 300K

6. 6 Atmospheric Profiles US Standard Atmosphere 1962 Temperature Density Pressure P= nRT/V=  air RT/M or P o e -z/H 3 or

7. 7 Water Vapor Profile Depends on factors like weather, seasons, time of the day. It’s a function of air temperature. Cold air can’t hold water Hot air can support higher humidities.(P dependence)  v (z)  o e -z/H 4 [g/m 3 ] where  o averages 7.72 in mid latitudes and the total mass of water vapor in a column of unit cross section is

8. 8 Water vapor molecule emissions

9. 9 EM interaction with Molecules Total internal energy state for a molecule –electronic energy corresponding to atomic level –vibration of atoms about their equilibrium position –rotation of atoms about center of molecule – E = E e + E v + E r Bohr condition f ml = ( E m - E l ) /h Values for energy differences for –electronic: 2 to 10 eV –vibrational-rotational: 0.1 to 2 eV –pure rotational: 10 -4 to 5 x 10 -2 eV ( microwaves)

10. 10 Line Shapes where, –S lm is the line strength –F(f,f lm ) is the line shape LINE SHAPES Absorption frequency One molecule Many molecules: pressure broaden* *caused by collision between molecules

11. 11 Line shapes Lorentz Gross Van-Vleck-Weisskopt

12. 12 Absorption Bands Mainly water and oxygen for microwaves Brightness Temperature [K] Frequency [GHz]

13. 13 Total Atmospheric Absorption  g, Opacity  , Loss factor L 

14. 14 Atmospheric Emission For clear atmosphere where Also there is some background radiation T cos =2.7K from the Big Bang and T gal ~0 above 5GHz

15. 15 Relative Humidity Air Temperature Vapor air can hold Actual Vapor in the air [gr per kg dry air] Relative humidity 86 o F27.610.8339% 77 o F20.410.8353% 68 o F14.910.8372% 59 o F10.810.83100%

16. 16 Aviris