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

2. 2 Atmosphere composition Other components: Carbon dioxide (CO 2 ), Neon (Ne), Helium (He), Methane (CH 4 ), Krypton (Kr), Hydrogen (H 2 ) and Water vapor (highly variable)

3. 3 Atmospheric Constituents Atmospheric Constituents

4. 4 Introduction  Up to now, we have assumed lossless atm.  For 1 GHz< f< 15 GHz ~lossless  For higher frequencies, =>absorption bands H2OH2O O2O2 22.235 GHz 183.3 GHz IR & visible 50-70GHz 118.7GHz IR & visible

5. 5 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 Sky Temp= cosmic + galaxy

6. 6 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 10km P= 1013 mbars = 1013 HPa T= 300K

7. 7 Atmospheric Profiles US Standard Atmosphere 1962  Temperature  Density  Pressure  Pressure P= nRT/V=  air RT/M nRT/V=  air RT/M or P o e -z/H 3 or

8. 8 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

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10. 10 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)

11. 11 Line Shapes where, –S lm is the line strength –F(f,f lm ) is the line shape LINE SHAPES –Lorentz –Gross –Van-Vleck-Weisskopt Absorption frequency One molecule Many molecules: pressure broaden* *caused by collision between molecules

12. 12 Line shapes  Lorentz  Gross  Van-Vleck-Weisskopt

13. 13 Absorption Bands Absorption Bands  Mainly water and oxygen for microwaves Brightness Temperature [K] Frequency [GHz]

14. 14 Total Atmospheric  Absorption  g,  Opacity  ,  Loss factor L 

15. 15 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

16. 16 Relative Humidity Air Temperature Vapor air can hold Actual Vapor in the air [gr per kg dry air] Relative humidity 86 o F 27.610.8339% 77 o F 20.410.8353% 68 o F 14.910.8372% 59 o F 10.810.83100%

17. 17 Aviris