Atmospheric Composition הרכב האטמוספירה

Transcription

1 Atmospheric Composition הרכב האטמוספירה

2 N 2 O 2 Trace Gases Water Vapor (H 2 O) Argon (Ar) Carbon Dioxide (CO 2 ) Neon (Ne) Helium (He) Methane (CH 4 ) Nitrous Oxide (N 2 O) Ozone (O 3 )

3 Nitrogen and oxygen concentrations experience little change, but carbon dioxide, methane, nitrous oxides, and chlorofluorocarbons are gases experiencing great increases in concentration.

4 ppm % Real Air

5 Why so much Nitrogen? It is volatile in most forms Eg. Ammonia gas It is unreactive with most solid earth material It is stable in sunlight. Why so much Oxygen? Produced by photosynthesis. Why so much Argon? It slowly degasses from rocks It is unreactive so stays in the atmosphere Argon is a noble gas Why so little carbon dioxide? Original atmosphere was probably about 25% CO 2 It dissolves in water It is used by plants in photosynthesis

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7 מחזור ההידרולוגי Water : The Hydrological Cycle

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9 m Mixing Ratio w v = m d mass of water vapour mass of dry air [g/kg] Dry regions ~ 1 g/kg Tropics ~ 20g/kg Specific Humidity: q = w + m d 1 + w m v m v Since m v << m d q~w Specific humidity NOT dependent on volume

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11 When evaporation = condensation w = w s w w s Saturation mixing ratio: PV = ρrt e s = water vapor pressure w s m vs m d ρ vs = ρ = e / s (p-e s ) d (R v T) (R d T) (mass of w.v. in saturated volume) w s = e s /(p-e s ) ~ e s /p e s depends only on T, and therefore w s depends only on T and p For a given T, w s is inversely proportional to the total pressure

12 Relative Humidity RH = 100 w = 100 e [%] w s e s RH is the ratio between the mixing ratio measured relative to the saturation value Dewpoint condensation of vapour to liquid

13 Dew Point Temperature: The temperature the air needs to be cooled in order to reach saturation (RH=100%) is called the dew point temp T d. Dry air T T d large Moist air T T d small Rule of thumb: T T d (100-RH) 5 If RH=85% T-Td ~3C Td T If T-Td = 15C RH ~ 25%

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15 Clausius Clapeyron Equation The CC equation describes the phase transition between water vapour and water liquid (drops), and indicates the dependence of the saturation vapour pressure on temperature. L v de s dt = Td = L v T( 2-1 ) L v = latent heat of vaporization per unit mass (2.25x10 6 Jkg -1 ) 2, 1 = specific volume of vapour and water d = increase in volume as unit mass changes from liquid to gas >0 2 >> 1 d ~ 2 = 1/ρ = R v T/p de s = L e s dt R v T 2 P, T are values at the phase change

16 de s L e = s L~ const. dt R v T 2 e s = e o exp (- L ) R v T or for the relationship between vapour pressure and dew point: de dt = L e d R v T 2 d ( L ) e = e o exp - R v T d

17 RH=100%. הטל נקודת Dew point Temp.. D.. E.

18 Measurements of the Vapor Pressure in the Atmosphere

19 Warm air can absorb more vapor than cold air, so for a given parcel of air, specific humidity declines from its highest in the tropics to its lowest in the colder poles. Desert air, at 30, however, is not more saturated than polar air.

20 C How can we reach saturation in atmosphere? B A

21 CONTRAILS (Condensation Trails)

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23 Evaporation Fog

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25 Fog ערפל

26 Radiation Fog Occurs at the ground when dew point temperature is reached by radiational cooling.

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28 Advection Fog Warm moist air that moves, or advects, above a cold surface may become cooled to its dew point temperature, creating an advection fog.

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30 Dew טל frost קרה T>0C T<0C

31 Mixing of Air Parcels

32 טמפרטורה הגולה הלחה ) w Wet Bulb Temperature (T The measurement of the temperature air will cool by evaporation until it reaches saturation. Heat from the air evaporates water on the wet bulb, increasing the mixing ratio around the thermometer cooling air. At saturation T w there is no more evaporation, and therefore the air stops cooling RH=69% T d T w T In practice T w ~ T d +T 2

33 Psychrometer

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