Basic Properties of the Atmosphere

1. Basic Properties of the Atmosphere

2. Heat and Temperature • Temperature: Average energy of molecules or atoms in a material • Heat: Total energy of molecules or atoms in a material • Can have large amount of heat but low temperatures • Can have high temperatures but little heat

3. Heat and Temperature • The Arctic Ocean has a large amount of heat (because of large mass) even though the temperature is low. • Air in an oven at 500 F has high temperature but little heat. • However, touch anything solid in the oven, and you’ll get burned. Same temperature, much larger amount of heat.

4. Heat and Temperature • The earth’s outermost atmosphere is extremely “hot” but its heat content is negligible • The surface of the moon can reach 250 F in sunlight and -200 F in shadow, but the vacuum around the Apollo astronauts contained no heat. • It takes time for things to warm up and cool off.

5. Temperature Scales • Fahrenheit • Water Freezes at 32 F • Water Boils at 212 F • Centigrade or Celsius • Water Freezes at 0 C • Water Boils at 100 C • Two scales exactly equal at -40

6. Converting C to F – In Your Head • Double the Centigrade • Subtract the first Digit • Add 32

7. Converting F to C – In Your Head • Subtract 32 • Add the first Digit • Divide by two

8. Absolute Temperature • Once atoms stop moving, that’s as cold as it can get • Absolute Zero = -273 C = -459 F • Kelvin scale uses Celsius degrees and starts at absolute zero • Most formulas involving temperature use the Kelvin Scale

9. Electromagnetic Radiation • Radio: cm to km wavelength • Microwaves: 0.1 mm to cm • Infrared: 0.001 to 0.1 mm • Visible light 0.0004 – 0.0007 mm • Ultraviolet 10-9 – 4 x 10-7 m • X-rays 10-13 – 10-9 m • Gamma Rays 10-15 –10-11 m

10. Composition of the Atmosphere • Nitrogen 78.08% • Oxygen 20.95% • Argon 0.93% (9300 ppm) • Carbon Dioxide 0.035% (350 ppm) • Neon 18 ppm • Helium 5.2 ppm • Methane 1.4 ppm • Ozone 0.07 ppm

11. Other Components of the Atmosphere • Water Droplets • Ice Crystals • Sulfuric Acid Aerosols • Volcanic Ash • Windblown Dust • Sea Salt • Human Pollutants

12. Structure of the Atmosphere • Defined by Temperature Profiles • Troposphere • Where Weather Happens • Stratosphere • Ozone Layer • Mesosphere • Thermosphere • Ionosphere

13. Troposphere • Heating of the Surface creates warm air at surface • Warm air rises, but air expands as it rises and cools as it expands (Adiabatic cooling) • Heating + Adiabatic Cooling = Warm air at surface, cooler air above • Buoyancy = Cool air at surface, warmer air above • Two opposing tendencies = constant turnover

14. Stratosphere • Altitude 11-50 km • Temperature increases with altitude • -60 C at base to 0 C at top • Reason: absorption of solar energy to make ozone at upper levels (ozone layer) • Ozone (O3) is effective at absorbing solar ultraviolet radiation

15. Mesosphere • 50 – 80 km altitude • Temperature decreases with altitude • 0 C at base, -95 C at top • Top is coldest region of atmosphere

16. Thermosphere • 80 km and above • Temperature increases with altitude as atoms accelerated by solar radiation • -95 C at base to 100 C at 120 km • Heat content negligible • Traces of atmosphere to 1000 km • Formerly called Ionosphere

17. Structure of Atmosphere From Cunningham & Cunningham, 2004, Fig. 9.1

18. What are the 3 ways heat can be transferred? • Radiation: transfer by electromagnetic waves. • Conduction: transfer by molecular collisions. • Convection: transfer by circulation of a fluid. Image from: http://www.uwsp.edu/geo/faculty/ritter/geog101/uwsp_lectures/ lecture_radiation_energy_concepts.html#Radiation

19. Why is the Mesosphere so Cold? • Stratosphere warmed because of ozone layer • Thermosphere warmed by atoms being accelerated by sunlight • Mesosphere is sandwiched between two warmer layers

20. Composition and Altitude • Up to about 80 km, atmospheric composition is uniform (troposphere, stratosphere, mesosphere) • This zone is called the homosphere • Above 80 km light atoms rise • This zone is sometimes called the heterosphere

21. Mean Free Path • Below 80 km, an atom accelerated by solar radiation will very soon hit another atom • Energy gets evenly distributed • Above 80 km atoms rarely hit other atoms • Light atoms get accelerated more and fly higher • Few atoms escape entirely

22. Planets and Atmospheres • At top of atmosphere, an atom behaves like any ballistic object • Velocity increases with temperature • If velocity exceeds escape velocity, atom or molecule escapes • Earth escape velocity 11 km/sec. • Moon escape velocity 2.4 km/sec

23. Atmospheric Measurements • Temperature • Pressure • Humidity • Wind Velocity and Direction

24. Weather Instruments • Temperature: Thermometer • Pressure: Barometer • Humidity: Hygrometer • Wind Velocity and Direction: Anemometer and Wind Vane

25. Thermometers • Fluid • Mercury • Alcohol • Use expansion of fluid • Bimetallic • Differential expansion of different metals • Electronic • Electrical resistance change with temperature

26. Barometers • Mercury • Air pressure will support 10 meters of water • Mercury is 13 times denser • Air pressure will support 76 cm of mercury • Aneroid • Air pressure deforms an evacuated chamber

27. Hygrometers • Filament • Hair expands and contracts with humidity • Sling Psychrometer • Measures cooling by evaporation • Two thermometers • Wet bulb and Dry bulb • Electrical • Chemicals change resistance as they absorb moisture

28. Sounding • Balloons carry radiosondes • Thermometer • Barometer • Hygrometer • Transmitter • Typically reach 30 km before balloon breaks

29. Radar • Detect precipitation types and amounts • Doppler radar measures velocity of winds

30. Satellite Studies • Visual imagery • Infrared imagery • Laser spectroscopy

31. Earth’s Radiation Budget • What comes in must go out • Direct Reflectance (Short Wave) • 31% • Infrared Re-emission (Long Wave) • 69%

32. How Heat Moves • Radiation • Conduction • Convection

33. Albedo Albedo = % incident energy reflected by a body • Fresh snow: 75 – 95% • Old snow: 40 – 60% • Desert: 25 – 30% • Deciduous forest, grassland: 15 – 20% • Conifer forest: 5 – 15% • Camera light meters set to 18%

34. Global Albedo

35. Air Pressure • By lucky coincidence, earth’s atmospheric pressure is approximately neat round numbers in metric terms • 14.7 pounds per square inch (1 kg/cm2) • Pressure of ten meters of water • Approximately one bar or 100 kPa • Weather reports use millibars (mb) • One mb = pressure of one cm water

36. Pressure and Altitude • Average at sea level 1013 mb • 500 mb at 5 km (upper limit of human settlement) • 280 mb at 10 km • 56 mb at 20 km • 1 mb at 50 km • 0.00056 mb at 100 km

37. Pressure and Altitude