Temperature, Heat, and Energy Transfer

1. Temperature, Heat, and Energy Transfer Wx 201 Henry Robinson

2. Philosophy • It is mused that in physics, when you know the mass, momentum (including rotational) and energy changes in a system, you have the problem solved.

3. Energy - Definitions • Energy - Ability to do work. • Work - move matter over some distance • Potential Energy – the ability to do work because of position (usually height) • PE = mgh: m = mass g = gravity h = height • Kinetic Energy – the energy of a mass because of its motion: v = velocity

4. Potential Energy Maximum Kinetic Energy Maximum

5. Conservation of Energy • Energy can not be created or destroyed • Energy is changed from one form into another. • Since the atmosphere is a big heat engine, most of our discussions of energy will center around heat transfer • Heat and Temperature are not the same thing

6. Temperature • Temperature is the average kinetic energy of the air molecules. • High temperature – faster molecules. • Low temperature – slower molecules. • animation

7. Fahrenheit Scale • Developed in early 1700s by G. David Fahrenheit. • Zero is the lowest temperature that salt water will freeze. • 32 Fo is freezing point of pure water. • 100 Fo was to be body temperature (slight mistake) • 212 Fo is boiling point of pure water. • F scale used in US for surface temperature except in METARS. • Absolute scale is the Rankine scale

8. Celsius Scale • Developed in late 1700s • Also called Centigrade scale. • Zero is the freezing point of pure water. • 100 is the boiling point of pure water at sea level. • A change of 1 Co = 1.8 Fo • 1.8 C = F - 32

9. Kelvin Temperature Scale Absolute zero – molecules stop moving. • Lowest possible temperature. • Zero K. = –459 degrees F. • Zero K. = -273 degrees C. • 1 degree K = 1 degree C. • oK=oC+273 • Kelvin scale used for all scientific equations, such as gas law, etc.

10. Temperature Scales

11. US Meteorology Temperature Units • Most of the world uses Celsius (metric unit) • U.S. uses F for surface air temperature. • U.S. uses C for air temperature above surface

12. Kinetic Theory of Gases • Perfect Gas Law (animation)

13. Heat • In the absence of chemical or phase changes, Heat is the total Kinetic Energy of the molecules. (Temperature is the average kinetic energy of the air molecules.) • First law of thermodynamics. • Animation

14. Heat • Transfer of heat energy to a mass changes its temperature and its dimensions. • Specific Heat – amount of heat needed to raise one gram of material one degree Celsius. • 1 calorie of heat will raise 1 gram of water one degree C.

15. First Law of Thermodynamics • Add heat to something and it goes into raising the temperature AND expanding the something • Adiabatic means not adding heat so compress a gas means the temperature goes up • The adiabatic lapse rate for air is 10 degrees C per kilometer (5.5 F/ 1000 ft)

16. Specific Heat of Substances

17. Radiation • Incoming solar radiation is important to life, weather • We receive 2 cal/cm min to warm us during daylight hours Area = 2pr

18. Surface absorption of heat • Sunlight not reflected by clouds or surface moves molecules in surface to increase total and average kinetic energy of molecules • Some heat moves downward but the rest warms the air just above the surface • Primary reason for the climatological lapse rate of 3.5 degrees F/1000 ft

19. Cooling the Earth • All warm bodies emit infrared radiation • Earth radiates heat through infrared all the time from entire surface Area = pr2

20. Balance • When incoming solar radiation equals outgoing infrared, the climate is in balance • If it is not in balance, the climate warms up or cools down. • We have had periodic imbalances in the past • We are probably going through a change right now

21. Greenhouse Effect • In the news - but it is a natural process in the atmosphere

23. At Issue • If the balance is changing, what sort of weather will result • Consensus seems to be Global Warming will result in • Sea level rise • Increased bacterial activity • Stronger storms • Faster winds

24. Radiation Transfers • Greenhouse effect is only one transfer • Reflection from clouds • Reflection from the oceans, lakes, etc. • Reflection from the surface • Direct absorption of air

25. Radiation Effects on Matter • Absorption • Conduction • Convection • Latent Heat conversion

27. Some Consequences of Specific Heat • Land areas warm up more rapidly than water areas for same heat input. Average air temperature near sea level in July (oF)

28. Latent Heat • Latent heat is the heat required to change state (solid to liquid or liquid to gas) • Latent heat of fusion (melting or freezing) • Water latent heat of fusion = 80 cal/g. • Latent heat of evaporation (or condensation) • Water latent heat of evaporation = 600 cal/g

29. Latent Heat Transfer to/from Environment • When water evaporates it takes heat from the environment (example: sweating cools body). When it condenses it releases heat to the environment. • Latent Heat of evaporation/condensation is an important sink/source of atmospheric energy • Latent heat drives hurricanes and thunderstorms.

30. Heat Transfer in Atmosphere • Conduction – transfer of heat from molecule to next molecule. Slow process. • Air is a poor conductor of heat. • Convection –vertical transfer of heat by fluid motions. (warm air rises by buoyancy) • Advection – horizontal transfer of heat by fluid motions. • Mixing of air is more efficient process of heat transfer than conduction.

31. Buoyancy • If a parcel is lighter than the fluid it displaces, it will rise. Gravity causes the heavier fluid to sink which forces the lighter parcel to rise. • Recall gas law PV=RT. • If all the air at a level warms, nothing will happen. Buoyancy requires localized differences in density caused by temperature differences.

32. Thermals • Differences in ground temperature caused hot and cool spots. • Warm air is forced up by cool air. • Rising air parcel goes to lower pressure. • Air parcel expands and cools (gas law). • If air parcel is still warmer (buoyant) than environment, it will continue to rise. • If air parcel is the same (or cooler) temperature than environment, it will stop rising.

33. Thermals and Clouds Clouds are cause by rising air parcels.

34. Thermals and Latent Heat • Rising parcel cools. If the air temperature reaches the dew point temperature (later chapter), droplets will condense out of the air. • Condensation will release latent heat (600 calories/gram). • Latent heat will warm air parcel making it buoyant relative to surrounding air.

35. Summary • Definitions: • Energy - Ability to do work. • Work - move matter over some distance • Kinetic Energy – mass moving =1/2 mv2 • Temperature - average kinetic energy of the air molecules.

43. Summary (cont 1) • Temperature Scales – Kelvin (oK) Fahrenheit (oF), and Celsius (oC) scales • 0 oK = -273 oC = -459 oF absolute zero • 273 oK = 0 oC = 32 oF water freezing • 373 oK = 100 oC = 212 oF water boiling • C=5/9(F-32) ; oK=oC+273

44. Summary (cont 2) • Heat -transfer of energy to a mass which changes its temperature. • Specific Heat – amount of heat needed to raise one gram of material one degree Celsius. • 1 Calorie of heat will raise 1 gram of water one degree C. • Specific heat: water=1.0; air=.24; sand=.19

45. Summary (cont 3) • Latent heat is the heat required to change state (solid to liquid or liquid to gas) • Water latent heat of fusion = 80 cal/g. • Water latent heat of evaporation = 600 cal/g • Latent heat drives hurricanes and thunderstorms • Convection –vertical transfer of heat by fluid motions. (warm air rises by buoyancy) • Buoyancy requires localized differences in density caused by temperature differences. • Advection – horizontal transfer of heat by fluid motions.