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Temperature, Heat, and Energy Transfer

Temperature, Heat, and Energy Transfer. Wx 201 Henry Robinson. 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. Energy - Definitions. Energy - Ability to do work.

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Temperature, Heat, and Energy Transfer

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  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

  22. 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

  23. 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

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

  25. 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)

  26. 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

  27. 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.

  28. 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.

  29. 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.

  30. 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.

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

  32. 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.

  33. 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.

  34. 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

  35. 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

  36. 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.

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