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# Temperature,Heat, and Energy Transfer

## on Sep 18, 2008

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## Temperature,Heat, and Energy TransferPresentation Transcript

• 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.
• 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
• Potential Energy Maximum Kinetic Energy Maximum
• 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
• Temperature
• Temperature is the average kinetic energy of the air molecules.
• High temperature – faster molecules.
• Low temperature – slower molecules.
• animation
• Fahrenheit Scale
• Developed in early 1700s by G. David Fahrenheit.
• Zero is the lowest temperature that salt water will freeze.
• 32 F o is freezing point of pure water.
• 100 F o was to be body temperature (slight mistake)
• 212 F o is boiling point of pure water.
• F scale used in US for surface temperature except in METARS.
• Absolute scale is the Rankine scale
• Celsius Scale
• Developed in late 1700s
• Zero is the freezing point of pure water.
• 100 is the boiling point of pure water at sea level.
• A change of 1 C o = 1.8 F o
• 1.8 C = F - 32
• 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.
• o K= o C+273
• Kelvin scale used for all scientific equations, such as gas law, etc.
• Temperature Scales 311 38 100 300 27 80 289 16 60 277 4 40 273 0 32 266 -7 20 255 -18 0 244 -29 -20 233 -40 -40 K C F
• 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
• Kinetic Theory of Gases
• Perfect Gas Law (animation)
• 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
• 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.
• 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)
• Specific Heat of Substances Sand Dry Air Clay Ice Water Substance 0.19 0.24 0.33 0.5 1.0 Specific Heat 5.2 C 4.2 C 3.0 C 2.0 C 1.0 C Temp rise for 1 calorie of heat added to one gram of material
• Incoming solar radiation is important to life, weather
• We receive 2 cal/cm min to warm us during daylight hours Area = 2  r
• 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
• Cooling the Earth
• All warm bodies emit infrared radiation
• Earth radiates heat through infrared all the time from entire surface Area =  r 2
• 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
• Greenhouse Effect
• In the news - but it is a natural process in the atmosphere
•
• 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
• Greenhouse effect is only one transfer
• Reflection from clouds
• Reflection from the oceans, lakes, etc.
• Reflection from the surface
• Direct absorption of air
• Absorption
• Conduction
• Convection
• Latent Heat conversion
•
• 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 ( o F)
• 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
• 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.
• 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.
• 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.
• 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.
• Thermals and Clouds Clouds are cause by rising air parcels.
• 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.
• Summary
• Definitions:
• Energy - Ability to do work.
• Work - move matter over some distance
• Kinetic Energy – mass moving =1/2 mv 2
• Temperature - average kinetic energy of the air molecules.
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• Summary (cont 1)
• Temperature Scales – Kelvin ( o K) Fahrenheit ( o F), and Celsius ( o C) scales
• 0 o K = -273 o C = -459 o F absolute zero
• 273 o K = 0 o C = 32 o F water freezing
• 373 o K = 100 o C = 212 o F water boiling
• C=5/9(F-32) ; o K= o C+273
• 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
• 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.