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Chapter 4

Chapter 4. Heat, Temperature, and Circulation. Driving Question. What is the consequence of heat transfer in the Earth-Atmosphere System?. Temperature Versus Heat. Kinetic Energy – energy within a body that is a result of motion

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Chapter 4

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  1. Chapter 4 Heat, Temperature, and Circulation

  2. Driving Question • What is the consequence of heat transfer in the Earth-Atmosphere System?

  3. Temperature Versus Heat • Kinetic Energy – energy within a body that is a result of motion • Heat – the total energy (kinetic and potential) of atoms or molecules composing a substance • Temperature – a measure of the average kinetic energy of the individual atoms or molecules composing a system • Temperature is NOT a type of energy, but a number used to measure kinetic energy

  4. Temperature Scales • Absolute Zero – temperature in which an objects emits no radiation • 0 K • -273.15 oC • -459.67 oF • Some atomic level activity still occurs

  5. Temperature Scales • oF = 9/5 oC + 32o • oC = 5/9 (oF - 32o) • K = 5/9 (oF + 459.67) • K = oC + 273.15

  6. Heat Units • Calorie (cal) – amount of heat needed to raise the temperature of 1 gram of water by 1 Celsius degree • This calorie has nothing to do with food calories • British Thermal Unit (Btu) – amount of heat needed to raise the temperature of 1 pound of water by 1 Fahrenheit degree • Joule (J) – most common form of measuring heat • 1 cal = 4.1868J • 1 Btu = 252cal = 1055J

  7. Measuring Air Temperature • Thermometer – device used to measure variations in temperature • Liquid in Glass • Invented in 1593 by Galileo • Filled with mercury (freezes at –38oF) or alcohol (freezes at -179oF) • Bimetallic • Two metals bonded together (brass and iron) that bend when heated or cooled • Rigged to a pen and drum that records continuous temperature - thermograph

  8. Measuring Air Temperature • Electrical Conductor – variations in electrical resistance calibrated in terms of temperature • Used in radiosondes • Replacing liquid in glass thermometers in NWS • Cricket chirps: temperature must be above 54oF • # of chirps in 8 seconds + 4 = oC • Methods for accurate measurements • Ventilation • Shielded from precipitation, direct sunlight, night sky • Located away from obstacles such as trees, buildings

  9. Heat Transfer • Temperature Gradient – a change in temperature with distance • Second Law of Thermodynamics (Entropy) – all systems tend towards disorder trying to eliminate gradients • Heat flows from higher temperature to lower temperature to erase the gradient • Larger the gradient the faster the rate of change • Types – Conduction, Convection, Radiation

  10. Radiation • Form of energy and energy transfer • Can travel through a vacuum • Principal means of EA system gaining heat from the sun and heat escaping to space • Radiational Heating – absorption > emission • Radiational Cooling – absorption < emission • Radiational Equilibrium - absorption = emission • In equilibrium temperature is constant, though different parts may be different temperatures

  11. Convection • Convection – transport of heat within a fluid due to the motions of the fluid itself • Only occurs in liquids and gases • In the atmosphere, it is caused by differences in air density • Cold dry air sinks because it is more dense • Warm moist air rises • This air expands, cools, and sinks again

  12. Conduction • Conduction – transfer of kinetic energy (heat) through collisions of molecules • Heat Conductivity – ratio of the rate of heat transport to a temperature gradient • Solids are best conductors, gases are worst • Poor conductors are good insulators (still air)

  13. Thermal Response • Specific Heat – the amount of heat needed to raise 1 gram of a substance 1 degree Celsius (a calorie) • Q = mc(ΔT) • Q: change in heat (calories, Joules) • m: mass of object (grams) • c: specific heat capacity (calories/gram oC) • ΔT: change in temperature (oC) • A higher value of “c” indicates a greater ability to store heat and resist temperature change • Thermal Inertia – resistance to temperature change

  14. Thermal Response • Because water retains heat better than land, areas near the coast have less temperature variation throughout the year

  15. Global Radiation Balance

  16. Heat Imbalance (Surface v. Atmosphere) • Earth’s surface undergoes net radiational heating • Earth’s atmosphere undergoes net radiational cooling • In response to this unbalance heat is transferred to the atmosphere from the surface

  17. Sensible Heating • Transport of heat from one place to another by way of conduction, convection, or both • Examples • Warm winds blowing over snow covered ground • Warm winds blowing over relatively cool ocean surface

  18. Latent Heat • Latent Heat – the heat that is involved in the phase changes of water • Example: cloud formation • Latent Heat of Melting (s  l) • 80 cal/g added • Latent Heat of Fusion (freezing: l  s) • 80 cal/g released

  19. Latent Heat • Latent Heat of Vaporization (evaporation: l  g) • 540 cal/g at 100oC added • 600 cal/g at 0oC added • 680 cal/g at 0oC added (sublimation: s  g) • Latent Heat of Fusion • Same numbers – except heat is released • condensation: g  l • deposition: g  s

  20. 680 cal/g 540-600 cal/g 80 cal/g

  21. Cooling of Earth’s Surface

  22. Bowen Ratio • BR = sensible heating/latent heating • For the earth BR = 7/23 = 0.3

  23. Heat Imbalance (Tropics v. Poles)

  24. Heat Imbalance (Tropics v. Poles) • Tropical Areas: incoming solar radiation is greater than outgoing IR radiation • Polar Areas: incoming solar radiation is less than outgoing IR radiation • Global Radiative Equilibrium: surplus of solar radiation = deficit of IR radiation • Excess heat in tropics is transported to higher latitudes by air masses

  25. Response to Heat Imbalance: Weather • Heat imbalances create temperature gradients • Between surface and troposphere • Between tropics and polar latitudes • Heat is transported by conduction, convection, clouds, air masses, storms • Circulation of the atmosphere • Circulation brings about changes in the state of the atmosphere • WEATHER

  26. Temperature Variations • Time and Day of year: solar intensity, angle • Cloud Cover • Surface Characteristics (albedo) • Temperature is warmer when • It is daylight • Under clear skies during the day • Under cloudy skies during the night • When the ground is not snow covered • When the ground is dry

  27. Air Mass Advection • Movement of an air mass from one place to another • Warm Air Advection (WAA) • Movement of warm air • Cold Air Advection (CAA) • Movement of cold air • Advection occurs when isobars and isotherms are NOT parallel

  28. Degree Days • Based on 65oF • Approximations of residential fuel demands for heating and cooling • Heating Degree Day • HDD = 65oF – Average Daily Temperature • Cooling Degree Day • CDD = Average Daily Temperature - 65oF

  29. Wind Chill • WC = 35.74 + 0.6215T – 35.75(V0.16) + 0.4275T(V0.16) • T = Temperature in Fahrenheit • V = wind velocity in miles per hour

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