Chapter 4 Heat, Temperature, and Circulation. Driving Question What is the consequence of heat...
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Transcript of Chapter 4 Heat, Temperature, and Circulation. Driving Question What is the consequence of heat...
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
• 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
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
Temperature Scales
• oF = 9/5 oC + 32o
• oC = 5/9 (oF - 32o)
• K = 5/9 (oF + 459.67)
• K = oC + 273.15
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
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
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
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
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 (blackbody)• In equilibrium temperature is constant, though
different parts may be different temperatures
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
• Ex) Low pressure system
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)
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
Thermal Response
• Because water retains heat better than land, areas near the coast have less temperature variation throughout the year
Sea Breeze Effect
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
Heat Imbalance (Tropics v. Poles)
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
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
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 (2012)
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
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
Accumulated Degree Days for Corn Growing Degree Days
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