Chapter 4 Heat, Temperature, and Circulation

Temperature Scales As temperature cools, atom and molecules would move slower. Absolute Zero – temperature in which an objects emits no radiation and no thermal motion. –0 K (No negative numbers) – o C – o F Some atomic level activity still occurs

Temperature Scales o F = 9/5 o C + 32 o o C = 5/9 ( o F - 32 o ) K = 5/9 ( o F ) K = o C

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 = J 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 –38 o F) or alcohol (freezes at -179 o F) 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 54 o F –# of chirps in 8 seconds + 4 = o C 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 o C) –ΔT: change in temperature ( o C) 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)

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 65 o F Approximations of residential fuel demands for heating and cooling Heating Degree Day –HDD = 65 o F – Average Daily Temperature Cooling Degree Day –CDD = Average Daily Temperature - 65 o F

Accumulated Degree Days for Corn Growing Degree Days

Wind Chill WC = T – 35.75(V 0.16 ) T(V 0.16 ) –T = Temperature in Fahrenheit –V = wind velocity in miles per hour