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)

– -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