Transcript Chapter4HeatTempCirc

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
–0K
– -273.15 oC
– -459.67 oF
• Some atomic level activity still occurs
Temperature Scales
o
• F
oC
o
32
= 9/5
+
o
o
o
• C = 5/9 ( F - 32 )
o
• K = 5/9 ( F + 459.67)
o
• K = C + 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
• 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
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
Global Radiation Balance
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
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
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
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
680 cal/g
80 cal/g
540-600
cal/g
Cooling of Earth’s Surface
Bowen Ratio
• BR = sensible
heating/latent
heating
• For the earth
BR = 7/23 =
0.3
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
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
Wind 0.16
Chill
• WC = 35.74 + 0.6215T – 35.75(V
– T = Temperature in Fahrenheit
– V = wind velocity in miles per hour
) + 0.4275T(V0.16)