Heat and Energy Transport in the Atmosphere

RECAP: Energy, Temperature and Heat

•Energy♦ Kinetic♦ Potential, e.g. gravitational

•Temperature scales♦ Absolute temperature: K♦ Fahrenheit scale: F♦ Celsius scale: C♦ Freezing point: 273K<->0C<->32F♦ Boiling point: 373K<->100C<->212F

•Heat capacity and specific heat capacity♦ Large heat capacity: the object requires more energy/heat (and it takes longer) to warm up to a certain degree.

♦ Alternatively: given the same amount of heat, the object with the largest heat capacity warms up to the lowest T

♦ C(water) > C(soil) > C(air)

2

2

1mvEK =

mgHEp =

Heat Capacity

•Heat capacity: the amount of heat energy that is required to change the temperature of a body by 1 K.♦ Heat capacity= Heat energy/Temperature change

♦ It depends on the material and on the mass of the body

•Specific heat capacity: the amount of energy that is required to change the temperature of 1 gram of substance by 1 degree C.

♦ It does not depend on the mass of the body.♦ It depends only on the material of the body.

T

QC

ΔΔ

=

Tm

QC

ΔΔ

=

> >

= >

Table 2-1, p. 30

Thermal inertia

•Bodies with a large heat capacity cool and/or heat up very slowly.♦ Analogy with a heavy body (a big truck)

♦ Water has a high heat capacity (large thermal inertia) 1cal/gram/degree

♦ Regions near large bodies of water (rivers, lakes, oceans) do not experience sharp temperature changes. Their climate is mild.

♦ Air and land have smaller specific heats than water.

Figure 3.23

Phase Changes

Phase transitions (1 gram of water)

0C

100C

80 cal

ICE melting water boiling

100 cal

600 cal

HEAT IN

Tem

perature C

Latent heat

Sensible heat

vapor

Latent heat

•Latent heat: the heat required to change a substance from one state to another (phase change)♦ Evaporation/Melting (cools the environment)

♦ Condensation/Freezing (heats the environment)

The importance of latent heat

Heat: Q

•Heat is energy in the process of being transferred from one object to another.

•The amount of heat is equal to the change of energy that results from the process of energy transfer.

•Processes of heat transfer:♦ Conduction;♦ Convection;♦ Radiation.

Heat Conduction

•Description: ♦ Transfer of kinetic energy from one molecule to another

♦ Objects are in physical contact

•Necessary conditions:♦ Heat is conducted whenever there is a T difference.

♦ The energy flow is from the body of higher T towards the body with lower T

•Conducting materials:♦ Good conductors: metals♦ Insulators (poor conductors): water, air, wood.

•Conduction is NOT an efficient way to transport energy in the atmosphere.

•The heat transport through conduction near the ground is relevant only for a thin layer a few cm thick.

Table 2-2, p. 33

Convection

•Description:♦ Transfer of heat by mass movements of a fluid.

•Rising air cools and sinking air warms!!!

•Convection is a very efficient way to transport energy in fluids (gas, liquid).

•Advection: the horizontal movement of a parcel of fluid.

Fig. 2-6, p. 34

Development of a thermal

Radiation•Description

♦ Energy carried by electromagnetic waves.

♦ They are NOT mechanical waves!!!

•Characteristics:♦ Wavelength : the distance between two adjacent peaks.

♦ Units: 1 m (micron)= 10-6 m

♦ May propagate through vacuum and “transparent”

materials.♦ Visible: 0.4-0.7

m

Radiation and Temperature•All bodies with T>0K emit radiation (electromagnetic

energy).

•The origin of the emission is the transition of the atoms (molecules) from one energy state to another.

•The wavelength and the amount of energy emitted by the body depend on its temperature.

•Higher T -> larger internal energy -> atoms vibrate faster -> the radiation has shorter wavelength and higher energy.

The Sun’s electromagnetic spectrum

Black body radiationBlack body: it emits and absorbs at all wavelengths.

4TE σ=Stefan-Boltzmann Law:

T

const.max =Wien’s Law:

Temperature and Emitted Energy

4TE σ=

4TE σ=

T

const.max =

Energy

The Sun emits ~(6000/288)4~188,000 times more energy than the Earth!!!

Sun/Earth radiation

•Sun♦ T=6000 K

♦ max=0.5 m

♦ Maximum in visible

•Earth♦ T=288 K = 15 C

♦ max=10 m

♦ Maximum in IR

The Sun emits (6000/288)4~188,000 times more energy than the Earth!!!