Lecture 2: Energy in the Atmosphere

• Vertical structure of the static atmosphere• Basics from physics: force, work, heat• Transferring energy in the atmosphere

– Conduction– Convection

• Effects of latent heat IMPORTANT– Advection of heat– Radiation – the sun supplies energy

• Radiative properties of the atmosphere

Vertical structure in the atmosphereDensity (rho) as a function of height above the surface

Vertical structure in the atmosphere

• What about pressure?• Hydrostatic equation: balance between pressure gradient

force and gravity.– dp/dz = - rho g

• Ideal gas law:– p = rho R T

Let’s go to the board!

z = - H ln (p/p0), where H is scale height and is only constant if T is constant.In other words, p = p0 exp(- z/H)

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Some fundamentals• Force = mass x acceleration unit: kg m/s2 = N (Newton)• Work = distance x force unit: Nm = J (Joule)• Energy capacity to do work unit: J

– Potential energy– Kinetic energy– Heat

• Heat is energy, random motions of molecules & atoms• Power is the rate of energy transfer, unit: W (J/s)• Temperature is not heat! 3 temp scales Deg F, Deg C, K. It

is a measure of the intensity of heat.• Specific heat of a substance is the heat required to raise

the temp of one gram of substance by 1 deg C.– Examples: water, ice, air, sand decreasing specific heat

a

Fahrenheit, Celsius and Kelvin temperature scales

Transferring heat in the atmosphere

• Conduction – requires contact• Convection – hot air rises, cool air sinks

– Latent heat release when a parcel of air containing some water vapor is forced upward so it cools until it reaches saturation. Further cooling will result in condensation when latent heat is released.

– Adiabatic cooling, compressional warming

• Heat advection – horiz movement of air• Radiation

The shape of flame results from convection

What is a parcel of air?

• It is a hypothetical bubble of air, flexible but impermeable and can be quite large. The air inside a parcel of air has the same properties, such as temp, pressure, water vapor content etc. The air outside the parcel is called the environment.

Phase changes of water(blue indicates removal of energy,

red indicates addition of energy to atmosphere)

600 cal/g

80 cal/g

Important source of energy in the atmosphere

Adiabatic cooling and warming(occurs as a parcel of dry air moves up and down in

the atmosphere). Notice the lapse rate

Dry adiabatic lapse rate

What happens when water vapor is present in the parcel of air and it rises?

Moist ascent

Dry ascent

Cools less as it rises than when it is dry

Cold air advection: a cold front is moving from left to right

Radiative heat transfer:

The sun emits radiation that propagates through space or matter in the form of waves.

Electromagnetic spectrumthe important parts for us are solar (including UV,

visible) and terrestrial radiation 4—400 micron

All objects (T>0) emit radiation.

The amount of radiation emitted rises with temperature.Wavelength of maximum emission decreases with

temperatureThe so-called “blackbody” is a perfect emitter and absorber

of radiation

When radiation interacts with the atmosphere:

• It may get absorbed (increasing the temp)

• It may be reflected (clouds are highly refl)

• It may be transmitted (atmosphere pretty transparent to parts of the solar spectrum)

• The albedo of an object describes the proportion of light that it reflects– Fresh snow has albedo 90%– Ocean at low latitudes has albedo of 8—10 %

Absorption of shortwave and longwave radiation by the atmosphere

The atmosphere is a highly selective absorber

Note the Atmosphericwindow