Temperature, heat, and energy balance

The particles (atoms and molecules) that make up all substances are always in motion We call this energy of motion “kinetic energy” The particles will not all have the same energy, and the energy of the particles is constantly changing as they undergo changes in speed So, for a given sample of matter, we can only talk about the average kinetic energy of the particles. Temperature is a measure of the average kinetic energy of the particles in a substance.

But what do we really mean by “temperature”?

• When we refer to an object as being hot, we usually mean it has high temperature• Temperature is more easily measured and defined than heat• Heat is a measure of the total molecular energy of a substance• The heat of an object depends on three characteristics of a substance: its temperature, its mass, and its composition

To drive the atmosphere, heat must be transferred from place to place and from substance to substance.

There are three ways by which heat is transferred:

• Conduction (contact)• Convection (motion of “currents”)• Radiation (energy transfer)

Conduction occurs when energy is passed directly from one item to another.

- If you stirred a pan of soup on the stove with a metal spoon, the spoon will heat up through conduction

Metals are excellent conductors of heat energy. Wood or plastics are not. These "bad" conductors are called insulators. That's why a pan is usually made of metal while the handle is made of a strong plastic.

Conduction can occur in solids, liquids, or gases. However, because the molecules in gases are far apart, gases are much poorer conductors that either solids or liquids.

• Convection transfers heat energy as fluid parcels move.• Depends on the fact that, in general, fluids expand when heated and thus undergo a decrease in density

• Warmer less dense portion of the fluid will tend to rise through the surrounding cooler fluid. • If heat continues to be supplied, the cooler fluid that flows in to replace the rising warmer fluid will also become heated and also rise.

Convection can occur in liquids or gases

BOTH conduction & convection at work here:

Pavement warms faster than grass; air touching pavement heats through _______, then rises and transfers heat to air above it through _______

Depending if the rising air is saturated or not, convection can be dry or moist

Moist convection is associated with cloud formation (but not the only way clouds form, as we will see later)

Radiation is the process by which heat is transferred in wave form - this process transfers heat at the speed of light

We can feel heat transferred by radiation, even though we aren’t in direct contact with the hot substance

Earth – Sun energy balance sunlight energy reaches Earth via radiation it is also the way energy leaves Earth to return to space

The Electromagnetic Spectrum

Shorter wavelength = higher energy

“Rules” of Radiation:1. All objects with a temperature above absolute zero (0 K) emit radiation2. Wein’s law: The higher the temperature of the object, the shorter the

wavelength of radiation emitted

3. Stefan-Boltzman equation: The higher the temperature of the object, the greater the total radiation emitted

As temperature increases, peak wavelength gets shorter and total energy emitted increases

The Sun’s peak wavelength of emission is in visible light, and earth’s is in infrared (IR)

Fun with Stefan-Boltzman

It’s possible to rearrange the S-B equation (leave as an exercise for you), equating the amount of energy received by the earth and the amount of energy emitted by the earth. This yields an equilibrium temperature, Te, that can be solved for.

Fs= solar constant, 1355 W m-2

A= albedo (reflectivity) of earth, 30%

εe= emissivity of earth, 1.0 (blackbody)

σB= Boltzman constant, 5.67 x 10-8 W m-2 K-4

Substituting gives T = 255 K, or -18 C. Brrr! Cold! But what gives? We measure the earth to be 288 K, not 255 K.

Answer? The emissivity is not 1.0, but rather 0.75, due to greenhouse gasses. Furthermore, if concentration of these gasses increases, emissivity will decrease, and Te will increase. Now you have a basic equation to describe global warming. Go forth and impress your friends!

Atmospheric spectral transmission: implications

Just a couple of points:

1- UV waves are strongly absorbed

2- IR waves are either transmitted or absorbed, depending on wavelength

3- Weather satellites rely these spectral properties of the atmosphere: satellite tunes its sensor to a particular wavelength “band” to observe clouds, etc. on earth

The solar radiation that reaches Earth is affected in one of the following ways:• it can be reflected• it can be scattered• it can be absorbed

Why is the sky blue?

_________ gives us our blue sky• The small gas atoms and molecules that make up the atmosphere preferentially scatter the shorter wavelengths of light

• Blue is shorter than green or red (remember “ROY G BIV”?), and thus blue is scattered first

As the sun gets lower in the sky, the sunlight has to travel through more atmosphere.

This gives the orange and red light more of a chance to be scattered.

Most of the sun’s energy is absorbed by the upper levels of our atmosphere (even though not much air exists there, it absorbs almost 100% of the very harmful rays

Visible, Infrared, and some Ultraviolet (UV) light does reach the earth’s surface

Question: why are you more likely to be sunburned on a mountaintop than you are at sea level?

The Reason for the Seasons

Why is the sun more intense in the tropics?

The daily temperature cycle

Daily maximum temperatures (usually) occur several hours after the time of maximum incoming solar radiation? WHY?

Not only daily temperature lag but a seasonal temperature lag, too

Seasonal variation of temperature depends strongly on latitude – but also on location within the continent

Seasonal temperature variation: Graz

What comes in, must go out:

Over the course of a year, Earth must emit as much radiation as it has received from the Sun

Energy in = Energy out

The Atmospheric “Greenhouse” Effect

CO2 and water vapor are the primary greenhouse gasses

Net result of all this talk about energy: weather is primarily driven by energy transfer around the planet

Another process by which energy is transferred through the atmosphere is called “latent heat”

Hurricane energy source: comes from the evaporation of warm ocean water. When this water vapor condenses into clouds in the atmosphere, a tremendous amount of energy is released in the form of latent heat: 3 x 1012 Watts, equal to rate of power consumption in the US in the year 2000!

Ocean currents also play a significant role in transferring heat. Major currents, such as the northward flowing Gulf Stream, transport tremendous amounts of heat poleward and contribute to the development of many types of weather phenomena.