Slide 1

Transcript Slide 1

Energy

is defined as “the ability to do work.” The standard unit of energy in the International System (SI) used in scientific applications is the

joule

(J).

Power

is the rate at which energy is released, transferred, or received. The unit of power is the

watt

(W), which corresponds to 1 joule per second (1 joule = 0.239 calories).

All forms of energy fall into the general categories of

kinetic energy

and

potential energy

Kinetic energy can be viewed as energy in use and is often described as the energy of motion.

Potential energy is energy that has not yet been used, such as a cloud droplet that occupies some position above Earth’s surface. Like all other objects, the droplet is subject to the effect of gravity. The higher the droplet’s elevation, the greater its potential energy.

Energy can be transferred from one place to another by three processes: conduction, convection, and radiation.

Conduction

is the movement of heat through a substance without the movement of molecules in the direction of heat transfer. Conduction is most effective in solid materials, but it also is an important process in a very thin layer of air near Earth’s surface.

The transfer of heat by the mixing of a fluid is called

convection

. Unlike conduction, convection is accomplished by displacement (movement) of the medium.

During the daytime, heating of Earth’s surface warms a very thin layer of air in contact with the surface. Above this thin

laminar layer

, air heated from below expands and rises upward because of the inherent

buoyancy

of warm air (the tendency for a light fluid to float upward when surrounded by a heavier fluid).

Of the three energy transfer mechanisms,

radiation

is the only one that can be propagated without a transfer medium. Unlike conduction or convection, the transfer of energy by radiation can occur through empty space.

Virtually all the energy available on Earth originates from the Sun. However, radiation is emitted by all matter.

In the case of radiation, quantity is associated with the height of the wave, or its

amplitude

. Everything else being equal, the amount of energy carried is directly proportional to wave amplitude.

The quality, or “type,” of radiation is related to another property of the wave, the distance between wave crests or

wavelength

, which is the distance between any two corresponding points along the wave.

Electromagnetic radiation consists of an electric wave (E) and a magnetic wave (M). As radiation travels, the waves migrate in the direction shown by the pink arrow. The waves in (a) and (b) have the same amplitude, so the radiation intensity is the same. However, (a) has a shorter wavelength, so it is qualitatively different than (b). Depending on the exact wavelengths involved, the radiation in (a) might pass through the atmosphere, whereas that in (b) might be absorbed.

It is convenient to specify wavelengths using small units called

micrometers

(or

microns

1 micrometer equals one-millionth of a meter.

Perfect emitters of radiation, so-called

blackbodies

are purely hypothetical bodies that emit the maximum possible radiation at every wavelength.

Earth and the Sun are almost blackbodies.

The single factor that determines how much energy a blackbody radiates is its temperature. Hotter bodies emit more energy than do cooler ones. The intensity of energy radiated by a blackbody increases according to the fourth power of its absolute temperature.

This relationship is represented by the

Stefan-Boltzmann law

, expressed as

= σT 4

where

is the intensity of radiation in

watts per square meter, is a constant (5.67 x 10

-8

watts per square meter) and

is the temperature of the body in kelvins.

Celsius Temperature = ( o F - 32) / 1.8

Fahrenheit Temperature = (1.8 x o C) + 32 Kelvin Temperature = o C + 273

For any radiating body, the wavelength of peak emission (in micrometers) is given by

Wien’s law

: 

max = constant (2900)/T

where 

max

refers to the wavelength of energy radiated with greatest intensity.

Wien’s law tells us that hotter objects radiate energy at shorter wavelengths than do cooler bodies.

Solar radiation is most intense in the visible portion of the spectrum. Most of the radiation has wavelengths less than 4 micrometers which we generically refer to as

shortwave radiation

. Radiation emanating from Earth’s surface and atmosphere consists mainly of that having wavelengths longer than 4 micrometers. This type of electromagnetic energy is called

longwave radiation

Energy radiated by substances occurs over a wide range of wavelengths. Because of its higher temperature, emission from a unit of area of the Sun (a) is 160,000 times more intense than that of the same area on Earth (b).

Solar radiation is also composed of shorter wavelengths than that emitted by Earth.

As the distance from the Sun increases, the intensity of the radiation diminishes in proportion to the distance squared (

inverse square law

The

solar constant

is the amount of solar energy received by a surface perpendicular to the incoming rays at the mean Earth –Sun distance and is equal to

1367 W/m 2 .

Atmospheric gases, particulates, and droplets all reduce the intensity of solar radiation (

insolation

) by

absorption

, a process in which radiation is captured by a molecule. It is important to note that absorption represents an energy transfer to the absorber. This transfer has two effects: the absorber gains energy and warms, while the amount of energy delivered to the surface is reduced.

The

reflection

of energy is a process whereby radiation making contact with some material is simply redirected away from the surface without being absorbed. The percentage of visible light reflected by an object or substance is called its

albedo

. When light strikes a mirror, it is reflected back as a beam of equal intensity, in a manner known as

specular reflection

. When a beam is reflected from an object as a larger number of weaker rays traveling in different directions, it is called

diffuse reflection

, or

scattering

In addition to large solid surfaces, gas molecules, particulates, and small droplets scatter radiation.

Although much is scattered back to space, much is also redirected forward to the surface.

The scattered energy reaching Earth’s surface is thus

diffuse radiation

, which is in contrast to unscattered

direct radiation

Scattering agents smaller than about one-tenth the wavelength of incoming radiation disperse radiation through

Rayleigh scattering

, which is particularly effective for those colors with the shortest wavelengths.

Thus, blue light is more effectively scattered by air molecules than is longer-wavelength red light.

Microscopic aerosol particles are considerably larger than air molecules and scatter sunlight by a process known as

Mie scattering

, which does not have nearly the tendency to scatter shorter wavelength radiation that Rayleigh scattering does. Mie scattering causes sunrises and sunsets to be redder than they would due to Rayleigh scattering alone, so episodes of heavy air pollution often result in spectacular sunsets.

The sky appears blue because gases and particles in the atmosphere scatter some of the incoming solar radiation in all directions. Air molecules scatter shorter wavelengths most effectively. Thus, we perceive blue light, the shortest wavelength of the visible portion of the spectrum.

Sunrises and sunsets appear red because sunlight travels a longer path through the atmosphere. This causes a high amount of scattering to remove shorter wavelengths from the incoming beam radiation. The result is sunlight consisting almost entirely of longer (e.g., red) wavelengths.

The water droplets in clouds are considerably larger than suspended particulates reflecting all wavelengths of incoming radiation about equally, which is why clouds appear white or gray. Because of the absence of preference for any particular wavelength, scattering by clouds is sometimes called

nonselective scattering

Incoming solar radiation available is subject to a number of processes as it passes through the atmosphere. The clouds and gases of the atmosphere reflect 19 and 6 units, respectively, of insolation back to space. The atmosphere absorbs another 25 units. Only half of the insolation available at the top of the atmosphere actually reaches the surface, of which another 5 units are reflected back to space. The net solar radiation absorbed by the surface is 45 units.

The difference between absorbed and emitted longwave radiation is referred to as the

net longwave radiation

Shortwave and longwave radiation are not separate entities as far as the heating of the atmosphere and surface are concerned. When either is absorbed, the absorber is warmed. It is therefore natural to combine longwave and shortwave into

net allwave radiation

, or simply

net radiation

, defined as the difference between absorbed and emitted radiation, or equivalently, the net energy gained or lost by radiation.

Net radiation is the end result of the absorption of insolation and the absorption and radiation of longwave radiation. The surface has a net radiation surplus of 29 units, while the atmosphere has a deficit of 29 units.

Convection is a heat transfer mechanism involving the mixing of a fluid. In free convection, local heating can cause a parcel of air to rise and be replaced by adjacent air.

Forced convection

(also called

mechanical turbulence

) occurs when a fluid breaks into disorganized swirling motions as it undergoes a large-scale flow. Air is forced to mix vertically because of its low viscosity and the deflection of wind by surface features.

When energy is added to a substance, an increase in temperature occurs that we physically sense (

sensible heat

). The magnitude of temperature increase is related to two factors, the first of which is

specific heat

, defined as the amount of energy needed to produce a given temperature change per unit mass of the substance.

The temperature increase resulting from a surplus of energy receipt also depends on the

mass

of a substance.

Latent heat

is the energy required to change the phase of a substance (solid, liquid, or gas).

In meteorology we are concerned with the heat involved in the phase changes of water.

In the case of melting ice, the energy is called the

latent heat of fusion

. For the change of phase from liquid to gas, the energy is called the

latent heat of evaporation

Both the surface and atmosphere lose exactly as much energy as they gain. The surface has a surplus of 29 units of net radiation, which is offset by the transfer of sensible and latent heat to the atmosphere.

The atmosphere offsets its 29 units of radiation deficit by the receipt of sensible and latent heat from the surface.

The interactions that warm the atmosphere are often collectively referred to as the

greenhouse effect

, but the analogy to a greenhouse is not strictly accurate.

The greenhouse gases of the atmosphere do not impede the transfer of latent and sensible heat. Thus, it would be more accurate if the term “greenhouse effect” were replaced by “atmospheric effect.” The greenhouse effect keeps Earth warmer by absorbing most of the longwave radiation emitted by the surface, thereby warming the lower atmosphere, which in turn emits radiation downward.

One of the most immediate and obvious outcomes of radiation gain or loss is a change in the air temperature. The map depicts differences between mean temperatures in January and July through the use of

isotherms

, which are lines that connect points of equal temperature.

Certain geographical factors combine to influence temperature patterns across the globe. These factors include latitude, altitude, atmospheric circulation patterns, local conditions, continentality, (the effect of an inland location that favors greater temperature extremes) and ocean current characteristics along coastal locations.

The

daily mean

is defined as the average of the maximum and minimum temperature for a day.

The

daily temperature range

is obtained by subtracting the minimum temperature from the maximum.

The

monthly mean temperature

is found by summing the daily means and dividing by the number of days in the month. The

annual mean temperature

is obtained by summing the monthly means for a year and dividing by 12.

The

annual range

is obtained as the difference between the highest and lowest monthly mean temperatures.

If low temperatures are accompanied by windy conditions, a person’s body loses heat much more rapidly than it would under calm conditions due to an increase in sensible heat loss.

It is common for weather reports to state both the actual temperature and how cold that temperature actually feels, the

wind chill temperature index

Thermodynamic diagrams

(such as the Stuve above) depict the vertical profiles of temperature and humidity with height above the surface enabling forecasters to determine the height and thickness of existing clouds and the ease with which the air can be mixed vertically. The data on the charts are obtained from radiosondes that are carried aloft by weather balloons twice a day at weather stations across the globe.

Conduction - Heat Transfer Conduction of heat energy occurs as warmer molecules transmit vibration, and hence heat, to adjacent cooler molecules.

Warm ground surfaces heat overlying air by conduction.

Figure 2.5

Convection - Heat Transfer Figure 2.6

Convection is heat energy moving as a fluid from hotter to cooler areas.

Warm air at the ground surface rises as a thermal bubble, expends energy to expand, and hence cools.

Radiation travels as waves of photons that release energy when absorbed.

All objects above 0° K release radiation, and its heat energy value increases to the 4th power of its temperature.

Radiation - Heat Transfer Figure 2.7

Longwave & Shortwave Radiation The hot sun radiates at shorter wavelengths that carry more energy, and the fraction absorbed by the cooler earth is then re-radiated at longer wavelengths, as predicted by Wein's law.

Figure 2.8

Electromagnetic Spectrum Figure 2.9

Solar radiation has peak intensities in the shorter wavelengths, dominant in the region we know as visible, but extends at low intensity into longwave regions.

Absorption & Emission Figure 2.10

Solar radiation is selectively absorbed by earth's surface cover.

Darker objects absorb shortwave and emit longwave with high efficiency (e.g. Kirchoff's law).

In a forest, this longwave energy melts snow.

Atmospheric Absorption Solar radiation passes rather freely through earth's atmosphere, but earth's re-emitted longwave energy either fits through a narrow window or is absorbed by greenhouse gases and re-radiated toward earth.

Figure 2.11

Greenhouse Effect Figure 2.12B

Figure 2.12A

Earth's energy balance requires that absorbed solar radiation is emitted to maintain a constant temperature.

Without natural levels of greenhouse gases absorbing and emitting, this surface temperature would be 33°C cooler than the observed temperature.

Warming Earth's Atmosphere Figure 2.13

Solar radiation passes first through the upper atmosphere, but only after absorption by earth's surface does it generate sensible heat to warm the ground and generate longwave energy.

This heat and energy at the surface then warms the atmosphere from below.

Solar radiation passing through earth's atmosphere is scattered by gases, aerosols, and dust.

At the horizon sunlight passes through more scatterers, leaving longer wavelengths and redder colors revealed.

Scattered Light Figure 2.14

Incoming Solar Radiation Figure 2.15

Solar radiation is scattered and reflected by the atmosphere, clouds, and earth's surface, creating an average albedo of 30%.

Atmospheric gases and clouds absorb another 19 units, leaving 51 units of shortwave absorbed by the earth's surface.

Earth-Atmosphere Energy Balance Figure 2.16

Earth's surface absorbs the 51 units of shortwave and 96 more of longwave energy units from atmospheric gases and clouds.

These 147 units gained by earth are due to shortwave and longwave greenhouse gas absorption and emittance.

Earth's surface loses these 147 units through conduction, evaporation, and radiation.

Convection:

Air or Water Moving Upward by Heating

Vertical motion of Air (gases) or water Caused by heating is Called convection (Convectional Cells) Convection in a pan of water Warmer materials Move upwards & Cooler materials fall

Convection

Heat is transferred by the movement, from place to place, of a gas or liquid. The principal motion is vertical. All heated gases and liquids experience this kind of “mixing” heat exchange.

Classifying Radiation

• Classified based on wavelength, 

Radiation Laws

#1 Stefan-Boltzmann law:

• All objects emit radiation.

more radiation less radiation

• Hot objects emit more radiation (per unit area) than cold objects.

#2 Wien’s law:

The hotter the radiating body, the shorter the wavelength of maximum radiation.

Sun:

6000ºC (11,000ºF)

Earth:

15ºC (60ºF)

The hotter sun radiates more energy than the cooler earth & radiates the majority of its energy at much shorter wavelengths.

Absorption:

Assimilation & Conversion of Energy

Absorption is the processes of changing sunlight into heat at Earth’s surface Darker surfaces absorb

more sunlight & convert it to heat (they have a low Albedo, like asphalt and basaltic lava flows)

Lighter surfaces reflect

more light & therefore convert little Light energy to heat (like Snow)

Albedo:

Reflection as a Percentage

**Earth’s Average Albedo is 31%**

Fresh Snow 80% - 95% Forests 10%-20% Grass 25% - 30% Asphalt 5% - 10% Water Bodies 10% - 60% (depends on angle)

Absorption

of radiation by gases in the atmosphere. The shaded area represents the percent of radiation absorbed. The

strongest absorbers

of infrared radiation are water vapor & carbon dioxide.

Sunlight warms earth's surface

only

during the day surface

constantly

emits infrared radiation

With

greenhouse gases…

Air in the lower atmosphere is heated from below...

Scattering

Scattering & light

•

short





scatter

•

result?

Scattering (diffuse radiation) Backscattering (albedo)



distance ,

