section 0101 lecture:tuesday-thursday 9:30-10:45 (css 2400) discussion: wednesday 2:00-2:50 (css...

Section 0101 Lecture: Tuesday-Thursday 9:30-10:45 (CSS 2400)

Discussion: Wednesday 2:00-2:50 (CSS 2428)

Section 0102 Lecture: Tuesday-Thursday 9:30-10:45 (CSS 2400)

Discussion: Wednesday 1:00-1:50 (CSS 2428)

Final Grades will be determined by: 

Homework 10%Midterms (1 & 2) 50%

Final 40%

Times and Places for Discussion

Announcements

Grading

Test 1

Test 2

Homework

Final

25%

10%

40%

25%

TentativeGrading ScaleA 90+B 80’sC 70’sD 60’sF <60

Web CT

• We’ll be posting our lectures on Web CT every week after they’ve been given.

Getting an A

• Read the book before class

• Do all the homework• Homework isn’t

enough—answer all review questions at the end of the chapter

• Come to class

• Stop us for questions!• Follow the weather

every day• The first test will be

the easiest—free points!

2/4/2003

Outline for Lecture 3

• Composition of the Atmosphere

• Earth-Sun Relationship

• Energy, Heat and Temperature

• Mechanisms of Heat Transfer

• The Fate of Incoming Solar Radiation

• What the Heck was THAT?

Composition of the Atmosphere

Proportional volume of gases composing dry air.

78.1%

20.9%

N2

Ar CO2

O2

Inert Gases

H2

N2O

CH4 TraceGasesO3 CO

SO2,NO2C FC’s

Tropospheric Composition

•Solar Radiation is responsible for >99% of the energy that heats the earth.

•The amount of energy received from the sun varies with: latitude, time of day, season of the year and optical thickness of the Atmosphere (urban pollution may also be important).

•The Earth is heated unequally by this system and it is this unequalheating which drives the “weather machine”.

General Comments:Earth-Sun Relationship

•Revolution: Earth’s movement about the sun. Completes the journey in 365 days…..

Aphelion (Apogee)Perihelion (Perigee)

91 million miles 94 million miles

•Rotation: Earth’s movement about its axis. Completes the journey in 24 hours; spinning about the axis is responsible for diurnal (“daily”) variations in the atmosphere.

NORTHERN HEMISPHERE

Earth’s Motions

Interesting that the sun is farthest from the earth when the northern hemisphere experiences summer.

Clearly the distance from the sun is not the dominant factor which determines the seasons. (~7% difference)

What are the controlling factors which make the seasons change?

There are other motions which we will not consider in this class(Axial Precession, “wobble”)

Earth’s Motions

The Sun’s rays are at ahigher angle to the earth in the summerthan in the winter.

Three things to keep in mind when trying to understand the seasons:1. The higher the angle of the sun, the more intense (concentrated)

the sun’s rays. At 39 °N, this is a 52% difference!2. The angle of the sun determines the thickness of the

atmosphere that the sun must penetrate.

3. The length of day determines theamount of energy that the earth is receiving (see figure2-6 in textbook).

The Seasons

0

5

10

15

20

25

30

35

40

45

50

0 10 20 30 40 50 60 70 80 90 100

Angle of the Sun Above the Horizon

Eq

Nu

m o

f A

tmo

s S

un

lig

ht

mu

st

pa

ss

th

rou

gh

Equivalent Number of Atmospheres Sunlight Must Pass Through

What causes the fluctuations in the sun’s angle?

Earth’s orientation to the sun is constantly changing

Earth’s axis is not perpendicular to the plane of the ecliptic (23.5°).

This tilt of 23.5 ° is called the inclination of the axis.

….no inclination, no seasonal variation in the weather.

Earth’s Orientation

Earth only

Most Energy Less Energy Least Energy

Angle of the sun

Earth + Sun

The farther away (north orsouth) from the latitude where the suns rays are 90° overhead, the less intense the radiation will be.

Equator

23.5° N

23.5° S

Fall

Summer

Winter

Spring

Migration of position where sun’s rays are 90° overhead

Solstices- Means to halt. The accession when the sun reaches its farthest northern position (summer solstice) or farthest southern position (winter solstice). Equinox—equal night. Time when the sun’s vertical rays are striking the equator. Lengths of daylight and darkness are equal at all latitudes.

Summer solstice

Winter Solstice

Equinox

Equator

23.5° N

23.5° S

Solstices and Equinoxes

Migrating vertical (90° from the horizon) rays of the sun and the resulting variations the sun’s angles determine the amount of energy that reaches earth….and in doing socreates the seasonal changes in the weather.

Summary of the Seasons

Energy- the ability to do work

Kinetic-energy in motion

Potential- stored energy, the ability to do work

Energy, Heat and Temperature

Heat is the total kinetic energy while temperature is a measure of that energy.

When we specifically speak of “heat energy” we are referring to the motion of atoms and molecules.

Therefore, temperature is a measure of the average kinetic energy of the individual molecules of a substance.

Temperature is measure of intensity whereas energy is a measure of quantity.

Heat Energy versus Temperature

Mechanisms of Energy Transfer

Conduction is the movement of heat through a substance without the direct movement of molecules in the direction of heat transfer. The energy is transferred by collisions from one molecule to another with the flow of heat from hotter to cooler substances.

There is much variability in substances that conduct heat.

Metals are good conductors (because of their atomic structure).Air is a poor conductor, a.k.a. an insulator (there is a lot of empty space between air molecules).

ConductionProcess in theAtmosphere

energy

Conduction

air,water

Convection is the transfer of heat by the mixing of a fluid. Transfer is accomplished by the displacement of a medium.

Convection is more important to weather processes than conduction because little energy is transferred in the atmosphere by conduction.

Meteorologists think of convection as the vertical movement of air and advection as the horizontal movement of air.

ConvectionProcess in theAtmosphere

Convection and Advection

Radiation is the only method of heat transfer that does not require a medium (i.e. air, water, metal….etc).

Radiation is perhaps the most important energy transfer process to weather given that most of the energy which drives our weather andatmosphere comes from the sun.

Radiation

We classify radiation by looking at different wavelengths.

Wavelengths are oftenmeasured with the units of micrometers (m) 10-6 meters.

Intensity and wavelengths of Electromagnetic Radiation

1. All objects with temp. greater than –273° C emit radiant energy— everything: you, me, the earth, the sun, pigs, dogs, everything!

2. Hotter objects emit more total energy per unit area than do cooler objects.

3. The hotter the radiating body, the shorter (and hence more energetic) the wavelength of light.• Earth has an average temperature of 59°F (~10m)!

• Surface of sun has an average temperature of 10,000°F (~0.5m)

• Air surrounding a lightning bolt ~ 18,000 °F (~0.16m)

Laws of Radiation

Three things can happen to solar radiation in the atmosphere.

Absorbed

TransmittedReflected/Scattered

Energy consumed by molecule Energy passes

through molecule

Energy redirected by molecule

The Fate of Incoming Radiation

Reflection is the process whereby light bounces back from the object at the same angle and intensity as the incoming radiation.

The scattering process produces a larger number of weaker energy rays which radiate in different directions. More energy associated with forward scattering than backward scattering.

Albedo is the fraction of radiation that is reflected by a surface; the albedo of an object (for visible light) is dependent on its color…..Fresh snow is a good reflector of sunlight and thus has a high albedo, asphalt “blacktop”absorbs sunlight and therefore has a low albedo…Albedo for clouds varies considerably depending on type.

Reflection and Scattering

More radiation absorbed by the Earth’s surface than the atmosphere. This explains why the air is heated from below by the Earth.

Average distribution of incoming solar radiation by percentage

Absorption

•Ozone absorbs UV radiation

•Water Vapor, CO2 absorb IR Radiation

Examples of gases which absorb radiation in the atmosphere

Transmittance

•Nitrogen and Oxygen are mostly transparent to both IR and UV radiation…but they do absorb higher forms of radiation…

Examples of gases that allow radiation to transmit through them

•Aerosols and clouds scatter and reflect radiation

Back scattered light (same wavelength, same intensity)

Gas molecules more efficiently scatter the shorter wavelengths of visible light (blue and violet) than the longer wavelengths (red and orange).

Scattering is responsible for Blue Skies and Red Sunsets

By the evening, when the sun is low on the horizon, all the blue light is scattered out leaving mostly red and orange light.

Blue skies are produced as shorter wavelengths of the incoming visible light (violet and blue) are selectively scatteredby N2 and O2 – which are much smaller than the wavelength of the light

Larger particles (haze, fog, or smog) scatter light more equally in all wavelengths. When there is a fog event, the sky appears white because no wavelength of light is preferentially scattered.

The size and characteristics of a “particle” are important in determining its optical properties.

This is why gases are selective in what types of radiation they will absorb, and as fate would have it, many of the gases in our atmosphere do not absorb incoming visible radiation.

The Earth’s surface absorbs incoming radiation more readily than does the atmosphere.

Absorption by Earth’s Surface and Atmosphere

Taken all together, this helps account for the fact that the half of the solar radiation that reaches the earth’s surface is absorbed while only 20% of this energy is absorbed directly by the atmosphere.

Green Flash

The Green Flash

What the Heck was that?

• Every day we’ll look at some aspect of the weather

• Today, take a look at different forecast models and a little on what goes into a forecast—in particular, the system passing through today.

Intellicast current

Intellicast last night

Intellicast today

Analysis—Data!

Eta

Eta

Eta

Eta

Eta

Eta

Eta

Eta

Eta

GPS (formerly MRF)

GPS/MRF

GPS/MRF

GPS/MRF

GPS/MRF

Eta Temp

Eta Precip

AVN Precip

NGM Precip