CHAPTER 2

Solar Radiation & Earth’s Seasons

CHAPTER 2

Solar Radiation & Earth’s Seasons

Temperature measures the average speed of air molecules◦ This also means it is a measure of kinetic

energy

Three ways for heat to be transferred:◦ Conduction: Heat transfer within a substance:

touching a metal pan

Energy travels from hot to cold

Metal is a good conductor, air is a poor conductor

Three ways for heat to be transferred:◦ Conduction: Heat transfer within a substance:

touching a metal pan◦ Convection: Heat transfer by a fluid (such as

water or air): Warm, less-dense air rising In meteorology, we only call vertical motions

“convection”, and we use “advection” for horizontal motions such as the wind

Remember: at the same pressure, warm air is less dense than cold air

Energy has been transported upward

As a parcel of air (think of a large balloon) is lifted up a mountain, the pressure surrounding it decreases – it must expand

The energy that goes into the expansion is lost, and the parcel cools

As it sinks, the pressure outside the parcel increases – it is compressed

As it compresses, the molecules inside move faster, leading to a higher temperature

Rising air expands and cools, sinking air compresses and warms

Three ways for heat to be transferred:◦ Conduction: Heat transfer within a substance:

touching a metal pan◦ Convection: Heat transfer by a fluid (such as

water or air): Warm, less-dense air rising In meteorology, we only call vertical motions

“convection”, and we use “advection” for horizontal motions such as the wind

◦ Radiation: Heat transfer that does not require the substances touching or a fluid between them: energy from the sun

Conduction: Only important very near the ground (air is a poor conductor)

Convection: Many clouds form as a result of convection, as warm, moist air rises

Radiation: Energy from the sun warms the planet; causes daily changes in temperature, and much more

(The scale on the left is 100,000 times greater than the scale on the right)

Solar radiation is often called “shortwave” radiation◦ Much of the solar radiation is in the visible part of

the spectrum – we can see the sun, and the reflection and absorption of solar radiation allows us to see other things

Earth’s radiation is “infrared” or “longwave” radiation◦ Not visible to our eyes◦ Transfers much less energy

If the Earth is radiating energy all the time, why is it not extremely cold and always getting colder?

Objects with a temperature don’t just emit, they also absorb!

If something emits more than it absorbs, it will cool, if it absorbs more than it emits, it will warm

Objects that are good absorbers are also generally good emitters

Consider an asphalt road: During the day the

asphalt absorbs solar radiation and warms

At night the asphalt emits infrared radiation and cools relative to its surroundings

Asphalt Road(warms due to solar radiation)

Asphalt Road(cools by IR radiation)

Day

Night

Warm

Cool

Averaged over a long period of time, the amount of shortwave energy received from the sun is equal to the amount of longwave energy emitted by the earth’s surface – the planet is in radiative equilibrium – on average, the planet does not heat or cool

But this calculation gives an average temperature of 255 K (0° F) – a frozen earth!

What we actually observe, however, is an average surface temperature of 288 K (59° F) – much more livable. Why?

Radiative equilibrium: incoming = outgoing

Radiation surplus in the Tropics; deficit near the poles

Do the poles get colder and colder, and the tropics hotter and hotter every year?

No! Circulations in the atmosphere and ocean transfer heat from the Tropics to the poles.

Radiation travels in the form of waves, which move at the speed of light in a vacuum (186,000 miles per second)

The shorter the wave, the more energy it carries!

Our eyes can only see radiation between 0.4-0.7 μm 1 um = 0.001 mm

Objects that absorb all radiation hitting them and emit all possible radiation

They don’t need to be black

The sun and the earth’s surface behave as blackbodies, but the atmosphere does not

The intensity of energy radiated by a blackbody increases to the fourth power of its absolute temperature.

Stefan-Boltzmann Lawexpressed as

I = σT4

where I is the intensity of radiation in watts per square meter,

σ is a constant (5.67 x 10-8 watts per square meter)and T is the temperature of the body in kelvins.

Celsius Temperature = (oF - 32) / 1.8

Fahrenheit Temperature = 1.8 x oC + 32

Kelvin Temperature = oC + 273

Determines the wavelength of peak emission for any radiating body

(in micrometers):

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 energyat shorter wavelengths than do cooler bodies.

Wien’s law

Can be: Absorbed by the atmosphere (19% of

incoming radiation: atmosphere is relatively transparent to solar radiation)

Reflected back to space by clouds, aerosols, and the atmosphere (26%)

Transmitted down to the surface◦ This can be reflected (4%)◦ Or absorbed by the surface (51%)

Reflectivity of a surface (such as Earth’s ground).

Average Earth albedo: 30%

Each surface has a different albedo – snow and clouds are very reflective, water and dark ground are not

More reflective =more albedo

This downward longwave radiation warms the surface

When this is accounted for, we can calculate the average temperature of 288 K

Without the greenhouse effect, Earth’s temperature would not be suitable for life!

Fig. 2.13, p. 50

Heat energy is required to change the phase of water – this heat is “hidden” or “latent” – we can’t measure it with a thermometer

Instead of being used to change the temperature of the substance, the heat is used to change the phase

The evaporation of water from oceans and lakes transfers heat from the surface to the atmosphere

When warm, moist air rises and clouds form, latent heat is released (condensation) – This is “moist convection”, and is another way that things are brought back into balance

“If you graduated from Harvard, do you think you would know why it is warmer in summer than in winter? Educators who surveyed Harvard students on their graduation day in 1986 discovered that most of them could not correctly answer this question.”

-- Harvard Gazette, 1997

When the sun is directly overhead, the radiation is concentrated over a smaller area

When at an angle, that same energy is spread out over a much larger area

…at an angle of about 23.5°

The tilt of Earth on its axis is the primary reason there are seasons

In December, the Southern Hemisphere is strongly tilted toward the sun; they get longer days and the sun is high in the sky◦ The Northern Hemisphere is tilted away from the

sun; we have shorter days and winter In June, the opposite is true March and September are the “equinoxes”,

when the solar energy is maximized at the equator

If the sun is out for all 24 hours in Alaska, why isn’t it hotter there than in College Station where it’s only light for 14 hours?

Fig. 2.19, p. 56

Stepped Art

Fig. 3-8, p. 63

(incoming minus outgoing)http://profhorn.meteor.wisc.edu/wxwise/AckermanKnox/chap2/ERBE%20Net.html

Equinox, “equal night” ◦ Day and night are the same length; sun is directly

over the equator (March 20 and September 22) Solstice, “sun stands still”

◦ Summer solstice: June 21 – longest day of year in northern hemisphere

◦ Winter solstice: December 21 – shortest day of year in northern hemisphere

In meteorology, seasons are DJF (winter), MAM (spring), JJA (summer), SON (autumn)

The “first official day of winter” on December 21 is the astronomical definition

Average high

Average low

Record high

Record low

http://www.srh.noaa.gov/hgx/?n=climate_cll

MarchMarch JuneJune Sept.Sept. Dec.Dec.

SouthSouth

12 hrs daylight15 hrs daylight

12 hrs daylight

9 hrs daylight

The core: estimated to be ~15 million degrees Celsius

The photosphere (what we see) is about 6000°C

Sunspots: cooler, dark regions

Corona: much hotter (2 million °C)

Chromosphere: cooler region between the photosphere and the corona

Solar flares and prominences: jets of gas that shoot up into the corona

Solar flares can disrupt Earth’s magnetic field, causing problems with radio and satellite communications

Much like a bar magnet, Earth has a magnetic field

Charged particles from the sun, called the “solar wind”, distorts its shape

Charged particle from the solar wind “excites” atoms or molecules in the upper atmosphere (thermosphere)

This causes the electron to jump to a higher energy level

When it returns to normal, it emits light

(Solar wind)

(Air molecule or other atom)

Different elements give off different color light (oxygen is red or green, nitrogen is red or violet)

Northern hemisphere = “aurora borealis” (northern lights)

Southern hemisphere = “aurora australis” (southern lights)

Auroras tend to happen where magnetic field lines intersect the earth’s surface (at high latitudes)

Number of nights per year with aurora

UV-B radiation responsible for most sunburn, though UV-A can also cause it

11 am to 3 pm: biggest threat of sunburn NWS UV forecast:

http://www.nws.noaa.gov/view/national.php?prodtype=ultraviolet