The incoming solar energy

Review of last lecture– The standard units of measurements (SI)– Earth’s three atmospheres: 1st: 4.6 billion years ago, H, He, magnetic field,

volcano 2nd: 4 billion years ago, CO2, H2O, N2, life, ocean

3rd: 400 million years ago, O2, land

– What is residence time? What is the difference between the permanent and variable gases? Name 3 of each.

– Given that variable gases are so rare, why are they considered at all? How are CO2 and O3 changing?

– Earth’s climate history: ice ages (at least 5 have occurred so far. We’re in an ice age!), 100,000-year cycle, little ice age (1350AD-1850AD)

• Energy: the ability to do work

• Many forms: electrical, mechanical, thermal, chemical, nuclear, …

• Joule (J): standard unit of energy (1 J= 0.239 calories)

• Watt (W): rate of energy flow (W = 1 J/s)

Energy basics

Methods of Energy Transfer

• Conduction– Molecule to molecule transfer– Heat flow: warm to cold – e.g. leather seats in a car

• Convection– transferred by vertical movement– physical mixing – e.g. boiling water

• Radiation– propagated without medium (i.e. vacuum)– solar radiation provides nearly all energy – The rest of this chapter deals with radiation

Radiation• Everything continually

emits radiation

• Transfers energy in waves

• Waves are both electrical and magnetic, hence electromagnetic radiation

• Travels at the speed of light (300,000 km/s). It takes 8 minutes for light from the Sun to reach Earth, and 4.3 years for light from the next nearest star, Proxima Centauri to reach us.

Radiation Quantity and Quality

• Quantity: how much? Wave height (Intensity).

• Quality: what kind? Wavelength: distance between crest and crest (or trough and trough). generally reported in μm (microns)- one millionth of a meter.

The Electromagnetic Spectrum

The limitations of the human eye!

Sun

Wavelength of Sun and Earth Radiation

Sun = “shortwave” (0.4-0.7 μm) Peak 0.5 μm (green)

Earth = “longwave” (4-100 μm) Peak 10 μm (infrared)

blue red

Composition of sunlight

Video: The solar cycle

• http://www.youtube.com/watch?v=sASbVkK-p0w&playnext=1&list=PL8EDAE089117E3D77&feature=results_main

Earth’s orbit around the Sun

Distance from the sun accounts for only 7% change in radiation receipt, which is not the cause of seasons. Moreover, the Earth is farther away from the Sun in the northern summer!

Revolution vs. Rotation

• Revolution: about a body (the earth revolves around the sun; i.e., a year)

• Rotation: about an axis (The earth rotates about its axis; i.e., a day)

• So what causes the seasons?!?!

The 4-seasons is caused by the Earth’s 23.5o tilt from the line perpendicular to its

orbit plane (toward the sun during summer)

How the changing orientation of the Earth directly affects the receipt of insolation:

1. Length of Daylight period

2. Angle at which sunlight hits the surface (“Beam Spreading”)

3. Thickness of atmosphere through which sunlight must travel (“Beam Depletion)

Length of Daylight PeriodEquinoxes June Solstice

•Day length increases from equator to pole in summer hemisphere•opposite in winter•At least one day of 24-hour sunlight/darkness above Arctic/Antarctic circles, depending on season

Beam Spreading

• Beam spreading: the increase in surface area over which radiation is distributed in response to a decrease of solar angle

• The greater the spreading, the less intense the radiation (and vice versa)

Beam Depletion

Summary

– What is energy? 3 methods of energy transfer

– The names of the 6 wavelength categories in the electromagnetic radiation spectrum

– The wavelength range of Sun (shortwave) and Earth (longwave) radition

– The two basic motions of the Earth– What causes the four seasons: the 23.5

degree tilt of the Earth’s axis, and the 3 ways it affects the solar insolation (change of length of the day, beam spreading, beam depletion)