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

Transition: formation of magnetic field, volcano activities

2nd: 4 billion years ago, CO2, H2O, N2

Transition: emergence of life, formation of ocean

3rd: 400 million years ago, O2

Important event: formation of seven continents– What is the residence time? What is the difference

between the permanent and variable gases? Name 3 of each. What are the most and second most abundant gases?

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

• 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!

A man detected by different instruments

Infred device

Bare eyes

X-ray Microscope

Physical Representation of Radiation

• Blackbodies: purely hypothetical bodies that absorb and emit the maximum radiation at all wavelengths

• The Earth and the sun are close to blackbodies.

What determines the quantity (intensity)?The Stefan-Boltzmann Law

• States that the intensity of radiation emitted from a blackbody is a function ONLY of temperature

I=T4

where I is the intensity of the radiation, T is the temperature in K, and is the Stefan-Boltzmann constant, 5.67 x 10-8 W m-2 K-4)

• So, hotter bodies emit more energy than colder bodies (double T, 16x more radiation)

Earth (290K) I = 401 Wm-2

Sun (6000K) I = 7.3 x 106 Wm-2.

So ISun >> Iearth

What determines the quality (wavelength)?The Wein’s Law

• States that the peak wavelength of radiation emitted from a blackbody is a function ONLY of temperature

max = b/T

where max is the peak wavelength of the radiation, T is the temperature in K, and b is the Wein’s displacement constant (2.90 x 10-3 m K)

• So, hotter bodies emit at shorter wavelengths Double T, half wavelength

Earth (290K) max = 10 μm

Sun (6000K) max = 0.5 μm

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)

Composition of sunlight

The rainbows

The Sun also emits high energy particles: Northern light (Aurora)

• Caused by electrons and protons

• They can only penetrate into the atmosphere from the magnetic north pole and south pole

• They collide with the atmospheric molecules to generate the aurora

Video: The solar cycle

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

The sunspot number - A good index for solar

activity• Sunspots are temporary

phenomena on the surface of the sun that appear visibly as dark spots

• Caused by intense magnetic activity

• The number of sunspots changes significantly between solar maximum and solar minimum

The solar cycle

• is the periodic change in the sun's activity (including changes in the levels of solar radiation and ejection of solar material) and appearance (visible in changes in the number of sunspots, flares, and other visible manifestations).

• has a duration of about 11 years.• has been observed (by changes in the sun's appearance

and by changes seen on Earth, such as auroras) for hundreds of years.

The solar cycle and earth’s climate

Change of precipitation associated with solar maximum

Red-Increase, Blue-decrease (from Meehl et al. 1999, Science)

Solar cycle and US air temperature

Solar cycle and grape production in France

Summary

– What is energy? 3 methods of energy transfer

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

– Intensity of radiation (Stefan-Boltzman law): I=T4

– Wavelength of radiation (Wein’s law): max = b/T

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

– The 11-year solar cycle