Part 1. Energy and Mass Chapter 2. Solar Radiation and the Seasons

Introduction Solar Radiation Initiates atmospheric motions and weather processes Energy Classified as Kinetic or potential

Kinetic energy examples

Energy Transfer Mechanisms 1) Conduction Energy transfer through solids 2) Convection Energy transfer through fluids (gases, liquids) Gas molecules move at random

3) Electromagnetic (EM) Radiation Energy transferred by electromagnetic EM waves (made up of electric and magnetic fields) Electromagnetic (EM) radiation E = electric field wave M = magnetic field wave

EM Energy EM spectrum classifies types by wavelength (usually measured in micrometers,  m) The amount of energy transferred is proportional to the wave amplitude Amplitude Wavelength

Stefan-Boltzmann Law EM Energy emitted is proportional to temperature Hotter objects emit more EM energy Blackbodies are hypothetical perfect emitters Graybodies emit a percentage of the maximum possible for a temperature Wein’s Law Hotter objects have their spectrum peak at shorter wavelengths Solar (Sun) radiation = 0.5  m peak Terrestrial (Earth) radiation = 10  m peak

The Sun’s radiation spectrum peak is a.5  m (shortwave radiation). The Earth’s radiation spectrum peak is a 10  m (longwave radiation). Wavelength Energy

The Solar Constant is the amount of energy emitted from the Sun’s surface every second (1367 W/m 2 ) The energy intensity of solar radiation decreases with distance from the Sun. Mars receives less intense solar radiation than the Earth because it is farther from the Sun.

Causes of the Earth’s Seasons The Earth revolves around the Sun in the ecliptic plane Perihelion (Earth closest to the Sun) is January 3; 147 million km or 91 million miles Aphelion (Earth farthest from the Sun) is July 3; 152 million km or 94 million miles Seasonal radiation variation in solar radiation is ~7% Climate and the Earth’s Seasons

Earth Rotation Once every 24 hours Rotational axis offset by about 23.5 o from the ecliptic plane The rotation axis appears to point to a fixed point in the heavens (Polaris, also called the North Star) The part of the Earth that points directly toward the Sun receives the greatest solar intensity. This changes throughout the year, causing the seasons on the Northern and Southern Hemisphere.

Solstices and Equinoxes

This figures shows the variation in solar radiation intensity at the time of the June solstice. Tropic of Cancer Tropic of Capricorn

Differences in solar radiation intensity at the June and December Solstices.

Equinoxes Temporally centered between solstices ~ March 21 and ~ Sept 21 The subsolar point = 0 o

The length of day is controlled by the position of the Earth in its orbit.

Radiation intensity is proportional to solar angle Larger angles of incidence give reduced beam spreading and greater heating Beam Spreading and Radiation Intensity 90° angle of incidence (solar angle) 45° angle of incidence (solar angle) Less beam spreading = greater heating More beam spreading = less heating

The sun at sunrise and sunset has a lower solar angle and the radiation travels farther through the atmosphere, reducing the heating effects.