Understanding Weather and Climate 3rd Edition Edward Aguado and James E. Burt Anthony J. Vega

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

Introduction Solar radiation is transferred to the Earth’s surface where it is absorbed This radiation provides energy for atmospheric motion, weather processes, biologic activity, conversions of state, and many other activities Most forms of energy can be classified as either kinetic or potential energy Kinetic energy, the energy of motion, can occur from very large to small scales. At the molecular scale, this energy describes molecular vibration and determines an object’s temperature. Potential energy is energy in reserve.

Examples of kinetic energy Gas molecules have no bonds to other molecules and as such they move in random motion

Radiation An energy transfer mechanism which requires no physical medium Propagates energy transfer through the vacuum of space Continually emitted by all substances Types differ by electrical and magnetic wave properties In any type of radiation, electrical and magnetic waves, although closely coupled, are perpendicular to each other Electromagnetic radiation. E = electric wave M = magnetic wave

Radiation quantity Refers to the amount of energy transferred Expressed through wave amplitude Wave quality Relates to radiation wavelength Identifies the type of radiant energy Wavelength is expressed in μm, or micrometers All electromagnetic energy travels at a constant speed 300,000 km/sec (186,000 mi/sec), the speed of light Electromagnetic energy Comes in an infinite number of wavelengths Can simplify by categorizing wavelengths into just a few individual “bands” along the electromagnetic spectrum

Intensity and Wavelengths of Emitted Radiation All matter not only radiates energy, but energy is emitted over a wide range of electromagnetic wavelengths Physical laws which define the amount and the wavelength of emitted energy apply only to hypothetical perfect emitters of radiation known as blackbodies The Earth and Sun are similar to blackbodies

Stefan-Boltzmann Law The amount of energy emitted by an object is proportional to the object’s temperature Hotter objects emit more energy than cooler ones Energy emitted is proportional to the fourth power of the emitter’s absolute temperature Stefan-Boltzmann Law describes this mathematically as I =  T 4 I is the intensity of the radiation in watts/m 2,  is the Stefan- Boltzmann constant (5.67 x watts/m 2 /K 4 ) and T is the temperature of the body in K Graybodies denote objects which emit some percentage of the maximum amount of radiation possible at a given temperature Most solids and liquids True radiation emitted is a percentage relative to a blackbody and reflects the emissivity of the object

Wein’s Law Radiation emission is across a wide array of electromagnetic wavelengths Useful to determine the wavelength of peak emission, found through Wein’s Law Wein’s Law = λ max = 2900/T λ max = wavelength of energy radiates with greatest intensity, 2900 equals a constant, T is temperature in K Wein’s Law tells us that hotter objects radiate at shorter wavelengths than cooler bodies Solar radiation peaks at 0.5μm while terrestrial radiation peaks at 10μm Largest portion of solar radiation emitted in visible spectrum Largest percentage of terrestrial radiation emitted as longwave radiation

Solar Constant The intensity of electromagnetic radiation is not reduced with distance through the vacuum of space A reduction of intensity is proportional to increasing distance only as energy is distributed over a larger area Due to this, radiation intensity decreases in proportion to the distance squared Calculating this inverse square law for Earth’s average distance from the Sun yields a solar constant of 1367 W/m 2 Solar emission = x W divided by distance surrounding the Sun = 4  (1.5 x m) 2 = 1367 W/m 2

Causes of the Earth’s Seasons Variations in the relationship between Earth’s orbital alignment and the Sun are responsible for variations in incoming solar radiation at Earth’s surface Revolution Earth revolves about the Sun along an ecliptic plane Distance varies Perihelion (Jan 3; 147 mil km, 91 mil mi) Aphelion (July 3; 152 mil km, 94 mil mi) Total variation is about 3% Using the inverse square law, radiation intensity varies by about 7% between perihelion and aphelion

Earth Rotation Earth rotates on its axis once every 24 hours Axis of rotation is offset 23.5 o from a perpendicular plane through the ecliptic plane Because the axis of rotation is never changing, the northern axis aligns with the star Polaris Hemispheric orientation changes as the Earth orbits the Sun A particular hemisphere will either align toward or away from the Sun, or occupy a position between the extremes Examining Earth’s axis with a full 90 o tilt may help in the visualization of the changing circle of illumination relative to hemispheric lines of latitude through revolution

Solstices Maximum axial tilt in relation to the Sun occurs on only two dates for each hemisphere, however, the hemispheres are in opposition relative to the Sun The June solstice occurs on approximately June 21 The northern (southern) hemisphere axis of rotation is fully inclined toward (away from) the Sun This ensures maximum (minimum) solar radiation absorption through the hemisphere Exactly opposite conditions occur relative to the December solstice (on or around Dec 21) Astronomically, dates designate first days of winter or summer where appropriate

Northern hemisphere summer solstice On or about June 21 Subsolar point (solar rays at a right angle to the surface, the latitudinal position of which equals solar declination) is at 23.5 o N, the Tropic of Cancer Northern hemisphere winter solstice On or about Dec 21 Subsolar point is located at 23.5 o S, the Tropic of Capricorn Exactly opposite conditions occur relative to these dates for the southern hemisphere Overall, the subsolar point fluctuates 47 o between the Tropics

Equinoxes Temporally centered between the solstices March equinox, on or about March 21 September equinox, on or about Sept 21 The subsolar point (solar declination) lies at the equator

Solar Angle Incident radiation is directly proportional to solar angle Higher solar angles incorporate reduced radiational beam spreading which leads to greater heating through energy concentration Lower angles induce less intense illumination and heating per unit area Demonstration of the effect of angle of incidence on energy intensity

Period of Daylight Axial tilt influences day length During hemispheric alignment either toward or away from the Sun, lines of latitude are bisected unequally by the circle of illumination A hemisphere aligned toward (away from) the Sun will have constant daylight (night) poleward of 66.5 o This line of latitude is designated the Arctic Circle in the northern hemisphere and the Antarctic Circle in the southern hemisphere Due to this geometry, day length increases (decreases) from the equator to the pole of the summer (winter) hemisphere Lines of latitude are equally split for both hemispheres on the equinoxes ensuring equal day/night conditions everywhere

Beam depletion Solar radiation is diminished relative to the amount of atmosphere the radiation passes through High solar angles see little reduction in intensity as the path from the top of the atmosphere to the surface is short Significant beam reduction occurs where energy is diffused through larger amounts of atmosphere High latitudes - magnified during the winter season Changes in energy receipt with latitude Combined effects of solar angle, day length, and beam depletion cause winter hemisphere to run a deficit of energy, leading to cooler temperatures Summer hemispheres have a mean surplus of energy resulting in warmer temperatures

End of Chapter 2 Understanding Weather and Climate 3rd Edition Edward Aguado and James E. Burt