NAS 125: Meteorology

NAS 125: Meteorology Solar and Terrestrial Radiation

Importance of the sun • Ancient peoples had to know the cycles of the sun, because if they did not keep track of time, crops would not be planted and harvested on time and people would starve. • To keep track of time, ancient peoples devised various types of astronomical calculators. Solar and Terrestrial Radiation

Ancient calculators • Stonehenge, constructed about 2950 BCE, is an example of an early astronomical calculator. • The arrangement of stones points toward the direction of sunrise on the summer solstice, as well as the mid-winter sunset. • The site can be used to predict solar and lunar eclipses. • A Native American site at Cahokia, Ill., used wooden posts for the same purpose. • The oldest known site is near Nabta, Egypt, dating from 6000 years ago. Solar and Terrestrial Radiation

The role of the sun • How does energy flow into and out of the Earth-atmosphere system? • The sun drives the atmosphere, providing the energy that powers atmospheric and oceanic circulation and storms. • The sun emits electromagnetic energy which strikes the Earth’s atmosphere – some of which is absorbed and converted into other forms of energy, like heat and the kinetic energy of wind and water currents. • First law of thermodynamics: Energy can be transferred and transformed, but not created nor destroyed. Solar and Terrestrial Radiation

Electromagnetic spectrum • The energy from the sun that bathes the Earth is part of the electromagnetic spectrum, which is energy that possesses electrical and magnetic properties. • Electromagnetic energy exhibits properties of waves, thus wavelength or frequency are effective measures to use to classify energy bands within the spectrum. • Electromagnetic energy ranges from radio waves to gamma waves. • One energy band grades into another. Solar and Terrestrial Radiation

Solar and Terrestrial Radiation

Wave properties • Wavelength is the distance between successive wave crests or successive wave troughs. • Frequency is the number of waves that pass a given point in a specified amount of time. • Passage of a complete wave is called a cycle. • The typical measure of frequency is the number of cycles per second, or hertz (Hz). • Wavelength and frequency are inversely proportional. • The higher the frequency, the shorter the wavelength. Solar and Terrestrial Radiation

Radiation bands, part 1 • There are seven primary bands of electromagnetic radiation. • Gamma rays, x-rays, and ultraviolet radiation: These three bands occur naturally, but can also be produced artificially. • Gamma rays have the shortest wavelengths (and highest frequencies) and contain the most energy. • X-rays have slightly longer wavelengths (and shorter frequencies) • Ultraviolet waves are the short-wavelength band adjacent to the visible spectrum. • All three are too short to be seen by the human eye, and they can cause considerable damage to living organisms if they reached the surface, but the atmosphere filters them out. Solar and Terrestrial Radiation

Radiation bands, part 2 • Seven primary bands (continued): • The visible spectrum, ranging from wavelengths of from 0.4 to 0.7 micrometers, makes up only 3 percent of the electromagnetic spectrum, but it represents a large portion of solar energy. • Visible light is necessary for many activities of plants and animals, such as photosynthesis and daylength-induced control of reproduction. Solar and Terrestrial Radiation

Radiation bands, part 3 • Seven primary bands (continued): • Infrared light, the portion of the electromagnetic spectrum with wavelengths slightly longer than the visible spectrum, cannot be seen by the human eye. • It is emitted by hot objects and is thus sometimes called heat rays. • Radiation emitted by the Earth radiation is entirely in the infrared region, but it represents only a small fraction of total solar radiation • Microwave and radio waves are low wavelength, low energy waves (radio waves have the longest wavelengths) that are useful in communications, cooking, and weather radar.. Solar and Terrestrial Radiation

Blackbody • Blackbody: A blackbody is a perfect absorber and emitter of energy. • At a constant temperature, blackbodies absorb all wavelengths of energy incident upon them, and emit all wavelengths that they absorb. • Emissivity is a measure of how closely an object resembles a blackbody. Solar and Terrestrial Radiation

Wien’s displacement law, part 1 • Wien’s displacement law: The wavelength of the most intense radiation emitted by an object is proportional to its temperature. • max = C/T, where • max is the wavelength of most intense radiation emitted by an object • C is a proportionality constant, and equals 2,897 if max is expressed in micrometers • T is the temperature expressed in degrees Kelvin • In other words, hot objects emit radiation that peaks at short wavelengths, while cool objects do the opposite. Solar and Terrestrial Radiation

Wien’s displacement law, part 2 • The spectrum emitted by the sun is similar to that of a blackbody at about 6,000 K. • The spectrum emitted by the Earth is similar to that of a blackbody at 288 K. Solar and Terrestrial Radiation

Stefan-Boltzmann law • Stefan-Boltzmann law: The total energy flux, or flow, emitted by a blackbody across all wavelengths (E) is proportional to the absolute temperature of the object raised to the fourth power: • E = T4, where the temperature is given in degrees Kelvin. • In other words, a small change in the temperature of a blackbody results in a large change in the total amount of energy emitted by the object. Solar and Terrestrial Radiation

Inverse square law • Inverse square law: The intensity of radiation falling on an object diminishes with increasing distance from the source. • Doubling the distance reduces the intensity by 75 percent. Solar and Terrestrial Radiation

Global radiation equilibrium The global radiation equilibrium is a balance between radiation received by the Earth-atmosphere system and that energy lost from the Earth-atmosphere system. Solar and Terrestrial Radiation

The sun, part 1 • The sun is more than 80 percent hydrogen (by mass), with most of the rest of its mast as helium. • Its internal temperatures may exceed 20 million degrees C. • It produces energy by the fusion of four hydrogen atoms to form one helium atom. • The mass of the four hydrogens exceeds that of one helium by 0.7 percent; the remaining mass is converted to energy. • E = mc2, where • E is energy • m is mass • c is the speed of light (300,000 km per second) Solar and Terrestrial Radiation

The sun, part 2 • The visible surface of the sun is called the photosphere. • At about 6,000° C, the photosphere is much cooler than the sun’s interior. • Irregularly shaped convection cells, or granules, give the photosphere its honeycomb-like appearance. • Granules are about 1,000 km in diameter, with supergranules up to 50,000 km in diameter. • The photosphere also features dark, cool areas called sunspots. • They may be as much as 1,800° C cooler than the photosphere. Solar and Terrestrial Radiation

The sun, part 3 • The photosphere (continued): • Faculae are bright areas near sunspots. • The chromosphere, consisting of hydrogen and helium ions, extends outward from the photosphere. • Chromosphere temperatures range from 4,000° C to 40,000° C. • The outermost portion of the sun is the corona, a region of highly ionized gases with temperatures as high as 4 million degrees C. • The corona extends millions of kilometers into space. Solar and Terrestrial Radiation

The sun, part 4 • The solar wind originates in the corona and sweeps the solar system. • Solar flares that originate in the photosphere intensify the solar wind. • The Earth intercepts only a portion of the energy emitted by the sun. Solar and Terrestrial Radiation

Solar altitude • The intensity of solar radiation striking the Earth is a function of the solar altitude, or the angle that the sun is above the horizon. • The higher the angle of the sun, the more intense the radiation, with the most intense radiation striking areas where the sun is directly overhead (at a 90° angle). • It is the primary determinant of the amount of solar radiation reaching a particular place on Earth. Solar and Terrestrial Radiation

Daylength • Daylength is important because the longer the day, the more solar radiation can be received and the more heat can be absorbed. • Middle and high latitudes have pronounced seasonal variations in day length, while tropical areas have little variation. Solar and Terrestrial Radiation

Atmospheric obstructions • Atmospheric obstructions – such as clouds, particulate matter, and gas molecules – absorb, reflect, or scatter solar radiation. • How much effect they have depends on path length, the distance a ray must travel. • Because angle of incidence determines path length, atmospheric obstruction reinforces the pattern established by the varying angle of incidence. • Because they must pass through more atmosphere than high-angle rays, low-angle rays are subject to more depletion through reflection, scattering, and absorption. Solar and Terrestrial Radiation

Latitudinal differences • The latitudinal radiation balance occurs because the belt of maximum solar energy swings back and forth through tropics as the direct rays of sun shift northward and southward in course of a year. • Low latitudes (about between 28° N and 33° S) receive an energy surplus, with more incoming than outgoing radiation. • There is an energy deficit in latitudes north and south of these low latitudes. • This simple latitudinal pattern is interrupted principally by atmospheric obstruction. Solar and Terrestrial Radiation

Earth’s revolution, part 1 • The tropical year is the time it takes Earth to complete one revolution around the Sun; for practical purposes it can be simplified to 365.25 days. • Earth’s revolution is an ellipse, along which varies the Earth-Sun distance. • The varying distance between Earth and the Sun is not an important determinant of seasonal temperature fluctuations. Solar and Terrestrial Radiation

Earth’s revolution, part 2 • The two “end” points of the ellipse are the perihelion and aphelion. • The perihelion is the point in an orbit that takes a planet nearest to the Sun (for Earth, it is 147,166,480 kilometers or 91,455,000 miles, on January 3). • The aphelion is the point in an orbit that takes a planet furthest away from the Sun (for Earth, it is 152,171,500 kilometers or 94,555,000 miles, on July 4). Solar and Terrestrial Radiation

The seasons, part 1 • The plane of the ecliptic is the imaginary plane that passes through the Sun and through every point of Earth’s orbit around the Sun. • It is not perpendicular to Earth’s rotation axis, which allows for seasons to occur. • Inclination is the degree to which Earth’s rotation axis is tilted (about 23.5 degrees away from the perpendicular). Solar and Terrestrial Radiation

The seasons, part 2 • Polarity, also called parallelism, occurs because Earth’s axis always points toward Polaris, the North Star, no matter where Earth is in its orbit. • Insolation is incoming solar radiation. • The angle at which the Sun’s rays strike Earth determines the amount of insolation reaching any given point on Earth. • That angle is a result of the combined effect of rotation, revolution, inclination, and polarity. Solar and Terrestrial Radiation

The seasons, part 3 • Solstices • A solstice is one of two times during year in which the Sun’s perpendicular (vertical) rays hit the northernmost or southernmost latitudes (23.5°). • On or about December 21 (called the winter solstice in Northern Hemisphere). • On or about June 21 (called the summer solstice in Northern Hemisphere). Solar and Terrestrial Radiation

The seasons, part 4 • Solstices (continued) • The tropic of Cancer is the parallel of 23.5° north latitude, which marks the northernmost location reached by the vertical (perpendicular) rays of the Sun; occurs on or about June 21. • The Tropic of Capricorn is the parallel of 23.5° south latitude, which makes the southernmost location reached by the vertical (perpendicular) rays of the Sun; occurs on or about December 21. Solar and Terrestrial Radiation

The seasons, part 5 • Solstices (continued) • The Arctic Circle is the parallel of 66.5° north latitude; experiences 24 hours of either light (circa June 21) or dark (circa December 21). • The Antarctic Circle is the parallel of 66.5° south latitude; experiences 24 hours of either light (circa December 21) or dark (circa June 21). Solar and Terrestrial Radiation

The seasons, part 5 • Equinoxes • The Equinoxes are the times of year when the perpendicular rays of the Sun strike the equator, the circle of illumination just touches both poles, and the periods of daylight and darkness are each 12 hours long all over Earth. • On or about March 20 (called vernal equinox in Northern Hemisphere). • On or about September 22 (autumnal equinox in Northern Hemisphere). • The equinoxes represent the midpoints in the shifting of direct rays of the Sun between the Tropic of Cancer and the Tropic of Capricorn. Solar and Terrestrial Radiation

The seasons, part 6 • Changes in daylight and darkness • The period of daylight varies throughout the year, increasing everywhere north of the equator from the shortest day of the year on the December solstice until the longest day of the year of the June solstice. Then days begin to shorten again in Northern Hemisphere. (Southern Hemisphere experiences an opposite effect.) Solar and Terrestrial Radiation

The seasons, part 7 • Changes in daylight and darkness (continued) • Both day length and the angle at which the Sun’s rays strike Earth are principal determinants of the amount of insolation received at any particular latitude. • Tropic latitudes are always warm/hot because they always have high Sun angles and consistent days close to 12 hours long. • Polar regions are consistently cold because they always have low sun angles. Solar and Terrestrial Radiation

The solar constant, part 1 • The solar constant is the fairly constant amount of solar insolation received at the top of the atmosphere at a place perpendicular to the solar beam. • The solar constant – which actually fluctuates slightly during the year – is about 1.97 Langleys per minute, or 1368 Watts per square meter (W/m2). • A Langley is a unit of measure of radiation intensity that is 1 calorie per square centimeter (a calorie is the amount of heat required to raise the temperature of 1 gram of water by 1°C). Solar and Terrestrial Radiation

NAS 125: Meteorology