G304 – Physical Meteorology and Climatology Chapter Solar radiation and the seasons By Vu Thanh Hang, Department of Meteorology, HUS 2.1 Energy • Energy is defined as “the ability to work.” • The standard unit of energy in the International System (SI) used in scientific applications is the joule (J) • joule = 0.239 calories • Power is the rate at which energy is released, transferred, or received • The unit of power is the watt (W), which corresponds to joule per second • About one two-billionth of the energy emitted by the Sun is transferred to Earth as electromagnetic radiation, Æ absorbed by the atmosphere and surface Æ provide energy for the movement of the atmos., growth of plants, evaporation of water, ect 2.1 Energy (cont.) • All forms of energy fall into the general categories of kinetic energy and potential energy • Kinetic energy (energy in use) is often described as the energy of motion • Potential energy is energy that has not yet been used, such as a cloud droplet that occupies some position above Earth’s surface Æ the droplet is subject to the effect of gravity Æ as it falls toward Earth’s surface, the PE is converted to KE • The higher the droplet’s elevation, the greater its potential energy 2.1 Energy (cont.) 2.1 Energy (cont.) • Energy can be transferred from one place to another by three processes: conduction, convection, and radiation • Conduction is the movement of heat through a substance without the movement of molecules in the direction of heat transfer • Conduction is most effective in solid materials, but it also is an important process in a very thin layer of air near Earth’s surface 2.1 Energy (cont.) • Convection is the transfer of heat by the mixing of a fluid, is accomplished by displacement (movement) of the medium • During the daytime, heating of Earth’s surface warms a very thin layer of air in contact with the surface Æ air heated from below expands and rises upward because of the inherent buoyancy of warm air • The atmosphere can undergo convection without buoyancy Æ forced convection Æ the vertical mixing happens as the wind blows 2.1 Energy (cont.) Free convection due to strong heating from below Forced convection due to turbulence created by horizontal wind flow over rough surface 2.2 Radiation • Radiation is the only one that can be propagated without a transfer medium • The transfer of energy by radiation can occur through empty space • Virtually all the energy available on Earth originates from the Sun • Radiation is emitted by all matter • Different types of radiation have different effects, all are transmitted as a sequence of waves • Radiation consists of both an electrical and a magnetic wave • When an object emits radiation, both an electrical field and a magnetic field radiate outward • The electric and magnetic waves are perpendicular to one another; rise and fall in unison 2.2 Radiation (cont.) • Quantity is associated with the height of the wave, or its amplitude • Everything else being equal, the amount of energy carried is directly proportional to wave amplitude • The quality, or “type,” of radiation is the distance between wave crests or wavelength (crest - to - crest, trough - to trough) • All forms of electromagnetic radiation travel through space at the speed of light (~300,000km/s) Æ takes minutes for energy from the Sun to reach Earth • The energy received from the other, more distant stars take even longer to arrive at Earth 2.2 Radiation (cont.) Fig 2-5: Electromagnetic radiation consists of an electric wave (E) and a magnetic wave (M) As radiation travels, the waves migrate in the direction shown by the pink arrow The waves in (a) and (b) have the same amplitude, so the radiation intensity is the same However, (a) has a shorter wavelength, so it is qualitatively different than (b) Depending on the exact wavelengths involved, the radiation in (a) might pass through the atmosphere, whereas that in (b) might be absorbed 2.2 Radiation (cont.) • For any radiating body, the wavelength of peak emission (in micrometers) is given by Wien’s law: λmax = constant/T where λmax refers to the wavelength of energy radiated with greatest intensity; the constant rounds off to the value 2900 for T in Kelvins and λmax in micrometers • Wien’s law tells us that hotter objects radiate energy at shorter wavelengths than cooler bodies • Shorter wavelengths correspond to higher energies 2.2 Radiation (cont.) • Solar radiation is most intense in the visible portion of the spectrum Most of the radiation has wavelengths less than micrometers which we generically refer to as shortwave radiation • Earth’s surface and atmosphere radiations consist mainly of that having wavelengths longer than micrometers This type of electromagnetic energy is called longwave radiation • Hotter bodies radiate more energy than cooler bodies at all wavelengths • Weather satellites (infrared imagery) measure radiation intensity to determine the cloud top temperature and also the cloud thickness 2.2 Radiation (cont.) Fig 2-7: Energy radiated by substances occurs over a wide range of wavelengths Because of its higher temperature, emission from a unit of area of the Sun (a) is 160,000 times more intense than that of the same area on Earth (b) Solar radiation is also composed of shorter wavelengths than that emitted by Earth 2.3 The solar constant • The Sun is extremely hot and we are protected from its great heat by the distance from the solar surface • Radiation traveling through space carries the same amount of energy and has the same wavelength as when it left the solar surface • At greater distance from the Sun, it is distributed over a greater area Æ reduces its intensity • Consider a sphere completely surrounding the Sun, whose radius is equal to the mean distance between Earth and the Sun (= 1.5 x 1011m) • As the distance from the Sun increases, the intensity of the radiation diminishes in proportion to the distance squared Æ the inverse square law 2.3 The solar constant (cont.) • By dividing total solar emission (3.865 x 1026 W) by the area of our imaginary sphere surrounding the Sun (4πr2) Æ determine the amount of solar energy received bya surface perpendicular to the incoming rays at the mean Earth-Sun distance • This incoming radiation is: 3.865 ×10 26 W ( 4π 1.5 ×10 m 11 ) = 1367 W/m • This value is refered to as solar constant 2.3 The solar constant (cont.) Fig 2-9 The intensity of a beam of solar radiation does not weaken as it travels away from the Sun However, its intensity is reduced when it is distributed over a large area 2.4 The causes of Earth’s seasons • Although the Sun emits a nearly constant amount of radiation, on Earth we experience significant changes in the amount radiation received during a year Æ the seasons • We know that the low latitudes received more solar radiation per year at the top of the atmosphere than regions at higher latitudes • Earth orbits the Sun once every 365 1/4 days as if it were riding along a flat plane Æ refer to this imaginary surface as the ecliptic plane and to Earth’s annual trip about the plane as its revolution 2.4 The causes of Earth’s seasons (cont.) Fig 2-10 Earth’s orbit around the Sun is not perfectly circular but is an ellipse • Earth is nearest the Sun (perihelion) on or about January (147,000,000 km) • Earth is farthest from the Sun (aphelion) on or about July (152,000,000 km) 2.4 The causes of Earth’s seasons (cont.) • Earth also undergoes a spinning motion called rotation • Rotation occurs every 24 hours around an imaginary line called Earth’s axis, connecting the North and South Poles • The axis is not perpendicular to the plane of the orbit of Earth around the Sun but is tilted 23.5° from it • No matter what time of year it is, the axis is always tilted in the same direction and always points to a distant star called Polaris (the North Star) • The constant direction of the tilt means that for half the year the Northern Hemisphere is oriented somewhat toward the Sun, and for half the year it is directed away from the Sun Æ cause the seasons (not the varying distance between Earth and the Sun) 2.4 The causes of Earth’s seasons (cont.) • The Northern Hemisphere has its maximum tilt toward the Sun on or about June 21, (June solstice) • Six months later (on or about December 21), the Northern Hemisphere has its minimum availability of solar radiation on the December solstice • Intermediate between the two solstices are the March equinox on or about March 21, and the September equinox on or about September 21 • On the equinoxes, every place on Earth has 12 hours of day and night and both hemispheres receive equal amounts of energy 2.4 The causes of Earth’s seasons (cont.) • The 23.5° tilt of the Northern Hemisphere toward the Sun on the June solstice causes the subsolar point (where the Sun’s rays meet the surface at a right angle and the Sun appears directly overhead) to be located at 23.5° N • This is the most northward latitude at which the subsolar point is located (Tropic of Cancer) • On the December solstice, the sun is directly overhead at 23.5° S (Tropic of Capricorn) • On the two equinoxes, the subsolar point is on the equator 2.4 The causes of Earth’s seasons (cont.) Fig 2-12 Earth’s revolution around the Sun 2.4 The causes of Earth’s seasons (cont.) Fig 2-13 Because Earth’s axis is tilted 23.5o, the subsolar point is at 23.5oN during the summer solstice 2.4 The causes of Earth’s seasons (cont.) The latitudinal position of the subsolar point is the solar declination, which can be visualized as the latitude at which the noontime Sun appears directly overhead 2.4 The causes of Earth’s seasons (cont.) Beam spreading is the increase in the surface area over which radiation is distributed in response to a decrease of solar angle The greater the spreading, the less intense is the radiation In (a), the incoming light is received at a 90° angle In (b), the rays hit the surface more obliquely and the energy is distributed over a greater area A beam of light is more effective if it has a high angle of incidence [...]... solstice, the sun is directly overhead at 23 .5° S (Tropic of Capricorn) • On the two equinoxes, the subsolar point is on the equator 2. 4 The causes of Earth’s seasons (cont.) Fig 2- 12 Earth’s revolution around the Sun 2. 4 The causes of Earth’s seasons (cont.) Fig 2- 13 Because Earth’s axis is tilted 23 .5o, the subsolar point is at 23 .5oN during the summer solstice 2. 4 The causes of Earth’s seasons (cont.)... law 2. 3 The solar constant (cont.) • By dividing total solar emission (3.865 x 1 026 W) by the area of our imaginary sphere surrounding the Sun (4πr2) Æ determine the amount of solar energy received bya surface perpendicular to the incoming rays at the mean Earth-Sun distance • This incoming radiation is: 3.865 ×10 26 W ( 4π 1.5 ×10 m 11 ) 2 = 1367 W/m 2 • This value is refered to as solar constant 2. 3.. .2. 2 Radiation (cont.) Specify wavelengths using small units called micrometers (or microns) 1 micrometer equals one-millionth of a meter 2. 2 Radiation (cont.) Electromagnetic Spectrum Chart from: Berkeley Lab Berkeley • All objects radiate energy not merely at one single wavelength but over a wide range of wavelengths 2. 2 Radiation (cont.) • Perfect emitters... September 21 • On the equinoxes, every place on Earth has 12 hours of day and night and both hemispheres receive equal amounts of energy 2. 4 The causes of Earth’s seasons (cont.) • The 23 .5° tilt of the Northern Hemisphere toward the Sun on the June solstice causes the subsolar point (where the Sun’s rays meet the surface at a right angle and the Sun appears directly overhead) to be located at 23 .5° N... and to Earth’s annual trip about the plane as its revolution 2. 4 The causes of Earth’s seasons (cont.) Fig 2- 10 Earth’s orbit around the Sun is not perfectly circular but is an ellipse • Earth is nearest the Sun (perihelion) on or about January 3 (147,000,000 km) • Earth is farthest from the Sun (aphelion) on or about July 3 (1 52, 000,000 km) 2. 4 The causes of Earth’s seasons (cont.) • Earth also undergoes... Earth and the Sun) 2. 4 The causes of Earth’s seasons (cont.) • The Northern Hemisphere has its maximum tilt toward the Sun on or about June 21 , (June solstice) • Six months later (on or about December 21 ), the Northern Hemisphere has its minimum availability of solar radiation on the December solstice • Intermediate between the two solstices are the March equinox on or about March 21 , and the September... temperature and also the cloud thickness 2. 2 Radiation (cont.) Fig 2- 7: Energy radiated by substances occurs over a wide range of wavelengths Because of its higher temperature, emission from a unit of area of the Sun (a) is 160,000 times more intense than that of the same area on Earth (b) Solar radiation is also composed of shorter wavelengths than that emitted by Earth 2. 3 The solar constant • The Sun is... intensity of energy radiated by a blackbody increases according to the fourth power of its absolute temperature I = σT4 where I is the intensity of radiation (Wm -2) , σ is a constant (5.67 x 10-8 Wm-2K-4) and T is the temperature of the body (K) 2. 2 Radiation (cont.) • Blackbodies do not exists in nature Æ most liquids and solids can be treated as graybodies Æ they emit some percentage of the maximum amount... emissivity, range from just above zero to just below 100% (ε): I = ε σT4 2. 2 Radiation (cont.) • For any radiating body, the wavelength of peak emission (in micrometers) is given by Wien’s law: λmax = constant/T where λmax refers to the wavelength of energy radiated with greatest intensity; the constant rounds off to the value 29 00 for T in Kelvins and λmax in micrometers • Wien’s law tells us that... ( 4π 1.5 ×10 m 11 ) 2 = 1367 W/m 2 • This value is refered to as solar constant 2. 3 The solar constant (cont.) Fig 2- 9 The intensity of a beam of solar radiation does not weaken as it travels away from the Sun However, its intensity is reduced when it is distributed over a large area 2. 4 The causes of Earth’s seasons • Although the Sun emits a nearly constant amount of radiation, on Earth we experience