The Atmosphere - Radiation and Seasons PDF

The Atmosphere Radiation and the Seasons Lesson goals Understand the physical characteristics of radiation as the main means of energy input to the Earth-Atmosphere system Be able to describe how magnitude and wavelength of radiation depend on temperature of the emitting body Und...

The Atmosphere Radiation and the Seasons Lesson goals Understand the physical characteristics of radiation as the main means of energy input to the Earth-Atmosphere system Be able to describe how magnitude and wavelength of radiation depend on temperature of the emitting body Understand the simple radiation laws (two important relationships between temperature and energy emission) Understand what factors affect the amount of energy received at the Earth’s surface during a year (which causes the seasons)  Forms of Energy Energy Energy is traditionally described as “the ability to do work.” The simplest activity requires a transfer of Loading… energy. Kinds of energy Energy can be classified as either kinetic or potential. Kinetic energy is energy in use or motion. Potential energy is energy in reserve or stored.  Energy Transfer Energy Transfer Mechanisms Energy can be transferred from one place to another by three processes: 1. Conduction 2. Convection 3. Radiation  Energy Transfer Conduction Most effective energy transfer through solids Conduction is the movement of heat through a substance without the movement of molecules in the direction of the heat transfer (from molecule to molecule) Heat moves into the ground by conduction Loading… Convection Energy transfer through fluids (gases, liquids) by mixing A pot of boiling water is an example of convection Winds are natural convection currents  Energy Transfer Radiation No physical medium required for transmission (can occur through empty space). Works best through vacuum Types differ by wave properties Moves at the speed of light [299,792,458 m/s or 3 × 108 m/s) Continually emitted by all substances Electromagnetic radiation: E = electric wave M = magnetic wave  Electromagnetic Spectrum Intensity and Wavelengths of Emitted Radiation All matter radiates energy over a wide range of electromagnetic wavelengths. Distribution of radiative energy over different wavelengths or frequencies: Categorized into a few individual “bands” along the electromagnetic spectrum 4 µm Visible light is a narrow shortwave longwave band bounded by infrared and ultraviolet. Forms of Energy Solar Radiation Only way for Earth to receive energy from the sun Incoming Solar Radiation or “Insolation” Initiates atmospheric motions and weather processes  Electromagnetic Spectrum In meteorology only a small part of the EM-spectrum is of interest: Ultraviolet radiation (UV) Visible radiation Infrared radiation (IR) 4 µm shortwave longwave Wavelengths less than 4 µm are considered shortwave radiation. Wavelengths longer than 4 µm are considered longwave radiation. Shortwave radiation: Received as solar radiation only Longwave radiation: Emitted by Earth/Atmosphere-system  Blackbody Radiation Emission Energy radiated by substances occurs over a wide range of wavelengths. Physical laws defining amount and wavelength of emitted energy apply to hypothetical perfect emitters of radiation known as blackbodies. The Sun and Earth are similar to blackbodies Diagrams below are called radiation emission curves (which is Sun and which is Earth?) Loading…  Radiation Laws Intensity and Wavelengths of Emitted Radiation General principles All objects emit radiation. The amount and wavelengths depend primarily on the emission temperature. When radiation is absorbed by an object, an increase in molecular motion gives an increase in temperature Blackbodies are hypothetical perfect emitters (emissivity, = 1) Reflection - Absorption - Transmission Only 3 things can happen when radiation hits an object or substance: Part or all can be reflected Part or all can be absorbed (this part raises the temperature) Part or all can be transmitted All parts together must add to 100%  Radiation Laws Stefan-Boltzmann Law The intensity of radiation energy emitted is proportional to temperature raised to the 4th power Hotter objects emit more energy, F (in W m-2) F= T4 where, T = temperature in K = Stefan-Boltzmann constant (= 5.67 x 10-8 W m-2 K-4) Graybodies (actually, most objects) Graybodies emit only a percentage of the maximum possible for a given temperature Emissivity refers to the percentage of energy radiated by a substance relative to that of a blackbody Radiation intensity is a function of both emissivity and temperature. F= T4 = L (0 < < 1) Most natural surfaces have emissivity above 0.9 (that is, >90 % of a blackbody).]  Radiation Laws Wien’s Law For any radiating body, the peak wavelength of emission (in micrometers, m) is given by Wien’s law. = a/T where: a is a constant = 2900 m.K; T = temperature in K 4 µm shortwave longwave Thus, from the two laws shown, we can say: Warmer objects radiate more energy than do cooler objects at all wavelengths. Warmer objects radiate energy at shorter wavelengths than do cooler objects. Solar Constant The Solar Constant Solar emission = 3.865 x 1026 W Shortwave radiation emitted by the sun and traveling through space carries the same amount of energy and has the same wavelength as when it left the solar surface. Electromagnetic radiation is not depleted as it moves toward Earth.  Solar Constant However, radiation intensity is reduced because it is distributed over a greater area (sphere) as the distance from the sun increases. Radiation intensity decreases in proportion to distance squared. Calculating this inverse square law for Earth’s average distance from the Sun yields insolation rate of 1367 W/m2 (average amount received) Earth’s Relationship with the Sun Earth Revolution (Orbit) Revolution is the orbit around the Sun The ecliptic plane is the plane of orbit Perihelion – nearest to the Sun (~Jan 4; 147 million km) Aphelion – farthest from the Sun (~July 4; 152 million km)  Earth’s Seasons Earth Rotation Rotation is the spin of the Earth, giving day and night Spins once every 24 hours Rotational axis is offset by 23.5o from the ecliptic plane (plane of revolution). In other words the rotational axis is tilted Cause of the seasons The combination of axial tilt and orbit around the sun changes the Earth’s hemispheric orientation through the year (i.e. changes which hemisphere faces towards or away from the Sun) This causes the seasons Now watch the video clip  Earth’s Seasons Solstices Maximum tilt of the Earth’s axis in relation to the Sun 21st June and 21st December (mid-summer and mid-winter) North and South Hemispheres are inclined toward or away from Sun Causes maximum or minimum solar radiation receipt  Earth’s Seasons Equinoxes On the equinoxes (= “equal night”) every place on Earth has 12 hours of day and night Both northern and southern hemispheres receive equal amounts of energy Equinoxes are temporally centered between the solstices 21st March and 21st September (i.e. mid-Spring and mid-Autumn)  Earth’s Seasons Subsolar point Tropic of Cancer (23.5oN) Subsolar point Tropic of Capricorn (23.5oS) Energy Receipt on Earth Related influences that affect receipt of energy on Earth: 1. Period of Daylight (day length) Length of day is determined by the tilt of Earth’s axis Below images show day length at different latitudes on the June and December solstices: Energy Receipt on Earth 2. Solar Angle Affects the amount of beam spreading Radiation received per unit area is proportional to solar angle Higher sun angles equal reduced beam spreading = greater heating The height of the sun in the sky changes during the year  Energy Receipt on Earth 3. Beam Depletion Solar radiation diminishes relative to path length through the atmosphere High solar angles mean small energy reductions Low solar angles mean large energy reductions Energy Receipt on Earth Changes in Energy Receipt with Latitude due to 1. Day length, 2. Solar angle, and 3. Beam depletion result in: Energy surpluses (Summer) Energy deficits (Winter)