Solar Radiation and the Seasons

Chapter 2 Lecture Understanding Weather and Climate Seventh Edition Solar Radiation and the Seasons Frode Stordal, University of Oslo Redina L. Herman Western Illinois University

Energy Energy is traditionally described as the ability to do work. About one two-billionth of the energy emitted by the Sun is transferred to Earth as electromagnetic radiation. Some electromagnetic radiation is absorbed by the atmosphere and some by the Earth s surface.

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. Power is the rate at which energy is used, released, transferred, or received.

Energy Kinetic Gas molecules have no bonds to other molecules and move in random motion.

Energy Heat Transfer Mechanisms Energy can be transferred from one place to another by three processes: Conduction Convection Radiation

Energy Conduction 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 to the handle of a warmed pot and this is conduction. Heat moves into the ground by conduction. Conduction is most effective in solid materials.

Energy Convection Convection is the transfer of heat by mixing of a fluid. Both liquids and gases can move energy by convection. A pot of boiling water is an example of convection. Convection in the atmosphere occurs when the heating of the Earth s surface warms the 1 mm layer of air in contact with the surface. Winds are natural convection currents (forced convection).

Energy Radiation Radiation is the transfer of energy that requires no physical medium (can occur through empty space). Continually emitted by all substances

Characteristics of Radiation Radiation Quantity and Quality Radiation quantity Refers to the amount of energy transferred Associated with wave height, or amplitude Radiation quality Relates to radiation wavelength, or the distance between the wave crests Identifies the type of radiant energy

Characteristics of Radiation Radiation Quantity and Quality Electromagnetic radiation E = electric wave M = magnetic wave

Characteristics of Radiation Intensity and Wavelengths of Emitted Radiation Categorized into a few individual bands along the electromagnetic spectrum, visible light is a narrow band bounded by infrared and ultraviolet.

Characteristics of Radiation Intensity and Wavelengths of Emitted Radiation All matter radiates energy over a wide range of electromagnetic wavelengths. Physical laws defining amount and wavelength of emitted energy apply to hypothetical perfect emitters of radiation known as blackbodies. The Earth and Sun are similar to blackbodies.

Characteristics of Radiation Energy radiated by substances occurs over a wide range of wavelengths.

Characteristics of Radiation Intensity and Wavelengths of Emitted Radiation The intensity of radiation depends on the temperature raised to the fourth power (Stefan-Boltzmann law): I = σ T 4 I T σ intensity temperature Stefan Bolzmann s constant The surface of the Sun is about 5800 K (5500 C or 9900 F) and emits about 64 million watts per square meter. Most liquids and solids are graybodies, meaning they emit some percentage of the maximum amount of radiation possible at a given temperature.

Characteristics of Radiation Intensity and Wavelengths of Emitted Radiation 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 I = ε σ T 4 ε emissivity Most natural surfaces have emissivities above 0.9 (that is, above 90 percent of blackbody).

Characteristics of Radiation Intensity and Wavelengths of Emitted Radiation For any radiating body, the wavelength of peak emission (in micrometers) is given by Wien s law. Warmer objects radiate energy at shorter wavelengths than do cooler bodies. Wavelengths less than 4 µm are considered shortwave radiation. Wavelengths longer than 4 µm are considered longwave radiation. Warmer bodies radiate more energy than do cooler bodies at all wavelengths.

Characteristics of Radiation Wiens law λ max = c/t λ C wavelength constant

The Solar Constant Intensity of electromagnetic radiation is not depleted or reduced as it moves toward Earth. The intensity is reduced as a result of radiation being distributed over a large area, not because of the distance from the Sun. 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 = 3.865 x 10 26 W/distance surrounding the Sun = 4 (1.5 x 10 11 m) 2 = 1367 W/m 2.

The Solar Constant

The Causes of Earth s Seasons Earth s Revolution Earth revolves around the Sun once every 365.25 days. Earth revolves the Sun in an ecliptic plane annually, known as the revolution. Distance from the Sun varies. Perihelion (Jan 3; 147 million km Aphelion (July 3; 152 million km Using the inverse square law, radiation intensity varies by about 7 percent between perihelion and aphelion.

The Causes of Earth s Seasons Earth s Revolution

The Causes of Earth s Seasons Earth s Revolution The length of a day is defined by Earth s rotation, which occurs every 24 hours. Axis of rotation is offset 23.5 from the perpendicular plane. Northern axis aligns with the star Polaris. As Earth orbits the Sun, the hemispheres are impacted seasonally. A particular hemisphere aligns toward or away from the Sun or occupies a position between the extremes, creating our solstices and equinoxes.

The Causes of Earth s Seasons Earth s Revolution and Rotation

The Causes of Earth s Seasons Summer and Winter Solstices

The Causes of Earth s Seasons Equinoxes: March 21 and September 21

Effects of Earth s Changing Orientation Solar Angle Solar radiation is directly related to solar angle. Higher solar angles reduce beam spreading, which leads to warming. Lower angles induce less intense warming.

Effects of Earth s Changing Orientation Solar Angle

Effects of Earth s Changing Orientation Changes in Energy Receipt with Latitude and Season