Driving the Earth’s Weather Solar Radiation Driving the Earth’s Weather

Forms of Energy Energy can be grouped into two general categories. “the ability to do work.” Energy can be grouped into two general categories. Kinetic energy can be viewed as energy in use. Often described as the energy of motion. Wind Most important type is radiant energy Potential energy is energy that has not yet been used. A cloud droplet, for instance, has potential energy due to the position it occupies above Earth’s surface. Like all other objects, the droplet is subject to the effect of gravity. The higher the droplet’s elevation, the greater its potential energy. Food How about a lake behind a dam? – Destructive work

Forms of Energy

Transfer of Energy Heat (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. Transfer from molecule to molecule. Most effective in solid materials Also an important process in a very thin layer of air near Earth’s surface. Ex. Transfer of heat from hot stove to hand

Transfer of Energy Convection is the transfer of heat by the mixing of a fluid. 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. Above this thin laminar layer, air heated from below expands and rises upward because of the inherent buoyancy of warm air (thermals). Buoyancy is the tendency for a light fluid to float upward when surrounded by a heavier fluid.

Transfer of Energy Of the three energy transfer mechanisms, 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, however, is emitted by all matter. Face absorbs radiation on a sunny, summer day and is converted to thermal energy.

Electromagnetic Waves Electromagnetic Radiation (short wave) travels in the form of waves Energy transferred from the sun that releases energy when absorbed by an object (ozone)(face on a hot summer day) Radiation can vary. Measured in micrometers. Travel at speed of light. Amplitude Wavelength

EM Wave Characteristics The quantity of radiation or energy transported is associated with the height of the wave, or its amplitude. The distance between wave crests, or wavelength, determines the quality, or “type” of radiation Amplitude Wavelength

The Electromagnetic Spectrum Visible – Light that we can see. One hundredth of the diameter of a human hair. Eyes are sensitive to this wavelength and thus we get color. Some forms carry more energy (photons) than others. (sunburn) Sun emits at all wavelengths

Stefan-Boltzmann Law This relationship is represented by the Stefan-Boltzmann law, expressed as: I = σT4 Where : I is the intensity of radiation in watts per square meter, σ is a constant (5.67 x 10-8 watts per square meter) and T is the temperature of the body in kelvins. (Surface of Sun – 10,500° F, Earth - 59° F)

Temperature Relationships Celsius Temperature = (oF - 32) / 1.8 Fahrenheit Temperature = (1.8 x oC) + 32 Kelvin Temperature = oC + 273

Wien’s Law For any radiating body, the wavelength of peak emission (in micrometers) is given by Wien’s law: max = constant (2900)/T where max refers to the wavelength of energy radiated with greatest intensity. Wien’s law tells us that hotter objects radiate energy at shorter wavelengths than do cooler bodies.

Solar vs Terrestrial Radiation Solar radiation is most intense in the visible portion of the spectrum. Most of the radiation has wavelengths less than 4 micrometers which we generically refer to as shortwave radiation. Radiation emanating from Earth’s surface and atmosphere consists mainly of that having wavelengths longer than 4 micrometers. This type of electromagnetic energy is called longwave radiation.

Earth’s Orbit Earth orbits the Sun once every 365 1/4 days as if it were riding along a flat plane. We refer to this imaginary surface as the ecliptic plane and to Earth’s annual trip about the plane as its revolution. 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 (152,000,000 km).

Earth’s Tilt and Rotation 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. The axis is always tilted in the same direction and always points to a distant star called Polaris (the North Star).

Earth’s axis points in the same direction all year long.

Why is it colder in the winter if the earth is closer to the sun Why is it colder in the winter if the earth is closer to the sun? Tilt and Solar Altitude Less Daylight – decrease in amount of solar energy

Absorption of Radiation Process where radiation is captured by atmospheric gases, particulates, and droplets. Radiation striking the surface of the particle is converted to heat energy. This has two effects: the absorber gains energy and warms The amount of energy delivered to the surface is reduced.

Atmospheric Absorption

Reflection of Radiation Radiation making contact with some material is simply redirected away from the surface without being absorbed. Angle of incident radiation = angle of reflected radiation. Albedo = reflected radiation / incident radiation. When light strikes a mirror, it is reflected back as a beam of equal intensity, in a manner known as specular reflection.

Atmospheric Scattering Radiation is reflected from an object as a large number of weaker rays traveling in different directions. Scattering can be from large solid surfaces, gas molecules, particulates, and small droplets. Scattering is wavelength dependent. Radiation is scattered both back to space and toward the surface. Scattered energy reaching Earth’s surface is thus diffuse radiation, which is in contrast to unscattered direct radiation.

The sky appears blue because gases and particles in the atmosphere scatter some of the incoming solar radiation in all directions. Air molecules scatter shorter wavelengths most effectively. Thus, we perceive blue light, the shortest wavelength of the visible portion of the spectrum.

Sunrises and sunsets appear red because sunlight travels a longer path through the atmosphere. This causes a high amount of scattering to remove shorter wavelengths from the incoming beam radiation. The result is sunlight consisting almost entirely of longer (e.g., red) wavelengths.

Solar radiation and the earth’s surface The fraction of solar radiation that does make it to the earth’s surface is either reflected or absorbed (increasing the surface’s temperature) Common Albedos Urban area: 14-18 – daytime highs warmer during sunny days Cirrus clouds: 40-50 – keeps nighttime temperatures warmer – daytime cooler Fresh snow: 75-95 – daytime temps cooler

Sensible Heat When energy is added to a substance, an increase in temperature occurs that we physically sense. This is called (sensible heat). The magnitude of temperature increase is related to two factors: Specific heat, which is defined as the amount of energy needed to produce a given temperature change per unit mass of the substance. The temperature increase resulting from a surplus of energy receipt also depends on the mass of a substance.

Latent Heat Latent heat is the energy required to change the phase of a substance (solid, liquid, or gas). In meteorology we are concerned with the heat involved in the phase changes of water. In the case of melting ice, the energy is called the latent heat of fusion. For the change of phase from liquid to gas, the energy is called the latent heat of evaporation.