Chapters 2 and 19 Energy: Warming the earth and Atmosphere
Specific Heat In the atmosphere, heat is transferred by conduction, convection and radiation. • Heat capacity is the heat energy absorbed to raise a substance to a given temperature • Specific heat is the heat capacity of the substance per unit mass; or the amount of energy required to raise one gram of a substance 1°C • High specific heat equates to slow warming and vice versa
Latent Heat • Change of state or phase change represents change between solid, gas, and liquid. • Latent heat is the energy involved in the change of state. latent heat is an important source of atmospheric energy. • Ice to vapor: absorb energy, resulting in cooling the environment (melt, evaporation, sublimation) • Vapor to ice: release energy, heating the environment (freeze, condensation, deposition)
Heat Transfer in the Atmosphere • Conduction: transfer heat from one molecule to another in a substance • Energy travels from hot cold • Air a poor conductor, metal a good conductor • Convection: transfer of heat by the mass movement of a fluid (water or air) • Convection circulation happens naturally in the atmosphere: warm air expands and rises then cools and sinks; thermal cell
A thermal is a rising bubble of air • that carries heat energy upward • by convection convective circulation • Air that rises will expand and cool • Air that sinks is compressed and warms
Radiation • Energy from the sun travels through the space and the atmosphere in the form of a wave (electromagnetic waves). • Radiation and Temperature • All objects with a temperature greater than 00 K radiate energy. • As temperature of an object increases, the more total radiation is emitted by the object.
Radiation • Radiation of the Sun and Earth • Sun 6000 0K emits radiation, electromagnetic spectrum (most energy emitted at λ~ 0.5 µm) • Earth 288 0K radiates also energy (λ~ 10 µm) • Shortwave radiation (high energy) from the Sun • Longwave radiation (low energy) from the Earth
Radiation • Environmental Issue: Sunburn • UV index is a weather forecast product that indicates the potential for sun burn due to high energy or short wavelengths emitted by the sun. Shorter λ Longer λ
Balancing Act • If the Earth is radiating energy all the time, why is it not very cold? • Radiative equilibrium • Absorb > emit = warming • Emit > absorb = cooling
Selective Absorbers • Good absorbers are good emitters at a particular wavelength and vice versa. • Greenhouse effect: the atmosphere selectively absorbs infrared radiation from the Earth’s surface but acts as a window and transmits shortwave radiation
The earth’s surface would constantly emit IR radiation upward, during day and night. Incoming energy from the sun would equal outgoing energy from the surface, but the surface would receive virtually no IR radiation from its lower atmosphere. (No atmospheric greenhouse effect.) The earth’s surface air temperature would be quite low, and small amounts of water found on the planet would be in the form of ice. The earth’s surface not only receives energy from the sun but also IR energy from the atmosphere. Incoming energy still equals outgoing energy, but the added IR energy from the greenhouse gases raises the earth’s average surface temperature to a more habitable level.
Greenhouse Enhancement • The atmospheric greenhouse effect occurs because the greenhouse gases are selective absorbers keeps the temperature of our planet at a level where life can survive!!! • Global warming is attributed to an increase in greenhouse gases (see Fig. 2.12, page 43): • Carbon dioxide (CO2) • Water vapor (H20) • Molecular Oxigen (02) and Ozone (03) • Methane (CH4) • Nitrous Oxide (N20) • Chlorofluorocarbons • Positive feedbacks continue the warming trend. • Negative feedbacks decrease warming. • Two potentially largest and least understood feedbacks in the climate system are the clouds and the oceans.
Diffuse reflection is reflection from a rough surface • The reflected rays travel in a variety of directions • Diffuse reflection makes the dry road easy to see at night • Specular reflection is reflection from a smooth surface • The reflected rays are parallel to each other
Refraction Refraction in a prism
The Rainbow • A ray of light strikes a drop of water in the atmosphere • It undergoes both reflection and refraction • First refraction at the front of the drop • Violet light will deviate the most • Red light will deviate the least
Observing the Rainbow • If a raindrop high in the sky is observed, the red ray is seen • A drop lower in the sky would direct violet light to the observer • The other colors of the spectra lie in between the red and the violet
Incoming Solar Radiation • Conduction, convection, and infrared radiation warm the atmosphere from below, not sunlight or insolation from above. • Air molecules are << than λ of visible light more effective scatterers of shorter (blue) λ than the longer (red) λ • Scattering sun light (blue sky during daytime) • Reflection of sun light, albedo (clouds ~ 60% albedo; Water ~ 10% albedo; snow ~ 95% albedo) • White clouds scatter light • Black clouds have large cloud droplets which absorb light, rain likely
Further warming occurs during condensation as latent heat is given up to the air inside the cloud. Air in the lower atmosphere is heated from the ground upward. Sunlight warms the ground, and the air above is warmed by conduction, convection, and infrared radiation.
Cloud droplets scatter visible light in all directions; light from many droplets turns a cloud white.
The sky appears blue because billions of air molecules selectively scatter the shorter wavelengths of visible light more effectively than the longer ones.
The blue haze is caused by the scattering of blue light by extremely small particles (hydrocarbons) smaller than the λ’s of visible light the scattered blue light causes the most distant mountains to become almost indistinguishable from the sky. The Blue Ridge Mountains in Virginia.
Red Suns and Blue Moons • A thick atmosphere selectively scatters all but red sunlight. • A low solar angle (sunrise or sunset) causes light to travel through a greater distance or thicker atmosphere. • Same process for a blue moon.
Bright red sky over California produced by the sulfur-rich particles from the volcano Mt. Pinatubo during September,1992. The photo was taken about an hour after sunset.