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The Movement of Energy
1.1. The "cause" of weather
Weather is caused by the movement or transfer of energy. Energy is transferred wherever there is a temperature difference between two objects. There are three main ways energy can be transferred: radiation, conduction and convection.
1.2. Electromagnetic radiation
All bodies emit energy in the form of electromagnetic radiation. Light is a form of electromagnetic radiation. So is infrared heat and ultraviolet radiation. Different types of radiation are shown in Figure 1.1. The type of radiation emitted by a body depends upon its temperature. Hotter objects release more energetic radiation. The Sun, for example, emits light wave and ultraviolet energy. The Earth, being much cooler, emits radiation in the infrared part of the spectrum. Living things, including us, also emit radiation in the infrared part of the spectrum. This energy is known as heat energy.
1.3. Transfer of radiation: absorption
The transfer of radiation may be accomplished by direct absorption by the cooler object of energy emitted by the warmer one. Food placed under a grill absorbs energy radiated from the heating elements. Similarly, the Earth absorbs energy emitted from the Sun. How much the cooler object heats up depends upon the nature of that object. Certain materials like rock require much less energy to raise their temperature than others, such as water. That is to say, water has a larger heat capacity than rock (about 5 times larger).
1.4. Transfer of radiation: reflection and scattering
Transfer of radiation can occur in other ways besides absorption. Certain material bodies, for example glass or pure air, are transparent to visible radiation (light), allowing the energy to pass straight through without absorption. This is not the case for infrared radiation, which is trapped (absorbed) by glass and certain greenhouse gases. Other material bodies neither absorb nor allow through radiation, but reflect it back. Light surfaces such as snow and ice are good reflectors of visible radiation. Dust in the atmosphere scatters radiation. The blue of the sky is due to the scattering of the blue part of visible light from the Sun by air molecules, the reds and yellows of sunset are due to scattering by dust particles.
Transfer of energy can also be made by conduction, if two objects are in contact. A heating element of a kettle heats its water by conduction of energy. Metal is a good conductor, as is water. Air is a poor conductor of energy and therefore acts as a good insulator, as, for example, between the outer and inner skins of a cavity wall house, or between the human body and outside air by air spaces between fibres of clothing.
Convection of energy involves the movement of the body storing the energy. Clearly, convection can only occur in fluids - liquids and gases. Both air and water act as convecting agents for the transfer of energy. Because air is a poor conductor of energy, convection is a major process of energy movement in the Earth's atmosphere. In the atmosphere, convection occurs when a shallow layer of air in contact with a hot surface warms by conduction, acquires buoyancy (warmer air is less dense than colder air), and rises, taking with it the energy that it stores.
1.7. Movement of radiation in the atmosphere
In a weather context, energy radiated from the Sun is distributed at the Earth by absorption, conduction, convection, reflection and scattering. The Sun radiates energy at a rate which, for practical purposes, is unchanging. Most of this radiation is visible light and ultra - violet energy. At the outer limit of the Earth's atmosphere, the radiation received is 1370 watts per square metre. This is called the solar constant. Passing through the atmosphere it is depleted in various ways.
Air in an atmosphere free from clouds and pollution absorbs very little solar radiation, which passes through to the Earth's surface. Here, some of the radiation is absorbed and some is reflected. The absorbed part heats the surface, causing its temperature to rise. The Earth's surface also emits energy, but because it is much cooler than the Sun, it does so in the infrared part of the spectrum. This is heat energy. Often, the Sun's energy is called short-wave radiation, whilst radiation from the Earth's surface is called long-wave radiation. The more energetic the radiation, the shorter its wavelength. Thus hotter objects emit shorter wavelength radiation.
1.8. The greenhouse effect
Whilst the air allows short-wave light to pass through to the Earth's surface, certain gases in the atmosphere impede the passage of the long - wave heat energy by absorbing it. Water vapour and carbon dioxide are particularly effective at absorbing the long-wave heat radiation. This warms the air, which in turn radiates some of the energy back to the Earth's surface and some to space. Thus, the atmosphere, by trapping some of the outgoing radiation, keeps the Earth's surface warmer than it would otherwise be.
This warming effect is called the "greenhouse effect" (see Figure 1.2) because it is the same process as that which occurs in a greenhouse on a sunny day. The glass is transparent to short-wave radiation but absorbs the outgoing long-wave radiation, causing a rise in temperature inside the greenhouse. Water vapour and carbon dioxide are called "greenhouse gases". Other greenhouse gases include methane and CFCs from aerosol cans.
1.9. Convection in the atmosphere
Heat energy in the atmosphere is also transferred by convection as described above, with the heating and ascension of surface air. Such convection processes in a large part dominate the world weather, including the production of rain and snow, thunderstorms, hurricanes and frontal systems. How many of these weather phenomena originate will be the topic of the following lessons in this section.
a) All bodies emit electromagnetic radiation across a spectrum of wavelengths (l) as shown in Figure 1.1 of the information sheet. Using the information below calculate lmax (the wavelength of maximum radiation) for the Sun and the Earth. In which part of the electromagnetic spectrum do these wavelengths occur (see Figure 1.1 in the information sheet)?
lmax = 2897/T
where T = surface temperature (K); lmax in µm (microns)
TSun = 5800 K (6073ºC)
b) List three sources of electromagnetic radiation in the visible part of the spectrum.
c) List three examples of energy transference by conduction.
d) Why is convection so important in the context of weather?
e) Without an atmosphere the average surface temperature of the Earth would be 255K (-18ºC). In fact it is 288K (15ºC). How do greenhouse gases in the atmosphere help to keep the Earth's atmosphere warmer than it would otherwise be?
f) What do you think the average surface temperature of the moon would be?
Notes for Teachers
After lesson 1, students should understand:
Answers to questions
a)l max (Sun) = 0.5 µm (microns)
l max (Earth) = 10 µm (microns)
Note that l max is the wavelength that which most electromagnetic radiation is emitted.
b) Three source of electromagnetic radiation in the visible part of the spectrum are:
c) Three examples of conduction are:
d) Convection is the transference of energy by movement of the energy carrier. In a weather context, the carrier is air the atmosphere. All the world's weather is the result of convection processes, to a greater or lesser degree brought about by temperature differences at the Earth's surface and in the lower part of the atmosphere. Convection generates clouds, rain, snow, thunderstorms and hurricanes.
e) The greenhouse gases in the atmosphere absorb much of the outgoing terrestrial radiation, re-emitting it back to the earth. With more energy stored in the lower atmosphere the temperature of the Earth's surface rises.
f) The Moon, unlike the Earth, has no atmosphere, and therefore no greenhouse gases to trap the infrared radiation emitted from the surface. Therefore the average surface temperature will be -18°C or 255 K. [Of course, during the day the Moon's surface is much hotter, whilst at night it is much colder.]