Energy is very important to the climate system. It is energy which drives the system and is constantly being exchanged from one component to the next. Energy is defined as the ability to do work and in physics work occurs when an object moves over some distance due to a force acting along the line of the movement. Sears, Zemansky and Young (1978) put forward the following; example of work. Supposing someone asked you to move a heavy box for them. If you were to push it along a level floor you would be exerting a force on the box and causing it to move. Some component of the force would be directed in the direction of motion; this is work as defined in terms of physics. However, if you picked up the box and carried it you might well think thai: you were working hard but in physics terms work would only have been done when you picked up and put down the box. While carrying the box no work will have been done as the supporting force (supplied by you!) is in the verti cal while the box is moving horizontally. In this case there is no component of the force acting in the direction of movement.

Energy comes in lots of different forms and it can transform from one type to another. A system has an energy budget and because energy can neither be created nor destroyed, the amount of energy gained by the system must equal the amount of energy lost plus any change in the stored energy of the system.

Potential energy is the amount of energy stored by an object. There are various different forms of potential energy. Gravitational potential energy is the energy an object possesses due to its position. It depends on mass, gravity and height. Essentially the higher and heavier an object is, the more poten tial energy it has. Chemical potential energy is the work that can be done due to a chemical change within a substance. The energy we get from food is stored as chemical potential energy. Similarly, fossil fuels have chemical poten tial energy which is transformed to provide heat.

Kinetic energy is the energy that an object possesses due to its movement. It depends on both the mass of the object and its speed. Objects moving at a higher speed have a higher kinetic energy than those moving at slower speeds. Those objects which have more mass have a greater kinetic energy than those with less mass. In a cup of water, where some molecules will be moving faster than others, there will be a range of speeds and hence a range of kinetic energies.

Temperature is defined as the mean kinetic energy per molecule of all the molecules in an object: in essence the average speed of the molecules of a substance. Temperature can be measured on a variety of different scales and the two used internationally are Celsius (also called centigrade) denoted by °C and Kelvin denoted by a K. The intervals on these two scales are the same, that is a temperature rise of 1°C is equivalent to a rise of 1 K. The definition of the zero points, however, are different on each scale. On the Celsius scale 0°C is the freezing point of water whereas on the Kelvin scale 0 K (around —273.15°C) is defined as the temperature at which all molecular motion stops. Bohren (1987) states that it is more true to say that matter ceases to radiate, rather than motion ceases. Only in an ideal gas would motion stop. An ideal gas is, a hypothetical one, where the molecules are far enough apart so as not to interact with each other. The atmosphere approximates to an ideal gas at room temperature. At absolute zero (0 K), quantum, rather than classical mechanics takes over. Any object above 0 K will have kinetic energy. Internal energy is the amount of kinetic and potential energy the molecules of an object contain.

Energy can be transferred by the means of work. This would occur if a mechanical force was used to compress a gas. Once compressed the gas can do more work so energy has been transferred to it through work. An example of this is a bicycle pump. You provide a mechanical force which compresses the air in the pump, causing it to become hotter, increasing its amount of kinetic energy. Another way in which energy is transferred from one object to another is by heat due to a temperature difference. The result of this energy transfer will be changes in the properties of the object. The temperature may rise, or the object may change state (as energy is being added the state change will be to a higher one from solid to liquid or liquid to gas) or both. A temperature rise is referred to as sensible heat exchange, whereas a change of state is known as latent (hidden) heat.

Specific heat capacity is the heat required to raise the temperature of lg mass of a substance by 1°C. Different substances require different amounts of heat to raise their temperature by 1°C. The term calorie (cal) is defined as the heat needed to raise lg of water from 14.5°C to 15.5°C. The specific heat capacity of water is taken to be 1 cal g (°C) . Soils have a much lower specific heat capacity than water (about 0.2 cal). Consequently although water takes longer to heat up than the land, it also takes longer to cool down. This property has an effect on the world's climate. The interior of continents such as Europe and Asia experience much more extreme summers and winters than coastal regions. At the coast the oceans act as a thermal store, smoothing out the difference between winter and summer.

Latent heat is the heat required for 1 g of mass of substance to change from one state to another without a rise in temperature. Using water as an example, ice is the solid state. To melt ice into a liquid requires energy as the water molecules need to move more quickly. Turning the liquid water into a vapour also requires energy. So in both these cases energy is absorbed. Perspiration is the body's way of using latent heat to cool us down. The layer of perspiration on a hot body uses that energy to turn from a liquid to a vapour taking away energy, so cooling us down. When a gas becomes a liquid or a liquid becomes a solid there is a consequent release of energy.

The transference of energy by heat can be by any of three processes:

conduction, convection and radiation. Conduction is the transfer of energy by molecular collisions. For example, if you take a long metal rod and heat it at one end, the energy is conducted to the other end of the rod. The molecules at the heated end of the rod will be moving fast due to the high temperature. They will collide into their cooler and more slowly moving neighbours and pass on some of their energy. This processes continues along the length of the rod to the unheated end. The molecules do not change their position along the rod. In contrast, convection involves the transferral of energy by the actual movement of the heated material. This commonly occurs in the lower atmosphere (troposphere) with heated parcels of air moving to colder parts of the atmosphere. Forced convection occurs when the material is forced to move, a common example in the atmosphere is the movement of air parcels up mountains. Free convection occurs when the material is at a different density to its surroundings. This is because the less dense an object, the lighter it is.

Latent heat release and convection play an important combined role in the atmosphere, transferring energy from low to high altitudes. To understand this consider what happens in the atmosphere by imagining a balloon filled with warm moist air slightly warmer than the surrounding atmosphere. The balloon is special, its thin surface skin effectively isolates the air from the surrounding atmosphere and stops any heat exchange with it. The atmospheric mass decreases with height so as the balloon rises there is less mass of atmosphere on it; less to compress it. The air inside the balloon will expand, doing work, so the air will cool. Eventually the air will cool sufficiently that the water vapour contained in it will condense out releasing latent heat. The air will continue to cool as it rises but the rate of cooling will not be as large, as it is now gaining energy from the latent heat release. This increases its ability to rise still higher. The atmosphere surrounding the balloon will also be cooling with height but so long as the air inside the balloon remains warmer than the surrounding atmosphere (i.e. cools at a slower rate), it will remain less dense than the atmosphere and will continue to rise. However, should the air cool faster than the surrounding atmosphere eventually it will become more dense than the atmosphere and begin to sink. As the balloon sinks the pressure will increase and the balloon will contract. The air inside will warm and any liquid water will begin to evaporate. As the air inside the balloon exchanged no heat with its surroundings the process is said to be adiabatic. The rate of change of temperature with height is known as the lapse rate and effectively the lapse rate of the atmosphere and that of the balloon or parcel of air will determine whether convective motions take place. So long as the parcel of air cools more slowly than the surrounding atmosphere (i.e. has a smaller lapse rate), then it will be less dense and more buoyant and convective motion will occur. This type of convective cell is particularly important in the tropics.

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