CHAPTER 3 Heat and energy in the atmosphere In Chapter 2 we examined the nature of energy and its interactions with Earth. Here we concentrate initially on the way in which energy interacts with the atmosphere and how it becomes heat. For a variety of reasons, which will be discussed during the chapter, there is a spatially varied pattern of heat. Greatest amounts of energy are absorbed within the tropics, with decreasing amounts polewards. This imbalance in radiation absorption is extremely important, as it provides the basic driving mechanism for the circulation of the atmosphere and, indirectly, the pressure systems which give us our weather. If we consider only radiation, we find the ground surface is receiving more energy than it loses and the atmosphere is losing more than it is gaining. Such a situation would be unsustainable, and we have to stress that other energy transferring processes operate to bring about a balance. These are sensible heat and latent heat. Water needs energy to sustain evaporation which is later returned on condensation. With the vast amounts of water and water vapour involved in evaporation and condensation, there is considerable energy transfer from the surface to the atmosphere and horizontal movements within the atmosphere. The chapter concludes with a brief examination of the consequences of this differential energy input in terms of temperature and its seasonal and diurnal patterns. Local climatic data can be used to examine the seasonal and perhaps also the diurnal variations of temperature if these are available. If sunshine observations are also recorded it is interesting to note the variation in heating power of the sun throughout the year. An hour of sunshine in the winter season does not produce the same level of heating as an hour in summer. Cloud and wind have a dramatic effect on temperatures too. Compare measurements of the night-time minimum temperature on calm and clear nights with those on nights which are cloudy and windy. Their effects by day are even more obvious. The atmospheric energy system Short-wave radiation The ultra-violet component of the sun s radiation is absorbed in the stratosphere in a complex process involving ozone and resulting in heating. As the short-wave radiation passes through the troposphere it is reflected, scattered and absorbed. Scattering of light produces our blue skies. Some scattered light reaches the ground as diffuse radiation. It allows us to see objects not in direct sunlight. Reflection can take place from clouds or the ground. The term albedo is used to indicate the proportion of light reflected from a surface. Snow has a high albedo, pine forest has a low albedo. Water can have a high or a low albedo, depending upon the solar angle. Long-wave radiation The atmosphere and Earth s surface are the main sources of long-wave radiation.
Some atmospheric gases absorb particular wavelengths of long-wave radiation very effectively. Few gases absorb wavelengths between 8 µm and 12 µm, so radiation of this wavelength can escape to space through the radiation window. Clouds are very effective absorbers of long-wave radiation, so help to keep temperatures warmer than would otherwise be the case at night. The greenhouse effect The level of carbon dioxide in the atmosphere has increased since the Industrial Revolution. With other gases that have the same effect of absorbing long-wave radiation also increasing, the natural greenhouse effect of the atmosphere should increase. It must be stressed that the gases of the atmosphere, particularly water vapour and carbon dioxide, provide a natural greenhouse effect. The increases in gaseous pollution lead to an enhanced greenhouse effect. The global radiation balance Adding together the short and long-wave radiation received at the surface from the sun and the atmosphere, it would appear that the surface is gaining more energy than it loses through reflection of short-wave radiation and emission of long-wave radiation. It does not get hotter because other processes help it to lose heat. Spatial variability of radiation exchanges The angle of the sun s rays is very important. If the rays are at 90 to a surface, it will receive the maximum intensity of radiation. As the angle decreases, so the intensity of the solar beam declines. Longer path-lengths of sunlight through the atmosphere will decrease the intensity of sunlight by giving rise to more scattering and absorption. Long-wave radiation emission is less variable than short-wave radiation receipt, because it is based on absolute temperature. Taking the atmosphere and surface together, we find a radiation surplus between approximately 38 N and 38 S, and a deficit of radiation polewards of these latitudes. The energy balance The energy surplus at the surface is transferred to the atmosphere as sensible and latent heat.
Sensible heat is the convectional exchange of warm air down the temperature gradient. It is most important over heated land surfaces such as deserts. Latent heat transfer takes place through the convection of moist air containing evaporated water vapour. Eventually, after cooling, the latent heat is added to the atmosphere during condensation. This helps to compensate for the radiation deficit. It is most important over the tropical oceans. In the atmospheric circulation we can also include kinetic energy and potential energy as well as sensible and latent heat. Although we have stressed the role of the atmosphere in transferring energy vertically and horizontally, we must not forget that ocean currents also transfer heat. Temperature effects Temperatures respond to inputs of energy. Where some of the radiant energy is used in evaporation, less energy is available for heating. In the tropics, seasonal differences in temperature are small, and the diurnal variation is more important. The sun never strays far from an overhead position at noon. CASE STUDY Atmospheric Composition Our present atmosphere is a mixture of gases that we take for granted. We are conscious of it when strong winds blow and affect our walking and also at high altitude when exertion can lead to a shortness of breath, but normally it is just something that is there that we cannot do without. Oxygen is usually thought of as the most important gas as without it we would not be able to breathe. The nitrogen, that accounts for 78% of the dry atmosphere (see Table 3.1), might appear to be there to dilute the oxygen and prevent fires getting out of control as they would in an oxygen-rich atmosphere. Recently we have been made aware of the importance of carbon dioxide in our atmosphere as it has the capacity to allow short-wave radiation to pass through the atmosphere but absorbs much of the long-wave radiation given off by Earth. It has become known as one of the Greenhouse gases which contribute towards keeping our planet at its current temperature instead of -15ºC that would be expected without our greenhouse gases. The reason for the increased interest in carbon dioxide and the other greenhouse gases is that their proportion in the atmosphere is increasing and so may be expected to trap more long-wave radiation which in turn would lead to a warmer Earth. From evidence that we shall discuss in chapter 23, it appears that the carbon dioxide concentration in the atmosphere has increased from about 0.028% to its current value of 0.038% over the last few centuries. Methane similarly has increased to its current value of 0.00017% and may increase dramatically in the future if the methane trapped in the permafrost areas of Siberia and Canada is released through global warming.
The reason for stressing these concentrations that appear incredibly small compared with oxygen and nitrogen, is that even minor changes may have serious implications for the radiation and energy budgets of Earth. If we go back far into Earth history we find evidence that our atmosphere did not always have its present composition. Between about 3.8 and 2.5 billion years ago, during what geologists term the Archean period, the atmosphere is believed to have consisted largely of methane, ammonia and other gases which would be toxic to today s lifeforms. Most of these gases will have been emitted by volcanoes which were much more active when Earth was relatively young. However, the gases emitted by today s volcanoes are largely carbon dioxide, water vapour and nitrogen. Why were they so different and what effect did they have on the Archean atmosphere? Our understanding of the early atmosphere is still very limited. It is argued that the earliest volcanoes had their peculiar gaseous composition because Earth s core had not been fully formed so that the mantle (see Chapter 10) was rich in metallic iron. This would favour the release of what are called reducing gases with little oxygen. (Figure 1) Figure 1 (a) Reducing volcanic gases (with significant H 2, CH 4 and NH 3 ) were introduced into the atmosphere before core formation, when the mantle was rich in metallic iron. (b) A weakly reducing mixture of volcanic gases has fed the atmosphere for most of Erth history, after the Earth differentiated into core, mantle and crust. Source: "Reprinted from Earth & Planetary Science Letters 237, Catling, D.C. and Claire, How Earth s atmosphere evolved to an oxic state: A status report, p.1-20, Figure 4, Copyright 2005 with permission from Elsevier (http://www.sciencedirect.com/science/journal/0012821x). Nevertheless life began about 3.5 billion years ago and consisted largely of bacterial microanimals such as stromatolites that still survive in Australia. These bacteria would release oxygen as part of their photosynthesis. Geological evidence indicates that after about 2.4 billion years ago, the proportion of oxygen became more significant but we are not really sure why.
Recent investigations have shown that between 2.4-2.3 billion and 0.8-0.6 billion years ago there were significant increases in the levels of atmospheric oxygen. Interestingly these coincide with phases of snowball earth (see Chapter 9) when the carbon cycle was greatly disturbed. It is possible that these two factors may be related. Oxygen and methane cannot coexist in large quantities so, as the proportion of oxygen increases, that of methane will decrease. At this time, the sun was much less powerful than now, estimates suggest about 25-30% fainter than today, so that to maintain temperatures, a greenhouse gas like methane would be needed to sustain average temperatures. If the methane was being destroyed through the presence of more oxygen then Earth could plunge into a glacial state (see Chapter 9). Unfortunately we still do not really understand how the proportion of oxygen increased to its present levels. What is certain is that its increase was vital for the development of life forms that we know in geological history and today. Together oxygen and carbon dioxide played a vital role in maintaining the atmospheric composition and influencing Earth s climate. Essay and discussion questions 1 Earth s surface gains more radiation than it emits. Why does it not get hotter? 2 Why is there not a steady decrease of solar radiation receipt from equator to poles? 3 Why is the diurnal range of temperature greater than the annual range in equatorial areas and why is the reverse true in temperate latitudes? 4 Why do oceanic locations have smaller diurnal and annual temperature ranges than continental sites? 5 Deserts have relatively low totals of net radiation. Why are they hot? 6 Explain why some areas are more important as sources of sensible heat than others. 7 What is the importance of ozone in the radiation budget of the atmosphere? Further reading Barry, R.G. and Chorley, R.J. (2003) Atmosphere, Weather and Climate, sixth edition, London: Routledge. A popular textbook, now in its eighth edition, which covers the whole field of climatology in considerable detail. Chapter 3 covers heat and energy in the atmosphere but is not always easy to absorb. Hartmann, D.L. (1994) Global Physical Climatology, San Diego, Cal.: Academic Press. Chapters 2 4 provide a modern replacement of Sellers s classic Physical Climatology. It is pitched at quite an advanced level and includes recent data from satellites. Hidore, J.J. and Oliver, J.E. (2001) Climatology: An atmospheric science, second edition, New York: Macmillan. An elementary textbook which introduces the processes of
climate changes through time, especially chapters 2 4. Also looks at the human impact on the energy budget. Websites http://www.srh.noaa.gov/jetstream/atmos/heat.htm For more information about the basics of energy, energy transfer and Earth, this US National Weather Service website provides a useful, well-illustrated resource. There are also a large-number of educational websites where lecture courses on this theme provide web information intended as a learning experience. http://jwocky.gsfc.nasa.gov/ Official website for NASA s Total Ozone Mapping Spectrometer. Gives information about the recent levels of ozone in the stratosphere and trends over time. Also includes links to an electronic text about ozone, its properties and characteristics. http://asd-www.larc.nasa.gov/erbe/asderbe.html Another NASA site, this one is concerned with the Earth Radiation Budget Experiment. Information is available about a wide range of aspects of long- and short-wave, and net radiation. http://www.ucar.edu/learn/index.htm A teaching website that is intended to increase awareness of atmospheric science. Covers a wide range of subject matter in an authoritative manner.