TMJC 2025 JC1H2 C2 Lecture 2 Factors Affecting TC Temperature

Higher 2 (9173) Tropical Environments C2 Lecture 2 Factors affecting Temperature in Tropical Climates 2025 Page 1 of 16 LECTURE OBJECTIVES 1. Discuss the dynamism of natural environments in the tropics • Discuss the various factors affecting temperature in the tropics INTRODUCTION The Earth is constantly receiving incoming heat energy from the sun in the form of insolation (incoming solar radiation). This energy influences our planet’s climate and weather and which, when converted by photosynthesis in green plants, supports all forms of life. This heat energy received from the sun must be balanced by outgoing heat energy radiated back into space from the Earth as otherwise the Earth will experience a continuous increase in temperature. Since the Earth does not experience a continuous increase i n temperature (human induced global warming excluded), this means that incoming energy is matched by outgoing energy from the earth. HOW IS ENERGY TRANSFERRED? Before we understand the radiation balance, we will need to first understand how heat energy is transferred. Heat energy can be transferred via 3 mechanisms (Fig. 1): (i) Conduction Conduction is the transfer of heat through matter by molecular activity . The energy is transferred through collisions from one molecule to another, with the heat flowing from the higher temperature to the lower temperature. Hence, direct contact of molecules is required. T his implies that land (solids) can conduct heat better than water bodies (liquids) which in turn, conduct heat better than air (gases). (ii) Convection Convection is the transfer of heat by circulation within a fluid (i.e. liquids like ocean, gases like air) where atoms and molecules are free to move about. On a global scale, convection in the atmosphere creates a huge world-wide air circulation which is responsible for the redistribution of heat. Redistribution of heat also occurs in the form of ocean currents. (iii) Radiation Unlike conduction and convection, radiation is the only process of heat transfer that does not require a medium (solid or fluid) to travel through. Radiation is the only mechanism of energy transfer through the vacuum of space and thus, radiation is the heat-transfer mechanism by which solar radiation reaches the Earth from the sun. Fig. 1 – Three mechanisms of heat transfer: conduction, convection, radiation

Page 2 of 16 WHAT IS THE EARTH’S ENERGY BUDGET/ RADIATION BALANCE? The radiation balance (also known as energy budget / heat budget) shows the equilibrium between incoming energy from the sun and outgoing energy from the Earth (Fig. 2). The radiation balance thus prevents, at least in the short term, significant rises or falls of the average temperature of the Earth. Fig. 2 - Incoming and outgoing radiation 30% of the earth’s incoming insolation is reflected and scattered back to space. The remaining 70% of the solar radiation that is not reflected back is a) absorbed by the atmosphere (20%) or b) absorbed by the earth’s surface. (50%).

Page 3 of 16 What happens to incoming solar radiation? Insolation from the sun consists of mostly shortwave radiation which is not easily absorbed by the atmosphere. As solar radiation travels through the atmosphere to the earth’s surface, it may undergo several processes, namely: (i) Reflection Reflection refers to the process whereby solar radiation bounces directly back into space without being absorbed or performing any work. Objects and surfaces will reflect differing amounts of sunlight. This can be measured using albedo. Albedo refers to the reflective quality of a surface. It is the percentage of radiation returning from a given surface compared to the amount of radiation initially striking that surface. Light-coloured and smoother surfaces have higher albedo. Dark -coloured and rough surfaces have lower albedo. For example: • Fresh snow has the highest albedo at 75% - 95% (meaning 75% - 95% of the sunlight will be reflected) hence contributing to low temperatures at the polar areas. • Clear water surfaces, due to their transparency, have a low albedo compared to land. • Clouds have an average albedo of 60%, hence places with high amount of cloud cover may receive lesser amount of insolation. (ii) Scattering Scattering is the process whereby a beam of solar radiation is broken down into many weak rays by atmospheric gases or clouds. When sunlight strikes very small objects, such as air molecules and dust particles, the light itself is deflected in all directions. The scattered radiation will still be shortwave in nature but it will reach the earth’s surface in a diffused manner thus the name diffuse radiation. (iii) Absorption Absorption is the conversion of incoming shortwave radiation into heat energy. Only 20% of insolation is directly absorbed by the atmospheric molecules, dust and clouds. The rest is absorbed by the earth’s surface (both land and water). Once absorbed, the shortwave radiation is converted to sensible heat, which is subsequently re-radiated as long wavelength radiation. Upon absorbing shortwave radiation, the earth’s surface is heated up and is therefore able to radiate heat. This radiation is different from insolation in that it has a much longer wavelength and is known as terrestrial radiation or longwave radiation. As the atmosphere does not readily absorb direct solar radiation, it is mostly heated up indirectly by absorbing the longwave radiation that is emitted back into the atmosphere from the earth surface. This longwave radiation is more readily absorbed by the atmosphere than incoming insolation (iv) Re-radiation Re-radiation refers to emission of radiation after absorption of incoming radiation . Short wave radiation absorbed by Earth’s surface is re-radiated as longwave radiation back to the atmosphere. A portion of this longwave radiation that is absorbed by the atmosphere is re -radiated back to the earth’s surface. This is called counter-radiation.

Page 4 of 16 The amount of counter-radiation varies directly with the amount of cloud cover. The greater the cloud (and dust) cover, the greater the amount of counter -radiation. This would mean that more heat is trapped within the earth. Counter-radiation is an important process as it gives rise to the greenhouse effect. HOW IS THE EARTH KEPT WARM? –THE GREENHOUSE EFFECT The greenhouse effect (Fig. 3) refers to the warming of the Earth’s atmosphere due to outgoing longwave radiation being absorbed and trapped. The greenhouse effect occurs naturally on Earth due to the presence of clouds that re-radiate outgoing longwave radiation and greenhouse gases. Greenhouse gases are gases which absorb longwave radiation, leading to the greenhouse effect. Greenhouse gases include carbon dioxide, carbon monoxide, methane, nitrogen oxides and chloroflorocarbons (CFCs). The greenhouse effect should not be confused with the term global warming. Global warming occurs when there is an ‘enhanced’ greenhouse effect due to the build-up of greenhouse gases such as carbon dioxide and methane. (We will return to these in subsequent lectures) Fig. 3 - The Greenhouse and Enhanced Greenhouse effect

Page 5 of 16 HOW IS ENERGY TRANSFERRED BETWEEN THE EARTH-ATMOSPHERE SYSTEM? As mentioned earlier, energy is generally transferred via the processes of conduction, convection and radiation. Energy transfer between the Earth -Atmosphere system can occur via both horizontal and vertical heat transfer. The h eat energy from the earth’s surface can be transferred by the following mechanisms: (i) Terrestrial Longwave Radiation Terrestrial longwave radiation is radiation emitted upon heating of the Earth’s surface (via absorption of shortwave radiation). It is easily absorbed by the atmosphere providing a means in which the surplus heat on the earth surface c