TMJC 2025 JC1H2 C2 Lecture 3 Factors Affecting TC Precipitation

Higher 2 ((9173) Tropical Environments TMJC C2Lecture 3 Factors affecting Tropical Climates’ Precipitation 2025 Page 1 of 17 LECTURE OBJECTIVES 1. Explain how the Hadley Cell affect tropical climates through the Convergence of Trade Winds at the Intertropical Convergence Zone (ITCZ), and the resultant effects on atmospheric stability and rain formation 2. Explain why seasonal variations in wind direction occur through understanding of Asian and African Monsoons 3. Explain how weakening of the Walker Circulation and the El Nino Southern Oscillation (ENSO) phenomenon affect Tropical Climates INTRODUCTION rgy from the sun in the form of Atmospheric stability refers to the tendency of an air mass to either remain in place or change its vertical position by ascending or descending. An air mass is stable and will sink if it is cooler (and hence denser) than the surrounding air. Hence, a stable air mass resists replacement upwards or when disturbed, tends to return to its original position. An air mass is unstable and will rise if it is warmer (and hence less dense) than the surrounding air. An unstable air mass has the tendency to continue rising until it reaches an altitude where the surrounding air has a density similar to its own.

Page 2 of 17 (A) HOW DOES RAINFALL OCCUR? 1. Rainfall formation starts when an air mass containing moisture undergoes uplift. 2. The uplift of the air mass can occur in various ways such as: a) convectional uplift, b) orographic uplift, or c) convergent uplift. 3. The rising air mass will dynamically expand due to a decrease in pressure of surrounding air and result in adiabatic cooling1 of the air mass (Fig. 1). Fig. 1: Dynamic expansion / contraction of air mass causing adiabatic cooling / heating 4. When the air mass containing water vapour is cooled to its dew point 2 , it become s saturated3 (relative humidity4 of 100%). 5. Further cooling below the dew point causes the water vapour to condens e and form small water droplets which become clouds. However, condensation can only take place if there are particles such as dust, pollen, smoke and ash in the air to act as nuclei for condensation. Thus, in order for clouds to form, ( i) air must be cooled below its dew point , and (ii) cloud condensation nuclei must be present. 6. Water droplets in clouds coalesce over time to become larger. When the larger water droplets become too heavy to remain suspended by the rising air, they fall down from the atmosphere in the form of precipitation such as rain. 1 Adiabatic cooling refers to the reduction in temperature without the addition of heat energy. 2 The temperature at which air is saturated with water vapour. 3 The air is holding the maximum amount of moisture it can hold. The maximum amount of moisture air can hold increases with its temperature. 4 The amount of moisture contained in the air mass compared to the maximum amount of moisture that the air mass can hold, measured in terms of percentage.

Page 3 of 17 (B) WHAT ARE THE DIFFERENT TYPES OF RAINFALL? The types of rainfall commonly found in the tropics include: a. Convectional rainfall (uplift) b. Orographic (relief) rainfall (uplift) c. Convergent rainfall (uplift) Convectional rainfall (Fig. 2) occurs when the air layer near the ground or sea surface is heated by conduction and long wavelength radiation. (Temperature driven) Fig. 2: Formation of Convectional Rainfall a. Air in contact wi th the ground warms and expands (Thermal expansion) . Consequently, being lighter than the surrounding air, it becomes unstable and rises. b. The rising air mass will expand dynamically due to the decrease in surrounding air pressure with increasing altitude. As a result, adiabatic cooling occurs. c. When the air mass containing water vapour is cooled to its dew point, the air mass becomes saturated, and its relative humidity reaches 100%. Cooling below dew point causes the water vapour to condense and form small water droplets on condensation nuclei such as dust, pollen and ash5. d. Water droplets in clouds coalesce over time to become larger. When the larger water droplets become too heavy to remain suspended by the rising air, they fall down from the atmosphere in the form of precipitation such as rain. e. Convectional rain usually occurs in the early afternoon when the surface temperatures are high due to intense insolation present. It brings short -lived 5 Tall and towering clouds such as cumulonimbus clouds may form due to the rapid uprising of moisture - laden air, forming raindrops of considerable sizes.

Page 4 of 17 showers which last for minutes. Convectional rainfall thus commonly occurs in warmed or heated areas such as tropical lands and oceans. f. It is less frequent but can also occur in tropical desert regions and the interior of continents in the summer. 2. Orographic or relief rainfall (Fig. 3) occurs when moist air is forced to undergo uplift when meeting mountain barrier. This is known as orographic uplift. Fig. 3: Formation of Orographic/Relief Rainfall a. The rising air mass will expand dynamically due to a decrease in pressure of surrounding air with increasing altitude. As a result, adiabatic cooling occurs. b. When the moist air mass is cooled to its dew point, the air mass becomes saturated, and its relative humidity reaches 100%. c. Further cooling below the dew point causes the water vapour , in the presence of condensation nuclei, to condense and form small water droplets. d. When the larger water droplets become too heavy to remain suspended in the air, they fall down from the atmosphere as orographic rainfall. The side of the mountain on which rainfall occurs on is known as the windward slope. e. If the mountain is very high, air currents which are now dry continue to rise because they are pushed upward by continual air movement from below. After crossing the mountain, the descending dry air is compressed and becomes warmed adiabatically, making condensation and precipitation less likely to occur. This side of the mountain is known as the leeward slope. f. The descending air forms a warm dry wind called föhn wind or chinook wind, and brings very little moisture. The leeward side of mountain forms a rain shadow area, experiencing very little rainfall. This may result in the formation of a rain shadow desert.

Page 5 of 17 3. Convergent rainfall (Fig. 4) occurs when surface winds converge and air masses are forced to rise as a result. For example, this occurs at the Inter -tropical Convergence Zone (ITCZ) when trade winds from the northern and southern hemisphere converge together. a. As the surface winds converge, the respective air masses are forced to undergo uplift. The rising air mass will dynamically expand due to a decrease in pressure of surrounding air with increasing altitude. As a result, adiabatic cooling occurs. b. When the air mass containing water vapour is cooled to its dew point, the air mass becomes saturated, and its relative humidity reaches 100%. c. Further cooling below the dew point causes the water vapour, in the presence of condensation nuclei, to condense and form small water droplets. d. When the larger water droplets become too heavy to remain suspended in the air, they fall down from the atmosphere and are known as convergent rainfall. Fig. 4: Formation of Convergent Rainfall

Page 6 of 17 (C) WHAT ARE THE FACTORS INFLUENCING THE RAINFALL PATTERNS OF CLIMATE ZONES IN THE TROPICS? The various factors influencing rainfall patterns of climate zones in the tropics include: Factor How it affects rainfall Presence and Migration of Pressure Belts Total rainfall, seasonality of rainfall Offshore Oceanic current Total rainfall Monsoon Winds Total rainfall, seasonality of rainfal