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 co