SAJC Cluster 2 Lecture 3 Notes (Rainfall Patterns in The Tropics)

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2_Cluster 2_Lecture 3_2024/pg1 Lecture 3 Tropical Climates (IIb): Rainfall Patterns in the Tropics KEY QUESTIONS: How does global atmospheric circulation affect rainfall in the tropics? How does synoptic scale circulation affect rainfall in the tropics? With the completion of this lecture, attached readings and tutorial, you should be able to understand the: • Influence of atmospheric circulation in the Hadley cells on rainfall patterns in the tropics • Influence of seasonal migration of the Hadley cells on rainfall patterns in the tropics • Influence of distribution of heat over land and sea on summer and winter monsoon circulations in humid tropical Asia, and the resultant effects on rainfall patterns Lecture Outline 3.1 Introduction: Rainfall and the Tropics 3.2 Some basics to Understanding Rain Formation 3.2.1 Atmospheric Stability and Instability 3.2.2 Condensation, Clouds and Rain Box 1: Latent Heat 3.3 Influence of Hadley cells on Rainfall Patterns in the Tropics 3.3.1 Influence of Hadley cells on rainfall patterns of Af climates 3.3.2 Influence of Hadley cells on rainfall patterns of BWh and BSh climates 3.3.3 Influence of seasonal migration of Hadley cells on rainfall patterns of Am/Aw climates 3.4 Influence of distribution of heat over land and sea in humid tropical climates 3.5 Summary All these photographs are of places in the tropics. Why are some places in the tropics so wet all year round ( top left ), some dry all year round ( right), and others seasonally wet and dry (bottom left)?

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2_Cluster 2_Lecture 3_2024/pg2 3.1 Introduction: Rainfall and the Tropics • Recall from Lect 1 that the Tropics may be described as… A climatic region of solar radiation surplus delimited by boundaries fluctuating between 30 ˚ and 35 ˚ north and south latitudes. Common to all areas in this region (at sea level) is high temperature. Significant variations in the amount and pattern of rainfall differentiate these areas into different tropical climate zones, primarily the humid tropics and the arid tropics. • While mean annual temperature in the Tropics is generally high everywhere (as we saw in Lect 1 and 2), the definition provided above reminds us that rainfall in the Tropics is not as simple to explain. Indeed, as we shall see in thus lecture , the Tropics can be further categorised into different climatic regions, by virtue of how much rainfall they receive and how that varies over time and space. • In this lecture, we first focus on the basic conditions for rainfall to occur so that we can understand the role of Hadley Cells in influencing rainfall in this part of the world. For this, we must begin with the concepts of atmospheric stability and instability. 3.2 Some Basics to Understanding Rain Formation This section explains concepts that will not be explicitly assessed, but an understanding of it is assumed when learning content for this lecture as well as subsequent lectures. 3.2.1 Atmospheric Stability and Instability • An air parcel is defin ed as a quantity of air in which there is no exchanges of mass and energy between its interior and exterior ; that is, there is no (or negli gible) mixing of parcel air with surrounding air. It is an object with imagined boundaries , invented to aid understanding of atmospheric processes. • Suppose the air parcel is a physical object, and the surrounding air is fluid. Just as an object will rise or sink as a result of the density differences between itself and the fluid, this air parcel will also seek its own level. This means that a parcel of air moves vertically until it reaches a level at which the surrounding air is of equal density. • If an air parcel were to rise away from the Earth’s surface, it would expand in volume (since atmospheric pressure acting on the parcel would drop), and in the process, reduces its density and also its temperature (that is, it cools). • The important observation to make here is how different the temperature and density of this air parcel are compared to its surrounding air. (See Fig. 1) o If the parcel of air is cooler, and therefore denser than the surrounding air, it tends to either sink or at least to resist uplift. This parcel of air is said to be stable. The sinking movement of air creates high pressure areas, hence high press ure areas are associated with stable conditions. (See Section 3.3.2 later) o If the parcel of air is warmer, and thus less dense than the surrounding air, it tends to rise on its own. Thus, warm air is said to be unstable. With the upward movement of air, low pressure areas are created, hence low pressure areas are also associated with unstable conditions. (See Section 3.3.1 later)

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2_Cluster 2_Lecture 3_2024/pg3 o In summary, stability is a property of air that describes its tendency to resist uplift (stable), or to rise (unstable). Fig. 1 Assume that ELR = 6°C per 1000 m (left side) and the air parcel cools at 10°C per 1000 m (right side). Stability occurs when a rising parcel of air cools more rapidly than the air surrounding it. As it rises, it becomes colder and denser than its surroundings and therefore will sink back to its starting point. Instability occurs when a rising air parcel cools more slowly than the atmosphere and thus remains warmer and lighter 3.2.2 Condensation, Clouds and Rain • When a parcel of air is heated enough so that it is warmer than the surrounding air, i t becomes unstable (see Fig. 1). This is a typical condition on a warm summer afternoon. The air parcel rises until it reaches an altitude where the surrounding air has similar temperature and density. While ascending, it will be cooled. • For an ascending air parcel that contains water vapour, it will reach a height where it will be cooled to its dew point temperature, when saturation is reached and this water vapour will begin to condense (that is, water vapour is converted to liquid water). This height in the atmosphere is known as the condensation level. • During condensation, latent heat (see Box 1) is released. The parcel of air becomes warmer than the surrounding air. This enhances atmospheric instability and causes the air parcel to rise further on its own, which in turn leads to further cooling and condensation. • The collection of minute droplets of water is called clouds. The condensation level is therefore also the cloud base – the point at which the rising parcels of air become visible as clouds. • Not all clouds give rain, but all rain comes from clouds. In other words, if there are no clouds, it would not be possible to have rain. Box 1: Latent Heat  Even as heat is added, the temperature of water does not increase while it is undergoing a phase change. The energy exchanged during a phase change is called latent heat.  When water vapour condenses back to liquid water, the highly agitated water vapour molecules must give up some of their internal structural energy in order to revert to a less agitated liquid state. The energy released during condensation is called the latent heat of condensation.  Because latent heat must be released during condensation, condensation is, in effect, a warming process. Water vapour represents a “reservoir” of heat – whenever and wherever condensation takes place, this heat is added back to the atmosphere. The release of latent heat during condensation f