SAJC Cluster 2 Lecture 2 Notes (Hadley Cells)

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2_Cluster 2_Lecture 2_2024/pg1 Lecture 2 Tropical Climates (IIa): Hadley Cells KEY QUESTION: How does global atmospheric circulation affect rainfall in the tropics? With the completion of this lecture, attached readings and tutorial, you should be able to understand the: • Effect of high angle of incidence on surplus heat and temperature in the tropics • Role of surplus heat in driving atmospheric circulation in the Hadley Cells Lecture Outline 2.1 Introduction 2.2 Surplus Heat and High Temperature in the Tropics 2.2.1 How the Earth is heated up 2.2.2 Surplus heat in the Tropics 2.2.3 Effect of Angle of Incidence on Surplus Heat and Temperature in the Tropics 2.3 Atmospheric Circulation in the Tropics: The Hadley Cells 2.3.1 Equatorial Low and Subtropical High 2.3.2 Trade Winds and Inter-Tropical Convergence Zone (ITCZ) Box 1: Some basics – Atmospheric Pressure and Wind This is a map with data calculated by averaging daily temperatures for the years 1961–1990. There are clearly spatial variations in temperature. Why are the tropics warmer than other regions?

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2_Cluster 2_Lecture 2_2024/pg2 2.1 Introduction • In our previous lecture we saw that average annual temperature is high everywhere in the tropics, but rainfall amount and pattern vary. To understand why the rainfall characteristics are so, we have to understand the concepts of surplus heat in the tropics and formation of atmospheric circulation at the global (see this lecture) and synoptic (see Lect 3) scales. • In this lecture, we focus our attention on the formation of a global atmospheric circulation called the Hadley cells, and its role in influencing rainfall patterns in the tropics . But, before we can do so, we must become familiar with some basic processes which are responsible for surplus heat in the tropics (see Section 2.2) and how such heating results in the formation of the Hadley cells (see Section 2.3). 2.2 Surplus Heat and High Temperatures in the Tropics 2.2.1 How the Earth is heated up • The sun is the only source of energy on earth . Atmospheric temperature is determined by the balance between insolation received by the earth and terrestrial radiation released by the earth into the atmosphere. o The term “insolation” refers to incoming solar radiation . This input of shortwave solar radiation heats up only the earth’s surface. o The earth’s surface releases the heat in the form of longwave terrestrial radiation. Terrestrial radiation heats up the atmosphere as greenhouse gases like water vapour, carbon dioxide, methane, nitrous oxides absorb the heat (greenhous