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 (greenhouse effect; see Fig. 1). This increases the temperature of the atmosphere. o The atmosphere gets heated by a number of processes (see Fig. 2), before the energy returns to space as output. The difference between the input and output of energy is referred to as the energy balance (see Section 2.2.2). 1. Much of the incoming, short wavelength solar radiation penetrates the atmosphere and heats Earth’s surface. 3. Greenhouse gases absorb outgoing, long wavelength, radiation and re-radiate some of this energy Earthward, thus trapping heat in the lower atmosphere. 2. Objects on Earth’s surface emit long wavelength radiation skyward. EARTH’S SURFACE ATMOSPHERE Fig. 1 The (natural) greenhouse effect [Type a quote from the document

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2_Cluster 2_Lecture 2_2024/pg3 2.2.2 Surplus heat in the Tropics • The distribution of heat energy is uneven across the Earth. o Fig. 3 shows the amount of radiation reaching and lost at the different latitudes. o It suggests that there is a radiation surplus in the tropics (0° to 30°N and S), and this decreases towards the higher latitudes (30° to 90°N and S), resulting in a radiation deficit (or shortage) at and near the poles. Fig. 3: The amount of radiation reaching and lost at different latitudes o Average annual temperature is high everywhere within the tropics. Here, the region delimited by boundaries fluctuating about 30-35° N/S receives higher insolation and thus gives out more heat as terrestrial radiation and thus experiences higher temperature than say, the polar region (again, see Fig. 3). The variation can be attributed to the angle of incidence (see Section 2.2.3). 2.2.3 Effect of Angle of Incidence on Surplus Heat and Temperature in the Tropics • The angle at which rays from the Sun strike Earth is called the angle of incidence. Low latitudes experiences high angle of incidence where insolation falls vertically (90°) on the earth. • The higher the angle of incidence, the more concentrated the solar radiation that reaches the surface which results in higher terrestrial radiation which heats the atmosphere more causing high average annual temperature at the tropics. Thus, there is surplus heat at the low latitudes. • However, places in the high latitudes experience lower angle of incidence, the insolation that reaches the surface is more spread out and less intense. Thus, higher latitudes experience lower i.e. insolation [Type a quote from the document or the summary of an interestin g point. You can position the text box anywhere in the document . Use the Drawing Tools tab to change the formattin g of the pull quote text box.] i.e. terrestial radiation [Type a quote from the document or the summary of an interesting point. You can position the text box anywhere in the document. Use the Drawing Tools tab to change the formatting of the pull quote text box.] Fig. 2 The energy balance [Type a quote from the document or the summary of an interesting point. You can position the text box anywhere in the document. Use the Drawing Tools tab to change the formatting of the pull quote text box.] NP 60N 30N EQ 30S 60S SP [Type a quote from the document or the summary of an interesting point. You can position the text box anywhere in the document. Use the Drawing Tools tab to change the formatting of the pull quote text box.]

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2_Cluster 2_Lecture 2_2024/pg4 average annual temperature and are cooler as the oblique rays of the sun spread over a larger area and heat gets distributed. Thus, there is deficit heat in the high latitudes. • There is therefore a need for the redistribution of energy from areas of surplus to areas of deficit. The details of which will be examined in Section 2.3. 2.3 Atmospheric Circulation in the Tropics: The Hadley Cells • From Section 2.2.3, we can deduce that if there were no mechanisms of moving heat poleward in both hemispheres, the equatorial region would become progressively warmer, and the polar region would become progressively colder. • Such temperature trends do not occur because there is a persistent shifting of warmth toward the high latitudes and the consequent cooling of the low latitudes , thereby making both those latitude zones more habitable than they would otherwise be. One such means is through atmospheric circulation (the other is oceanic circulation, which is not our syllabus’ emphasis). • Over the major parts of the Earth's surface