1. Monsoons of Asia Physical Processes

2. Objective  By the time you finish this lesson, you should be able to describe the physical processes which cause monsoons.  We will look at the effects of the monsoons in later lessons – this lesson is just the processes which cause them  This information works for South Asia (where we are now) and for East Asia and Southeast Asia (where we are going)

3. Vocabulary  Monsoon – has two meanings  Scientific (meteorology and geography): seasonal winds; winds that blow from particular directions at different times of the year  Colloquial (as people use it): “heavy rains” (as in “it’s a real monsoon out there”)  Most people associate monsoons with Asia, but they happen in other parts of the world, too (like El Paso)

4. Theories  What you are going to learn is a combination of two theories of the monsoon mechanism, called the “traditional theory” and the “thermal theory”  These two theories focus on the temperature differences between land and ocean  There are other theories that also try to explain the monsoons  Dynamic Theory – involves the annual movement of “belts” of winds around the planet  Jet Stream Theory – involves the formation and direction of streams of air in the upper atmosphere  All of the theories explain pieces of the process, but none explains all of it  The short version is we know that monsoons happen, but do not completely understand why they happen the way that they do

5. Basic Concept 1 (Physical Science)  Land and water absorb heat at different rates  Land and water also release heat at different rates  This is why desert areas can get very hot in the daytime and very cool at night  Land absorbs heat much more quickly than water does, but it also releases heat much more quickly  This is also why areas with lots of water tend to have smaller temperature changes over a day than areas with little water  Water is much slower to heat, but it also releases its heat much more slowly

6. Basic Concept 2 (Physical Science)  Warm air is less dense than cool air, so it will rise

7. Basic Concept 3  Cool air is more dense than warm air, so it will sink

8. Basic Concept 4 (Meteorology)  Rising air creates a low pressure area  Sinking air creates a high pressure area

9. Big Idea 1  Temperature differences cause pressure differences and pressure differences cause winds  CAUTION: “warm” and “cold” are not absolute numbers – they are in comparison to the surrounding air, so “warmer” and “colder” are appropriate terms rather than x-degrees  Trivia: in the northern hemisphere, winds around a low-pressure area move counter-clockwise, but move clockwise in the southern hemisphere  It’s opposite for high-pressure areas: clockwise in the northern hemisphere and counter-clockwise in the southern hemisphere

11. Tropic of Cancer Equator Tropical Zone Temperate Zone

12. From the fall equinox (September) until the spring equinox (March) the sun is shining most directly on the southern hemisphere, so it is fall/winter in the northern hemisphere. The ocean is warmer than the land so there is low pressure over the ocean and high pressure over the land. The winds move from high pressure to low pressure. This is the “dry” season in South Asia and there is very little rain in most of the region. H L

13. From the spring equinox (March) until the fall equinox (September) the sun is shining most directly on the northern hemisphere, so it is spring/summer in the northern hemisphere. The land is warmer than the ocean so there is low pressure over the land and high pressure over the ocean. The winds move from high pressure to low pressure. This is the “wet” season in South Asia and there is rain over most of the region. The Himalaya Mountains keep the rains pretty much confined to the Indian subcontinent. H L

14. Using what we know of the geography of South Asia, let’s try to explain what is happening here. Why is there such a big difference in the amount of rainfall between the western (1.5 to 2.5 meters) and eastern (0.2 to 0.6 meters) parts of the area?

15. How much rain do you think Nepal gets? How much rain do you think Bhutan gets? How much rain do you think Bangladesh gets?