1. Virtual Academy for the Semi Arid Tropics Course on Coping with Drought About Drought After completing this Lesson you have learned to answer: 1.List the causes for drought? 2.What causes weather patterns globally? 3.How clouds are formed? 4.How winds move? 5.What causes for the rise and fall of winds? 6.Which type of continued air pressure results in drought? Lesson 3: What causes Drought ?

2. Virtual Academy for the Semi Arid Tropics Course on Coping with Drought About Drought After completing this Lesson you have learned to answer: 7.Which type of air pressure results in rain (condensation of water vapor)? 8.What are the features of winds that bring rain? 9.How monsoon rains are received in India? 10.How mountains influence rainfall or no rains? Lesson 3: What causes Drought ?

3. Virtual Academy for the Semi Arid Tropics Course on Coping with Drought About Drought Drought occurs due to many causes. It could be a lack of rainfall; It could be a lack of snowfall from mountains far away (not in India but in colder regions of the world); or It could be caused when water supplies are not sufficient to meet everybody's needs. To understand why we do not always get the rainfall we need or expect, we first need to understand weather and climate. Lesson 3: What causes Drought ?

4. Virtual Academy for the Semi Arid Tropics Course on Coping with Drought About Drought Winds cause weather patterns to move around the globe, including the clouds that bring rain. Over the years, these patterns become routine, creating what we know as our climate. But sometimes these patterns change, and we get less (or more) rainfall than we are expecting. In general, clouds form when the wind forces air to move upward; clouds disappear when the wind forces air to move downward. Lesson 3: What causes Drought ?

5. Virtual Academy for the Semi Arid Tropics Course on Coping with Drought About Drought Differential heating between land and ocean and the resultant movement of winds. Ref.: http://ww2010.atmos.uiuc.edu/(Gh)/guid es/mtr/fw/sea/htg.rxml Lesson 3: What causes Drought ?

6. Virtual Academy for the Semi Arid Tropics Course on Coping with Drought About Drought Water vapor condenses only if air rises into the colder regions of the atmosphere. If the air does not rise, then no rain will form. When there is high air pressure, air falls instead of rising. With the air pressing down in a high pressure zone, no currents of water vapor are carried upward. As a result, no condensation occurs, and little rain falls to earth. In addition, high-pressure areas push clouds and air currents downward and away, resulting in sunny, cloudless weather. Low-pressure systems see more cloudy, stormy weather. Lesson 3: What causes Drought ?

7. Virtual Academy for the Semi Arid Tropics Course on Coping with Drought About Drought Usually, we experience both high- and low-pressure systems. It is normal for a high-pressure system to pass over an area and move on, being replaced by a low- pressure system. However, when a high-pressure system is stalled, the sunny weather can drag on for days. If it keeps on going, the result is a drought. High-pressure systems can be stalled by jet streams, wide bands of fast-moving air (up to 335 miles per hour), in the upper atmosphere. Masses of air that usually move from place to place can be locked in one area by jet streams. Lesson 3: What causes Drought ?

8. Virtual Academy for the Semi Arid Tropics Course on Coping with Drought About Drought Also, droughts occur because water vapor is not brought by air currents to the right areas at the right times. Water that evaporates from the oceans is brought inland by wind to regions where it is needed. However, sometimes those winds are not strong enough. This moisture is then pushed by other winds and this water then falls to the ground, supporting the farms in that region. Thus, if the winds do not blow at the right time, in the right direction, or with enough force, the moisture falls in other areas. Lesson 3: What causes Drought ?

9. Virtual Academy for the Semi Arid Tropics Course on Coping with Drought About Drought In India and other parts of south east Asia, summer winds known as monsoons carry water vapor north from the Indian Ocean inland, providing desperately needed rain (Fig.). Sometimes, however, instead of blowing from south to north, they blow east to west. When that happens, the water vapor does not leave the Indian Ocean and many people suffer from the resulting droughts. Lesson 3: What causes Drought ?

10. Virtual Academy for the Semi Arid Tropics Course on Coping with Drought About Drought Mountains can prevent wind from blowing moisture to needed regions. As air is moving past a mountain range, it is forced to rise in order to pass over the peaks. However, as the air rises, it becomes colder and the vapor condenses into rain or snow. The rain then falls on that side of the mountain, known as the windward side (the side that is turned toward the wind). Lesson 3: What causes Drought ?

11. Virtual Academy for the Semi Arid Tropics Course on Coping with Drought About Drought When the air mass finally makes it over the mountain, it has lost much of its vapor. This is another reason why many deserts are found on the side of a mountain facing away from the ocean. This phenomenon is known as the rain shadow effect. Lesson 3: What causes Drought ?

12. Virtual Academy for the Semi Arid Tropics Course on Coping with Drought About Drought Thus, drought is a different kind of extreme weather event. It starts and ends slowly. It is hard to tell when a drought begins and ends, but by watching various “indicators” of drought, we can keep track of droughts. Because droughts differ from one area to another, the indicators that we measure will depend on the area we are monitoring. Lesson 3: What causes Drought ?

13. Virtual Academy for the Semi Arid Tropics Course on Coping with Drought About Drought We also need a way to make sense of the measurements we collect. For example, rainfall measurements by themselves won't tell us if we are in a drought unless we can compare them to other measurements, particularly the area's average rainfall. For these reasons, scientists have created many tools and models to help people in different parts of the world determine whether they are in a drought, and how severe it is. Lesson 3: What causes Drought ?

14. Virtual Academy for the Semi Arid Tropics Course on Coping with Drought About Drought Many of these tools use some sort of process or formulae to turn a wide variety of data, such as stream flow, soil moisture, temperature, rainfall etc. into a single number, known as an index. Scientists and drought planners use such kinds of indices to categorize drought occurrence. Lesson 3: What causes Drought ?

15. Virtual Academy for the Semi Arid Tropics Course on Coping with Drought About Drought This concludes the Lesson 3 on “What causes Drought?” and also this Module I “About Drought”. If you would like to check your understanding about the causes of drought, click “Exercise” button. The next Module covers the subject on the impacts of drought. Click “Modules” button to select Module II About Drought Exercise Virtual Academy for the Semi Arid Tropics Course on Coping with Drought About Drought Modules Lessons