Droughts occur when a long period of abnormally dry weather leads to a severe water shortage. Extreme weather conditions are not the only cause of droughts, however - they are often caused by the activity of man as well. Human activities which can help trigger droughts include:
Constructing a dam on a large river may help provide electricity and water to irrigate farmland near the reservoir. But it may also cause drought downstream by severely reducing the flow of water.
Widespread cutting down of trees for fuel reduces the soil's ability to hold water, - drying out the ground, triggering desertification , and leading to drought.
Hurricanes need a lot of heat to form, which is why they usually occur over tropical seas. It works like this:
Rising warm air rises fast (by warm seas over 27 degrees), causing towering clouds, heavy rainfall, and intense low pressure.
The low pressure sucks in air, causing very strong winds which spiral - clockwise in the northern hemisphere (anti in the Southern) - around the centre of the low, at speeds of around 120 km/h (75 mph).
Seen from above hurricanes are huge circular bodies of thick cloud around 450 km (300miles) wide. The cloud brings heavy rain, thunder and lightning.
In the centre is the eye of the hurricane , about 45 km across (30miles) across. Often there will be no cloud in the eye. Seen from below it will seem calmer, with a circle of blue sky above. The eye is formed because this is the only part of the hurricane where air is sinking.
In the northern hemisphere, the prevailing easterly tropical winds tend to steer hurricanes toward land - although their course is unpredictable. As they move inshore their power gradually reduces, because their energy comes from sucking up moist sea air