Dictionary Definition

A long period of abnormally low rainfall, especially one that adversely affects growing or living conditions.

Causes

When an anticyclone (see below) lasts a long time, a drought occurs. The air in the anticyclones sinks and warms up to dissolve clouds, instead of rising and cooling to build rain clouds. When this happens, no rain can fall and everything dries up after a while. Fog can form, but no rain falls to replace the moisture that sinks into the ground or evaporates. This can have major effects on many plants and animals.

Droughts in the middle latitudes are special. These usually happen when a large area of high pressure gets stuck so that depressions bringing rain are sent to the north or south. The path of the depressions are blocked, and the area below the anticyclone is blocked from the rain, so the anticyclones are called "blocking" anticyclones.

Anticyclones

An anticyclone helps cause a drought. But what is an anticyclone? It is a region of high pressure in the atmosphere that winds blow around. In the northern hemisphere, the winds blow clockwise, and in the southern hemisphere they blow counterclockwise. Anticyclones usually bring good weather in mid-latitudes with hot, sunny summers and cold, cloudless winters. Look above at the causes to see how an anticyclone causes a drought.

Effects

Explanation

When there is a drought the rain doesn't fall, and therefore the moisture that sinks into the ground or evaporates into the air cannot be replaced. When this happens, the water dries up and short-rooted plants cannot survive. Grazing animals will then starve without their source of food. Also, without enough water, people, animals, and plants will get thirsty and have to conserve their water. Look at the experiment below to see what all water does when it is in contact with air. The difference when there is a drought is that no rain falls to replace this vaporization.

Experiment #1: All Dried Up

What You'll Need:

• two glass jars, one with a lid
• masking tape
• marking pen

Directions:

1. Put a strip of tape down the side of both jars from top to bottom.
2. Fill both jars half full with water.
3. Mark the top of the water level on each strip of tape.
4. Seal one jar with a lid and leave the other jar open.
5. Allow the jars to sit undisturbed for two weeks.
6. Look at the level of the water in each jar and mark the new level if it changed.

What's Happening?

Liquid water molecules on the surface change into vapor by absorbing energy from the surrounding air. In the open jar, water molecules on the surface vaporize and move up into the atmosphere. Each water molecule that vaporizes decreases the level of the water. Surface water also vaporized in the closed jar, but couldn't leave, so it condensed (changed back into liquid) as it hit the cool surface of the jar. Lakes dry up when the rising vapor isn't returned by rain.

Water Conservation

When there is a drought, it is very important to conserve water. Desert-inhabitants must always conserve their water, but other places who are used to a large supply of water must conform when a drought hits. Here are some tips to help save water. You can practice them even if you aren't having a drought.

• Use a low-flow showerhead and cut your showers down to five minutes.
• While waiting for water to get hot, catch the water in a bucket or watering can.
• Put a water displacement bag or plastic bottle in each toilet tank.
• Fix all leaks in toilets, faucets, and pipes.
• Flush toilets and use the garbage disposer only when necessary.
• Turn off the water while shaving, brushing your teeth, and lathering in the shower.
• Run only full loads in the dishwasher and washing machines.
• Water your lawn no more than once a week, and operate automatic sprinklers manually.
• Use a bucket of water and one short rinse to wash your car.
• Sweep the driveway, patio, and sidewalk instead of hosing it.

Targets

Droughts occur more often in some places than in others. Places that get a lot of rain, like the British Isles who have gotten some rain almost every month since 1855, don't have to worry about droughts. However, some countries get hardly any rainfall for years. India, China, and Africa are some common examples. The American Midwest had a drought in the 1930s that completely killed plants in the prairies. And California, as a desert, has had many droughts as well. In the mid-`970s, a drought hit Sahel, a strip of Africa just south of the Sahara Desert. Herds of cattle and thousands of people died of thirst and hunger. Then in the 1980s, Australia had the worst drought in many years and thousands of sheep died.