Rainy: How Do Raindrops Begin?
How Do Raindrops Begin?
- tap water
- sheet of black construction paper
- lid from a quart (liter) jar
- petroleum jelly
- table salt
- desk lamp
- magnifying lens
- 2-liter soda bottle with cap
- adult helper
- Place the saucer on a table. Pour enough water into the saucer to cover the bottom.
- Cut a circle of black paper large enough to cover the inside of the lid. Place the circle in the lid.
- Cover the paper with a thin layer of petroleum jelly. This will prevent the paper from absorbing water.
- Carefully sprinkle salt grains on the paper in the lid. The grains should be kept separate from each other.
- Hold the lid near a desk lamp. Use a magnifying lens to observe the salt grains.
- Set the lid in the saucer of water.
- Ask an adult to cut the bottom from the plastic soda bottle.
- Secure the cap on the bottle and stand it in the saucer over the lid.
- Lift the bottle and observe the salt grains through the magnifying lens every 30 minutes for 3 hours. Observe the grains again after 24 hours.
The grains are dry, white, and mostly cube-shaped before they are placed in the saucer of water. After 30 minutes in the saucer, the surface of the grains appears moist. As time passes, the grains look wetter, less white, and more transparent. Finally, no salt grains are present; only drops of liquid remain in the lid.
Rain droplets begin when water vapor condenses on tiny particles in the atmosphere, such as dust. These particles are called condensation nuclei. When such drops grow to a diameter greater than 0.02 inches (0.05 cm), they often fall as rain. Rain is a form of precipitation (liquid or solid water particles that form in the atmosphere and then fall to the earth's surface). Nearly all precipitation begins as water vapor that condenses around small particles in the air.
This experiment demonstrates the condensation of water vapor on salt, a condensation nuclei that dissolves in the water. The liquid water in the saucer evaporated, as does water from the earth's surface. The water vapor inside the bottle was attracted to the salt grains because salt is hygroscopic (able to absorb water from the air). The water vapor condenses on the salt grains and the grains dissolve in the water, forming drops of salt water.
Would drier air above the salt grains affect the results? Repeat the experiment placing the lid in a saucer without water. The result gives a clue to the formation of raindrops in areas of low humidity.
- In the atmosphere, the condensation of water on small particles produces cloud drops (drops of water forming clouds with diameters between 0.00004 and 0.002 inches [0.0001 and 0.005 em]) that are so small they remain suspended. The drops must grow before they become heavy enough to fall as raindrops. If the temperature in the cloud is above the freezing point of water (32 degrees Fahrenheit or 0 degrees Celsius), the growth of the drops occurs by accretion, which is the merging of water drops that bump into each other. Demonstrate how small water drops combine to form larger drops by covering a 4-inch (10-cm) square of cardboard with wax paper. The smooth side of the wax paper should be facing out. Secure the paper with tape and add small drops of water. Use a toothpick to move the drops around so that they bounce into each other, and observe the results.
- How big do the drops have to be before they fall? Prepare the drops as in the previous experiment. Quickly turn the cardboard upside down. Use the toothpick to move the small drops together until they become large enough to fall to the ground. While typical raindrops have a diameter of about 0.08 inches (0.2 cm), the falling drops from the wax paper will be larger because their weight must be great enough to overcome the attraction the water molecules have for the wax paper. Raindrops must be heavy enough to overcome the upward lift of air. Use photographs taken during this experiment to represent the results.
- Make a poster comparing the size of cloud drops and raindrops. Show drawings of cloud drops and raindrops under a magnifying lens. label the diameters of each.
- How does an updraft (upward-moving air) affect raindrops? Inflate a 9-inch (23-cm) round balloon. Adjust a fan so that it blows upward, and turn it to high speed. Place the balloon over the fan. The force of the moving air holds the balloon in the air just as strong updrafts can support raindrops.
Check it Out!
Cloud seeding encourages tiny cloud drops to grow. Find out more about this method of modifying the weather. Information about cloud seeding, terminal velocity, and other cloud facts can be found on pages 72–78 of The Weather Book, by Jack Williams (New York: Vintage Books, 1992).
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