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Measuring Raindrops

based on 8 ratings
Author: Tricia Edgar

When it’s raining hard, those raindrops look like they’re bouncing up and down on the sidewalk. What are they really doing, and how does the size and height each drop make it splash in different ways?

Problem: Observe and measure raindrops.

Materials

  • Cooking spray
  • Metal baking sheet
  • Water
  • Cooking spray
  • Freezer
  • Three identical eyedroppers
  • Ruler
  • Meter stick
  • Paper towel
  • Graduated cylinder
  • Heavy duty scissors
  • Notebook
  • Pencil

Procedure

  1. First, you will make a droplet catcher. Take your metal baking sheet and spray it lightly with cooking oil. Use your paper towel to wipe up any extra oil that pools on the surface, but make sure that the surface remains oily. Put the pan in the freezer. 
  2. Measure the diameter of the eyedropper hole with a ruler.
  3. Cut the plastic off one of the eyedroppers slightly above the existing hole so that you get a larger hole, at least ¼ inch in diameter.
  4. Cut the tube of one of the other eyedroppers so that you get a hole almost ½ inch in diameter.
  5. How much water is in a drop from each dropper? Fill up your droppers with water and then add drops to a graduated cylinder. When the water reaches 1 mL, stop adding water. How many drops did that take? Divide the number of drops by 1 mL to get the volume of each individual drop. Do this for all three eyedroppers and write it down in your notebook.
  6. After half an hour, remove your baking sheet from the fridge. It should be very cold and feel a little sticky to the touch.
  7. Have a friend hold a meter stick for you. Place the pan at the bottom of the stick. Drop one drop from each dropper onto different parts of the pan. Do this from the same height. The drops will freeze slightly on the pan, so it should be easy to see how they splashed. Before they thaw, measure the size of the splashes and record it in your notebook
  8. Now, fill up a single eyedropper again and send out one drop from ¼ meter, ½ meter, and 1 meter in height. Again, take a look at the splash patterns. Are they different? The same?
  9. Make a graph of your findings. Make one graph for the eyedroppers with different sizes of drops. On the bottom, write "Size of Eyedropper Opening" and on the side write "Diameter of Splash." You can experiment with even more different sizes of eyedroppers and plot them on the graph as well.
  10. Do the same sort of graph for the different heights. On the bottom of this graph, write "Height of Drop." On the side, write "Diameter of Splash." 

Results

Larger drops and drops that come from greater height make larger splashes.

Why?

How does a raindrop become a raindrop? Rain happens when water coalesces or comes together around a tiny speck of dust or dirt. This speck is so small that you can’t see it.

Warm air rises, and as it rises, it cools. As air cools down, water that is in the air cools down to what’s called its saturation point. It turns into liquid water and collects around these specks, forming drops. Together, all of these water drops make clouds. When there are enough droplets of water together and gravity begins to pull them down, you get rain.

Some raindrops are large and others are small. Some fall from a great height, while others fall onto the ground from shrubs. Studying splashes can be interesting to scientists in many fields, but it’s particularly useful in archaeology. Ancient raindrop patterns fossilized in rocks can tell us a lot about an area’s weather patterns.

When you tried dropping different amounts of water from the same height, the larger amounts of water made a bigger splash. This is because there is more material in the water droplet. It’s like dropping a cherry tomato and a large tomato on the ground: if they’re the same level of squishiness, the large tomato will make a larger splat.

Why do some droplets make a bigger splash than others when you drop them from a height? This is related to their velocity. When you drop the same amount of water from double the height, it has a longer time to speed up before it hits the ground. When an object is hanging in the air, it has potential energy. When you let go, that object has kinetic energy, the energy of a moving object. The higher the height it drops from, the greater the potential and kinetic energy it has. This means that the velocity when it hits the ground is higher, so the water makes a bigger splash. Think about the tomatoes again. What would happen if you dropped one tomato onto the ground from a foot away and one from the top window of your house? Which one would make a bigger splat?

Measure your water droplets. Can you figure out how much more velocity a droplet has when it’s dropped from one foot versus two or three feet? How might you be able to calculate this?  

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