Learning Lesson: Atmospheric Collisions


Collision and coalescence are the processes by which cloud droplets grow large enough to fall as rain in clouds. You will suspend two ping pong balls in the stream of air supplied by a hair dryer. The balls will bump into with each other, creating a clicking sound signifying the collision.

TOTAL TIME 2 minutes
SUPPLIES Hand held hair dryer; at least two ping pong balls.
It is better to have several ping pong balls on hand as occasionally they will fall out of the air stream.
SAFETY FOCUS Flood/Flash Flood Safety


  1. Point the nozzle of the hair drier up and turn the fan to full power. (You do not need any heat.)
  2. Place one ping pong ball into the stream of air.
  3. Carefully place the second ping pong ball in the air stream.
  4. Both balls will be suspended by the air and will occasionally collide with each other making a clicking sound.


Had the balls been raindrops, every time they collided, they would have joined each other, making a larger drop of water.

In rising (cooling) air, water vapor begins to condense on cloud condensation nuclei when the air has cooled to the dew-point temperature. As the air continues to rise and cool, water vapor will eventually condense onto the cloud condensation nuclei and form cloud droplets. Since there are many sizes of cloud condensation nuclei in any given air parcel, the cloud droplets that form will be different sizes as well.

As a result of the cloud condensation nuclei size distribution, some cloud droplets will be larger than the rest. Eventually, the largest cloud droplets begin to fall faster than the smaller droplets because large drops have faster terminal velocity than small drops. As these large cloud droplets fall, they collide with the small cloud droplets.

Many times, these droplets will stick together and become one large drop (coalesce). Eventually these droplets fall from the cloud as raindrops, reaching a diameter of approximately 10 mm (0.39 inches). After a raindrop reaches 10 mm, it becomes so large that it breaks up into smaller drops.

Building a Weather-Ready Nation

When a Flood WATCH Is Issued

  • Move your furniture and valuables to higher floors of your home if you live in a flood prone area.
  • Fill your car's gas tank, in case an evacuation notice is issued.

When a Flood WARNING Is Issued

  • Listen to the NOAA Weather Radio, local radio, and/or TV stations for information and advice. If told to evacuate, do so as soon as possible.

When a Flash Flood WATCH Is Issued

  • Be alert to signs of flash flooding and be ready to evacuate on a moment's notice.

When a Flash Flood WARNING Is Issued (or if you think it has already started) evacuate immediately. You may have only seconds to escape. Act quickly!

  • If your car stalls in rapidly rising waters, abandon it immediately and climb to higher ground.
  • Move to higher ground away from rivers, streams, creeks, and storm drains. Do not drive or walk around barricades as they are there for your safety. When you see a barricaded road, "Turn Around Don't Drown"