Why does touching the canvas of a tent during a rain storm make the tent leak?

Question Answered by Dr. Steven Shropshire, Physics Professor, Idaho State University

Tent in Rain StormWhen you touch wet canvas, surface tension will draw water to your finger. There will be a “wicking” action then that will draw water through the canvas in response to your drawing water away from the canvas. When you take your finger away, this wicking will stop. However, the drop left behind where you touched, like any irregular point on an overhead surface, will draw condensation from inside the tent if it is humid. Since it usually is humid when it is raining, the condensation will build until you get a drop, but not all of it falls. What is left will still draw condensation more than the rest of the inside tent surface, so it will appear to leak from that point as long as the humidity is high.

Science drips sometimes.

Condensation
There is water in the air around you all the time. Instead of liquid water, it’s a gas called water vapor. Warmer air can hold much more water vapor than colder air.
When water vapor in the air comes into contact with something cool, like the ceiling of a tent, its molecules slow down and get closer together. When that happens, the gaseous water vapor turns back into liquid water droplets. That’s condensation!

Materials: 2 clear plastic cups, room-temperature water, ice cubes, gallon-size ziplock plastic bag

  1. Place the water and ice cubes into two identical plastic cups.
  2. Immediately place one of the cups in a ziplock plastic bag and get as much air out of the bag as possible as you close it securely.
  3. Allow the cups to sit undisturbed about 10 minutes.
  4. Take the cup out of the plastic bag and observe both cups. Which cup has the most moisture on the outside of it?

Condensation ExperimentThe cup exposed to more air has the most moisture on the outside of it. This moisture came from water vapor in the air that condensed on the outside of the cup. The cup in the bag has very little to no moisture on it because it is exposed to much less air. Less air means less water vapor.

Some people think that the moisture that appears on the outside of a cold cup is water that has leaked through the cup. How does this demonstration prove that this idea is not true?

Because there is little to no moisture on the outside of the cup in the bag, water could not have leaked through the cup. If the moisture came from leaking, there would be water on the outside of both cups.

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