How Much Does a Cloud Weigh?


Imagine 300 midsize cars floating above your
head. That’s how much this cloud weighs. So how the heck is it floating? So, first thing’s first–how did this cumulus
cloud get up here? Well, it wouldn’t be up there without the
sun. Solar radiation heats air and water vapor
near the ground. That air and water vapor then start to rise
because they’re less dense than the air in our atmosphere. Which we know sounds weird, and we promise
to get back to in a second. So, as water vapor rises, it cools. As it cools it condenses back into tiny, tiny
droplets of water or ice. These droplets are spread throughout the cloud
and scatter sunlight, which is what creates that whitish sheen that we on Earth interpret
as…a cloud. Although each individual droplet doesn’t
weigh much because it’s so minuscule, together they add up. A lot. Meteorologists estimate that cumulus clouds
have a density of about 0.5 grams per 1,000 liters. The average cumulus cloud is about 2 kilometers
across, 2 and a half kilometers deep and around 200 meters tall. That turns out to be a volume of about a trillion
liters–giving us 500,000,000 grams of water, which is about 1.1 million pounds–the weight
of those 300 midsize cars. So how the heck is it floating? Let’s first quickly turn to the Ideal Gas
Law, which says that in a given volume of any gas at a given temperature and pressure,
there are the same number of gas molecules. So two liters of dry air — a mixture of mostly
nitrogen and oxygen, without any water in it — has exactly the same number of molecules
as two liters of water vapor. The molecular weight of dry air is around
29. Water has a molecular weight of about 18 — a
lot less. Since density is weight divided by volume,
the gas inside this two liter bottle is less dense than the gas in this one. So that’s why humid air is less dense than
dry air, and explains, along with solar heating, why air and water vapor rises. But, once it’s up in our atmosphere it will
expand, cool, and change into those tiny water droplets we mentioned earlier. And you’d think that they’d fall out of
the sky, but they don’t…and here’s why. When water vapor condenses into those tiny
droplets, heat is released. That heat causes the air in the cloud to expand,
making it less dense compared to the surrounding air, and so that’s part of the reason it
doesn’t fall out the sky. This whole process — the air and water vapor
near the ground rising up, the water condensing to a liquid and forming a cloud — this is
called a thermal, and it’s really the engine that drives cumulus cloud formation. And, fun side note, thermals aren’t just
useful for creating clouds and keeping them floating above us–they’re also used by
hang gliders and balloonists to stay lofted. So although by eye it looks like a fluffy
cloud isn’t doing much, that’s actually not true. It’s experiencing a constant cycling: updraft
and condensation, downdraft and evaporation, updraft and condensation, and so on. What that means is that all those water molecules
that make up the cloud are being cycled through gas – liquid – gas – liquid – gas – liquid
constantly, and you can throw ice crystals into that mix if the cloud grows vertically
enough to tap into that really cold air in our upper troposphere. And that’s how a million pound cloud floats. And it will eventually come down on us…just
very, very slowly. PBS Digital Studios wants to hear from YOU. We do a survey every year that asks about
what you’re into, your favorite PBS shows and things you’d like to see more from PBS
Digital Studios. You even get to vote on potential new shows. All of this helps us make more of the stuff
YOU want to see. The survey takes about ten minutes, and you
might win a sweet T-shirt. Link is in the description. Thanks!

20 thoughts on “How Much Does a Cloud Weigh?”

  1. Most cumulus clouds actually dissipate by evaporation. But in the right conditions, they can become cumulonimbus clouds… and that leads to rain, thunder, and spectacular storms.

  2. 😖 In a scientific presentation do not mix units of measurement as that creates a distorted account of the phenomenon. Either use SI forms or the more recognisable metric units. Grams and pounds do not sit comfortably together.

  3. I know intodays sensative climate around personal comments is bad to make them if you are a guy and giving a complement but I am a risk taker. so here it goes "I love the content this channel continues to put out. I am subed with my main youtube account and have been for a very long time before the split in content on the channel. I loved this video and the way it's presented. The videos are always easy to understand or go do a little more digging on the content if I missed some thing cause I don't presume to understand everything. That all said I really like this presenter, not that I like any of the other presenters less it's just my personal taste that this presenter is very attractive to me but not in that bad way but in that natural beauty kind of way. Ok yeah it sounds creepy to give a complement these days and I really don't care unless the recipient cares and then I am truly sorry and please delete this comment, I mean no harm. But sometimes it's just nice to know someone thinks you are cute and that's why I am risking say all this in case of misunderstanding and or massive hate from the interwebs, I am just trying to give a nice complement. "Keep up the good content Reactions. I Look forwards to your next video no matter who the presenter is."

  4. Very nice video–except for one thing. Why do you include background music with a science video? Any background music is distracting from the learning that goes on with this sort of video, or with lectures on science. That's why professors don't include background music when they lecture, and why famous scientists don't include background music when they give talks.

Leave a Reply

Your email address will not be published. Required fields are marked *