# Round Fluffy Tops

In article http://www.straightdope.com/classics/a980313a.html there is a long discussion about the weight of clouds or lack of it.

A very good reason why clouds do, as a practical matter, have weight is the round fluffy top ones.

These type of clouds are formed when warm moist air becomes buoyant and rises, rapidly. In the rising process it gains momentum. Eventually it starts to condense and become less buoyant than the surrounding air and slows down.

Does it stop there? No. The soggy stuff keeps on rising under its own momentum as well as from the pressure of the air underneath. The tremendous amount of energy contained in the weight of the uprising cloud forms the nice round tops.

If a cloud had no weight - as a practical matter - it would stop dead the instant it reached neutral buoyancy. All clouds would be flat topped.

Ergo. Clouds have weight.

I believe you have just confused the terms “mass” and “weight”.

The ergo in your post should have been “clouds have mass”. To clarify, mass is the intrinsic property of matter, weight is the force acting on that matter. Mass stays constant, weight differs by altitude or being relocated to the moon.

In fact, the term “neutral buoyancy” requires weight. The buoyant force is an upward push, needing a downward push to balance it. Weight is that downward force. (Actually, weight is also the driver for the upward force, by the difference in densities and pulling the denser air downward, which then pushes the less dense air upward, but now it’s getting technical, and we have to pull out free body diagrams and shudder do math. )

Whereas, momentum is mass times velocity. So if the momentum is driving the cloud tops higher than the neutral buoyancy point, then it is because of mass, not weight.

One other point - having mass in a gravity field by definition gives weight. (Oh no, somebody is going to bring up geodesics and Rheimann geometry and relativity - ack!)

It seems to me that they must have weight because if they didn’t have weight they would keep rising forever - there being no gravity to act on them and pull them back.

Perhaps the fact they do not flat top shows they have mass and the fact they do not keep on rising forever shows they have weight.