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How Much Does A Cloud Weigh?
MyNameIsFred writes "ABC News is running an article revealing unexpected facts about weather formations. Ever wonder how much a cloud weighs? What about a hurricane? A meteorologist has done some estimates and the results might surprise you..." Reports that include the phrase "more than all the elephants on the planet" are always welcome.
Cecil Adams answered this a few years back. Sure he uses 747's instead of elephants, but his answer is a bit more detailed.
Clouds are composed primarily of small water droplets and, if it's cold enough, ice crystals. The vast majority of clouds you see contain droplets and/or crystals that are too small to have any appreciable fall velocity. So the particles continue to float with the surrounding air. For an analogy closer to the ground, think of tiny dust particles that, when viewed against a shaft of sunlight, appear to float in the air. Indeed, the distance from the center of a typical water droplet to its edge--its radius--ranges from a few microns (thousandths of a millimeter) to a few tens of microns (ice crystals are often a bit larger). And the speed with which any object falls is related to its mass and surface area--which is why a feather falls more slowly than a pebble of the same weight. For particles that are roughly spherical, mass is proportional to the radius cubed (r3); the downward-facing surface area of such a particle is proportional to the radius squared (r2). Thus, as a tiny water droplet grows, its mass becomes more important than its shape and the droplet falls faster. Even a large droplet having a radius of 100 microns has a fall velocity of only about 27 centimeters per second (cm/s). And because ice crystals have more irregular shapes, their fall velocities are relatively smaller. Upward vertical motions, or updrafts, in the atmosphere also contribute to the floating appearance of clouds by offsetting the small fall velocities of their constituent particles. Clouds generally form, survive and grow in air that is moving upward. Rising air expands as the pressure on it decreases, and that expansion into thinner, high-altitude air causes cooling. Enough cooling eventually makes water vapor condense, which contributes to the survival and growth of the clouds. Stratiform clouds (those producing steady rain) typically form in an environment with widespread but weak upward motion (say, a few cm/s); convective clouds (those causing showers and thunderstorms) are associated with updrafts that exceed a few meters per second. In both cases, though, the atmospheric ascent is sufficient to negate the small fall velocities of cloud particles. Another way to illustrate the relative lightness of clouds is to compare the total mass of a cloud to the mass of the air in which it resides. Consider a hypothetical but typical small cloud at an altitude of 10,000 feet, comprising one cubic kilometer and having a liquid water content of 1.0 gram per cubic meter. The total mass of the cloud particles is about 1 million kilograms, which is roughly equivalent to the weight of 500 automobiles. But the total mass of the air in that same cubic kilometer is about 1 billion kilograms--1,000 times heavier than the liquid! So, even though typical clouds do contain a lot of water, this water is spread out for miles in the form of tiny water droplets or crystals, which are so small that the effect of gravity on them is negligible. Thus, from our vantage on the ground, clouds seem to float in the sky.
Try skydiving trough a cloud. The do indeed look fluffy and soft from an airplane, but when you fall trough them at 200-280kmh, it feels quite different.. All those small droplets hitting your bare skin feels like hundreds or thousands of small nails, and larger drops can be be painful trough thin clothing as well..
Not to mention hail within clouds. Hail is really, really painful. Skydivers really don't like hail. At all.
The do have weight because the have a downward force from the relation of their mass within the gravity of earth. But because of their low density they float in the air. Just like a feather has weight but still floats in the air currents.
All objects that have a mass have weight. Weight is related to the gravitational conditions the object is in. You are confusing weight and density / buoyancy.
Physics 101 please try this class again.
I was thinking of the immortal words of Socrates, who said: "I drank what?" - Chris Knight (Val Kilmer)- Real Genius
Clouds float because the water droplets in them are tiny, and have a large surface-to-volume ratio. If the force caused by the friction of rising air currents on the droplet's surface is larger than the weight of the droplet, the droplet rises with the air. When the droplets increase too much in size, it rains.
And what if the air in the cloud isn't rising? Then the water droplets fall, very slowly. If they are too small to cause rain, when they reach lower layers of the atmosphere they evaporate, because air lower down is, normally, warmer.