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 r 3 ; the downward-facing surface area of such a particle is proportional to the radius squared r 2.
Thus, as a tiny water droplet grows, its mass becomes more important than its shape and the droplet falls faster. 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. In both cases, though, the atmospheric ascent is sufficient to negate the small fall velocities of cloud particles. When air rises in the atmosphere it gets cooler and is under less pressure.
The vapor becomes small water droplets or ice crystals and a cloud is formed. These particles, such as dust and pollen, are called condensation nuclei. Eventually, enough water vapor condenses upon pieces of dust, pollen or other condensation nuclei to form a cloud. Some clouds form as air warms up near the Earth's surface and rises. Heated by sunshine, the ground heats the air just above it.
That warmed air starts to rise because, when warm, it is lighter and less dense than the air around it. As it rises, its pressure and temperature drop causing water vapor to condense. Eventually, enough moisture will condense out of the air to form a cloud. Several types of clouds form in this way including cumulus, cumulonimbus, mammatus, and stratocumulus clouds. So this water vapor also goes up along with the warm air in the sky. The average puffy cloud called a cumulus cloud floats about one mile above the surface of the earth.
Stratus clouds which are giant blankets that are lying across the sky, float about one mile. Water droplets in air behave the same way as dust 6. The second reason that clouds can float in the air is that there is a constant flow of warm air rising to meet the cloud: the warm air pushes up on the cloud and keeps it afloat.
Cold air cannot hold as much moisture as warm air: the vapor has to condense into a liquid. This is the beginning of a cloud. The sun heats the earth, and causes water on the ground to evaporate3.
The water rises, cools, and condenses.
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