When ice crystals form in clouds, the surrounding air

Douglas Wesley, a senior meteorologist in the Cooperative Program for Operational Meteorology, Education and Training (COMET) at the University Corporation for Atmospheric Research, explains:

FLOATING CLOUDS.The water and ice particles in the clouds we see are simply too small to feel the effects of gravity. As a result, clouds appear to float on air.

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.

Answer originally posted May 31, 1999

Have you ever heard someone say, “Clouds are just water vapor”? Next time, you’ll be able to correct them.

While it’s true that clouds contain water, they actually aren’t made of water vapor. If they were, you wouldn’t be able to see them. The water that makes up clouds is in liquid or ice form. The air around us is partially made up of invisible water vapor. It’s only when that water vapor cools and condenses into liquid water droplets or solid ice crystals that visible clouds form. 

Evaporation 

So how does that water get up into the sky? Consider the water on the surface of Earth—that means the oceans, lakes, and streams but also the soil and even the drops and puddles that collect on leaves, buildings, and rocks. Remember that water is made up of tiny particles and that those particles are in motion.

As long as the air above isn’t completely saturated with water vapor (meaning it has less than 100 percent humidity), some fraction of the particles in the liquid water have enough energy to “escape,” and they can rise into the air above the surface and evaporate. The warmer the water is, the more thermal energy the particles have. On average, as the temperature increases, the number of particles with enough energy to escape into the air increases. Likewise, the drier the air, the faster the water can evaporate.

Another important source of water vapor is plants. Plants draw water through their roots, stems, and leaves by regularly letting water vapor and other gases out of the pores (tiny holes) in their leaves. Because of the tendency of water particles to stick to each other (called cohesion), as this water exits the plant, it draws up the water behind it. This allows the roots to take in more water from the soil. The release of water vapor by plant pores is called transpiration. Together, evaporation and transpiration contribute the water vapor in the air that can eventually form clouds. 

Condensation 

Warm, moist air is less dense than the air around it, so it begins to rise higher into the sky. Wind can also push the parcel of air containing the water vapor to higher elevation or up the side of a mountain. Air temperatures tend to decrease the higher you go in the atmosphere. This is because the pressure decreases as you go higher, allowing the air to spread out and become thinner and, therefore, cooler.

Eventually, when the water rises to an elevation where the temperature is cool enough (the dew point, or point of saturation), it will start to condense into liquid form.

However, the water vapor will not readily condense without help from other particles. The air making up our atmosphere is full of microscopic floating particles of dust, soil, smoke, sea salts, and other matter. These particles are called condensation nuclei (singular: nucleus) when they assist in cloud formation. Just as water particles condense on grass to form dew, the

tiny airborne particles of water vapor condense into liquid or ice on the surfaces of dust particles in the air. As more water vapor condenses into water droplets, a visible cloud forms.

So if clouds are liquid or solid water, why don’t they immediately fall out of the sky as rain or snow? Think about the fine dust particles you can often see floating in a shaft of light. These particles are solid, but their mass is so small that they remain airborne with even the slightest of updrafts; that is, until they collide and merge with enough other particles that they get big enough to fall. Similarly, the liquid droplets or ice crystals making up clouds are tiny enough to stay aloft. Only when enough collect and collide to form larger droplets do they begin to fall as precipitation. 

Types of Clouds 

While the basic concepts of cloud formation apply to all clouds, everyone knows that no two clouds look exactly alike! However, meteorologists use a classification system to group clouds into types to help them understand and predict their effects on the weather.

You’ve probably heard some of the Latin names for cloud types. The most common one is cumulus, referring to the popular puffballs. While some of the names can be rather long and complicated, they have some basic components that will help you break them down. 

Look at some of the cloud types and their names in the illustration above and see if you can figure out how they might form and what kind of weather they might bring.  

This is an excerpt from the Weather and Climate Systems unit of our curriculum product line, Science and Technology ConceptsTM (STC). Please visit our publisher, Carolina Biological, to learn more. 

The problem is that there aren't many materials that can act as an ice nucleus.  Silver iodide is used in cloud seeding.  Kaolinite is a clay material (that was used at one time in Kaopectate for the treatment of diarrhea, bismuch subsalicilate is now used).  Certain bacteria also are effective ice nuclei (bacteria are added to water in snow-making operations at ski resorts to ensure that the water freezes when sprayed onto the slopes).

Screening layers are found at the top and sides of the cloud (2a and 2b in the figure).  These form because of the abrupt drop in conductivity as you move from outside the cloud into the cloud.

E fields under the thunderstorm at the ground are typically 1000s of V/m (100 to 300 V/m is typically found during fair weather).  Enhancement of the E field at the points of sharp objects on the ground often go into corona discharge and spray positive charge into the air near the ground.  The ground under the main part of the thunderstorm is also positively charged.