Clouds are part of the hydrologic cycle
Everyone is familiar with the general types of clouds. Depending on the conditions, clouds come in a variety of forms: fat, puffy cumulus clouds; thin, wispy cirrus clouds; or dark, lowering stratus clouds threatening rain. Clouds are an integral part of the hydrologic cycle, by which water moves from Earth’s surface into the atmosphere and back again through the processes of evaporation, condensation and precipitation.
How clouds are formed
Water exists in three phases: solid, liquid and gas. When liquid water from Earth’s surface evaporates, it becomes a gas, water vapor, and rises into the atmosphere. As it rises to higher altitudes where the temperature and pressure are lower, the water vapor expands and cools. If the conditions are right, a cloud will form.
Cold air can hold less water vapor than warm air. Once cooled to a low enough temperature, the molecules of water vapor will condense back into tiny drops of liquid. If the concentration of water vapor in the air is sufficiently high, greater than 100% relative humidity, it will be able to coalesce into drops. However, if the relative humidity is below 100%, which is often the case, the molecules of water vapor require a solid surface, called a cloud condensation nucleus (plural: nuclei) on which to attach before they are able to form drops. Condensation nuclei can be microscopic particles of dust, smoke, salt, or even bacteria in the atmosphere. If the condensed water droplets become large enough, they will be too heavy to remain airborne, and will fall back to Earth as precipitation.
Making a cloud in a bottle
It is quite simple to simulate this process and observe cloud formation happening. To create your cloud in a bottle, you will need:
A large, wide-mouthed clear glass bottle, such as a pickle jar
Ice cubes in a plastic bag
A source of dust or smoke, such as matches
First pour some warm water into the bottom of the bottle to a depth of 1-2 centimeters. The water should be quite warm but not boiling, as this may cause the glass to crack.
Next, light a match, place the burning match into the neck of the bottle, and then blow it out, so that smoke from the match remains inside the bottle.
Quickly place the plastic bag of ice over the open top of the bottle.
You will immediately see a cloud form inside the bottle.
This is a model of the process that takes place high in the atmosphere. The ice cools the air at the top of the bottle. When the water vapor rising from the warm water on the bottom of the bottle reaches the cooler air at the top, it begins to condense and form drops around the smoke particles, which serve as condensation nuclei, and a cloud is created.