The movement of water between the Earth’s surface and the atmosphere constitutes the water cycle, governing weather patterns worldwide. Clouds and the rain they produce are an observable part of this cycle, transforming invisible water vapor into liquid precipitation. Although the atmospheric scale is immense, the physics of cloud formation and rainfall can be replicated in a confined environment. This demonstration provides a tangible way to understand how water vapor cools, condenses, and eventually falls back to the surface.
The Atmospheric Process of Rain Formation
Rain formation begins with the transformation of liquid surface water into water vapor through evaporation. This gas rises into the atmosphere, often carried higher by updrafts of warm air. As the moist air ascends, it expands due to lower atmospheric pressure, causing its temperature to drop significantly.
This cooling brings the water vapor closer to its saturation point, the temperature required for it to change back into a liquid state. Pure water vapor is reluctant to condense without a surface to cling to, even when the air is saturated. In the atmosphere, this necessary surface is provided by microscopic airborne particles known as cloud condensation nuclei (CCN).
These CCN are tiny specks of dust, pollen, sea salt, or pollutants, serving as the foundation upon which water vapor condenses to form cloud droplets. Once formed, these droplets are too small to fall as rain, remaining suspended by air currents within the cloud. Precipitation occurs when the droplets grow large enough to overcome the upward drag of the air.
In warm clouds, droplets increase their size primarily through collision and coalescence, where they bump into and merge with other droplets. As this process continues, the growing droplets become heavy enough that gravity pulls them down toward the Earth’s surface as rain. In colder clouds, the Bergeron process, involving ice crystals and supercooled water, contributes significantly to droplet growth before melting into rain on their descent.
Materials and Setup for the DIY Cloud Experiment
Creating a cloud in a jar requires simple materials: a source of water vapor, a way to cool the air, and condensation nuclei. Adult supervision is recommended when handling hot water and aerosol spray. You will need:
- A clear glass jar with a wide mouth
- Very hot (but not boiling) water
- A metal plate or lid that can cover the jar opening
- Several ice cubes
- Aerosol hairspray or a match
Begin by carefully pouring about two inches of hot water into the glass jar, enough to warm the air inside and create visible steam. Let the jar sit for about a minute to allow the water vapor to rise and saturate the air inside. While holding the metal plate, place the ice cubes on top of it, preparing the cold surface.
The final action requires quick movements to trap the components inside the jar. Lift the ice-covered plate and quickly spray a very short burst of aerosol hairspray into the jar opening. Immediately cover the jar tightly with the cold, ice-covered plate. Observe the space just beneath the metal plate, where a distinct swirling mist should begin to form.
Explaining the Experiment’s Physics
The jar experiment directly models the physical requirements for cloud formation in the atmosphere. The hot water serves as the source of moisture, evaporating rapidly into the air inside the jar, representing the initial stage of the water cycle. This warm, moist air then rises toward the top of the jar, mimicking the updrafts that carry humid air to higher altitudes.
The metal plate topped with ice cubes acts as the cold upper atmosphere, drastically reducing the temperature of the air it contacts. When the warm, moist air meets this cold surface, it cools quickly, causing the water vapor to exceed its saturation point. Without the aerosol, this cooling would likely only result in condensation on the glass walls.
The aerosol hairspray provides the cloud condensation nuclei, the microscopic particles needed for water vapor to transition from gas to liquid. The water vapor molecules condense onto the hairspray particles, forming tiny, visible liquid droplets that collectively create the cloud, or mist, inside the jar. This visible cloud illustrates how water vapor requires a surface to condense upon, even when the air is supersaturated.
The mist will swirl and remain suspended because the water droplets are too small to fall and there is no mechanism for them to grow large enough to precipitate. The small scale of the experiment prevents the collision and coalescence process from occurring significantly, demonstrating the difference between forming a cloud (condensation) and producing rain (precipitation). By lifting the lid, the cloud escapes and disperses, showing how a change in pressure and temperature can quickly dissipate the condensed moisture.