Specifically, what is a cloud? Describe the droplets that form a cloud.

The life cycle of a cloud begins with evaporation, the process by which water is converted from its liquid state into a gaseous state, known as water vapor. This transformation occurs when energy, usually from the sun, heats the water on Earth's surface. Ponds, lakes, oceans, and even plants contribute to this vapor that rises into the atmosphere. Once the water vapor ascends, it encounters cooler temperatures at higher altitudes, and the process of condensation starts. During condensation, the water vapor cools and changes back into liquid droplets. This phase change occurs because cooler air can hold less moisture than warm air.

To facilitate condensation, the water vapor requires a surface to condense upon, which leads us to the role of atmospheric particles.

The formation of cloud droplets would be exceedingly difficult without the presence of atmospheric particles. These microscopic particles, known as cloud condensation nuclei (CCN), are essential for cloud formation. They can be composed of various substances like dust, pollen, sea salt, and soot. When the water vapor that has risen from the Earth's surface starts to cool down, it condenses around these CCN. The role of these particles is so crucial that without them, the relative humidity would have to be exceedingly high, around 400% rather than 100%, for water to condense and form droplets.

For students, understanding the dependency of cloud droplets on atmospheric particles is vital. It's a symbiotic relationship wherein the water vapor needs the CCN to transition back to its liquid form, and these particles are the building blocks for the growth of droplets that later form clouds.

In a cloud, the size of the droplets is a defining characteristic. Each droplet in a cloud is about one hundredth of a millimeter in diameter, incredibly small when compared to raindrops. They exist in such sizes because that scale allows them to be held aloft by the atmosphere's updrafts. If the droplets were considerably larger, gravity would cause them to fall as precipitation.

Visualizing the minuscule size of cloud droplets can be challenging, so consider this comparison—although there are countless droplets within a cubic meter of a cumulus cloud, their combined volume equates to only about a single raindrop. This illustration not only emphasizes the droplets' diminutive size but also the sheer number present in cloud formation. The disparity in size between a cloud droplet and a raindrop is crucial to understanding why clouds can float while rain falls.