Rain, a seemingly simple phenomenon of water falling from the sky, is actually the culmination of complex physical processes high up in the atmosphere. This common occurrence requires water to be collected, lifted, transformed into a visible cloud, and finally organized into drops heavy enough to descend to the ground. Understanding what makes it rain involves the delicate balance of energy, temperature, and microscopic particles that must align perfectly for precipitation to form.
Getting Water into the Atmosphere
The journey of a raindrop begins with the conversion of liquid water on the Earth’s surface into an invisible gas called water vapor. The sun’s energy drives this process, providing the heat necessary for molecules to break free from liquid bodies of water like oceans, lakes, and rivers through evaporation. Oceanic evaporation accounts for the vast majority of the moisture found in the atmosphere.
Plants release water vapor into the air through tiny pores on their leaves in a process known as transpiration. Collectively, the moisture transferred from the land and plants is known as evapotranspiration. Warmer air near the surface can hold substantially more water vapor, which is why tropical regions often experience high humidity.
This atmospheric water vapor is constantly being mixed and transported by air currents around the globe. Before rain can occur, this moist air must be lifted high enough into the atmosphere to cool down. The transformation into a cloud requires a significant change in the air’s temperature and pressure.
The Process of Cloud Formation
The invisible water vapor in the atmosphere must become visible liquid droplets to form a cloud, a process that is primarily achieved through cooling. As air rises from the Earth’s surface, it encounters lower atmospheric pressure, causing the air parcel to expand. This expansion, called adiabatic cooling, causes the air to lose energy and its temperature to drop without exchanging heat with the surrounding environment.
This cooling continues until the air parcel reaches its dew point, which is the temperature at which it becomes completely saturated with water vapor. At this point, the invisible gas is ready to condense, but it requires a surface to do so. These surfaces are provided by microscopic airborne particles known as cloud condensation nuclei.
These nuclei are minute specks of dust, pollen, soot, or sea salt, and are hygroscopic, meaning they readily attract water molecules. Water vapor condenses onto these particles, forming incredibly small liquid droplets, typically around 0.002 millimeters in radius. The cloud we see is simply a massive collection of these suspended water droplets or ice crystals, not yet rain.
How Water Droplets Become Rain
For a cloud droplet to grow large enough to fall as rain, it must increase its size by approximately a million times. This growth process happens through two primary mechanisms, depending on the temperature within the cloud. In warmer clouds, typically found in tropical regions where the temperature remains above freezing, the process is called collision and coalescence.
Within these clouds, larger droplets that formed on bigger condensation nuclei begin to fall faster than the smaller droplets. As they descend, they collide with and absorb the slower, smaller droplets in their path, a process known as coalescence. The drops continue to grow until they are too heavy for the cloud’s updrafts to support, and gravity pulls them to the ground as rain.
In mid-latitude and colder clouds, which often contain temperatures below freezing, the Bergeron process dominates the formation of precipitation. This mechanism relies on the fact that ice crystals and supercooled liquid water droplets—liquid water that remains unfrozen below the freezing point—can coexist at temperatures between 0°C and -40°C.
The saturation vapor pressure is lower over ice than over water, meaning the air is supersaturated relative to the ice crystals. Water vapor rapidly deposits directly onto the ice crystals, causing them to grow quickly at the expense of the supercooled droplets, which evaporate. As the ice crystals become heavy, they fall, often melting into rain before reaching the ground if they pass through a layer of air above freezing temperature.