Rain is often perceived as pure, a natural cleansing agent for the environment. However, the reality is that rainwater is rarely just pure H₂O. As it forms and falls through the atmosphere, rain naturally collects various substances, ranging from microscopic particles to dissolved gases. This interaction with the air means that what lands in your rain barrel or on your plants contains more than just water.
How Raindrops Are Formed
The formation of raindrops begins with tiny particles known as cloud condensation nuclei (CCN). Water vapor in the atmosphere needs a surface to condense upon to transition from a gas to a liquid. Without CCNs, water vapor would struggle to form the droplets that make up clouds and eventually rain.
As water vapor cools and reaches its saturation point, it condenses around these CCNs, forming minuscule cloud droplets, typically 10 to 20 micrometers in diameter. These droplets are light enough to remain suspended in the air. As more water condenses and droplets collide, they grow larger, eventually becoming heavy enough to fall as precipitation.
What is in the Atmosphere
The atmosphere is filled with a diverse array of particles and gases, both natural and human-made, that can become part of rainwater. Natural sources include dust from arid regions, pollen from plants, sea salt from ocean spray, and ash from volcanic eruptions or wildfires.
Human activities also contribute significantly to atmospheric content. Industrial emissions, vehicle exhaust, and the burning of fossil fuels release pollutants like sulfates, nitrates, and soot particles. These human-made substances can be absorbed by existing raindrops as they descend through the air.
Impacts of Atmospheric Particles in Rain
The presence of atmospheric particles in rain has various consequences, some beneficial and others concerning. Rainwater also naturally contains dissolved gases like carbon dioxide, which gives it a slightly acidic pH, and can carry beneficial nutrients such as nitrogen and potassium that support plant growth.
However, human-made pollutants can lead to detrimental effects, most notably acid rain. When sulfur dioxide and nitrogen oxides from burning fossil fuels dissolve in rainwater, they form sulfuric and nitric acids, lowering the rain’s pH. This acidic precipitation can harm ecosystems, leach aluminum from soil into waterways, and corrode buildings and monuments. Rain also cleanses the air by removing particles, though the deposited substances can impact surfaces and ecosystems.