The sky above Earth is a pathway for a variety of materials, ranging from microscopic particles to objects weighing many tons. These materials originate both within the atmosphere and from sources outside the solar system. The descent of this matter shapes the environment, influences climate, and occasionally presents startling phenomena.
The Hydrological Cycle’s Descent
The most common material falling from the sky is water, cycling through the atmosphere in various forms of precipitation. This process begins when water vapor cools and condenses around microscopic particles called condensation nuclei to form clouds. As these tiny droplets coalesce, they eventually become too heavy to remain suspended and fall.
The specific form the precipitation takes depends on the vertical temperature profile of the atmosphere. Rain occurs when temperatures remain above the freezing point (0 degrees Celsius) throughout the column of air from the cloud to the ground. Snow forms when the temperature is below freezing all the way down, allowing ice crystals to collect into visible flakes.
Other, more complex forms of precipitation result from layered temperature changes. Sleet, or ice pellets, forms when snow partially melts in a warm layer before re-freezing into small, spherical ice in a deep sub-freezing layer near the surface. Hail forms uniquely within intense thunderstorms, where powerful updrafts repeatedly lift ice pellets into extremely cold parts of the cloud, allowing them to accumulate layers of supercooled water until they become too heavy.
Particulates and Aerosols
Beyond water, the atmosphere constantly carries a suspension of fine solid and liquid particles known as aerosols, which eventually settle back to Earth. These microscopic materials originate from both natural sources and human activities. Natural sources include volcanic eruptions injecting ash high into the stratosphere and wind erosion lifting mineral dust from deserts.
The oceans are another major contributor, where breaking waves release sea spray that evaporates, leaving behind tiny particles of sea salt and organic compounds. On land, biogenic aerosols include pollen and spores released by plants and fungi. Human-made aerosols, such as sulfates from burning fossil fuels and soot, significantly contribute to this atmospheric load, influencing both air quality and cloud formation processes.
Objects from Beyond Earth
Materials originating from outside the planet frequently encounter the atmosphere, resulting in intense friction. A meteoroid is the term for a small chunk of rock or metal traveling through space, often originating from asteroids or comets. When a meteoroid enters Earth’s atmosphere at high velocity, the resulting heat causes it to burn up, creating a bright streak of light known as a meteor, commonly called a “shooting star.”
An estimated 44,000 kilograms of meteoritic material enters the atmosphere daily, though the vast majority vaporizes completely before reaching the surface. If the original meteoroid is large enough to survive the fiery passage and impacts the ground, the remnant is then classified as a meteorite. These surviving fragments provide scientists with direct samples of materials from the early solar system. Artificial space debris, such as defunct satellites or rocket components, also re-enter the atmosphere, creating bright, slow-moving fireballs before burning up or occasionally landing as fragments on Earth.
Anomalous and Biological Falls
Unusual reports of material descending from the sky involve non-standard events, including biological matter and large ice masses. Reports of “rain” involving fish, frogs, or insects are rare phenomena, but they have been documented across history. The scientific explanation for these incidents involves the displacement of animals from their original habitat, not their formation in the sky.
The mechanism is usually attributed to powerful atmospheric disturbances, such as tornadic waterspouts or strong updrafts within severe thunderstorms. These vortices can generate enough suction to lift small, light aquatic animals from shallow bodies of water, carrying them high into the air before depositing them back onto the ground. Another rare event is the fall of megacryometeors, which are unusually large chunks of ice, sometimes weighing over 50 kilograms, that fall from clear skies. Unlike hail, which forms in storm clouds, the formation of megacryometeors is not fully understood, though their composition suggests they are terrestrial in origin.