How Raindrops Trap Air in Soil—and Launch Tiny Droplets

A raindrop hitting dry ground seems like a simple event: water lands, then soaks in. But in the brief interval between those two steps, something surprising can happen. The drop can seal air beneath itself, grow bubbles, and send microscopic droplets into the air when those bubbles burst.

That helps explain how rain can lift material from soil into the atmosphere—and contribute to the familiar earthy smell after rain. The key lies in what it means for a raindrop to “trap air.”

The Air Is Already in the Ground

Dry soil is not a solid block. Between its particles and clumps are pores: small spaces that can hold water or air. In dry soil, many of those spaces contain air.

When a raindrop lands, it does not need to scoop up air from above. Instead, it can briefly seal air that was already at or just beneath the soil surface.

“Trapping air” means that spreading water covers soil-pore openings before the air inside has fully escaped.

Imagine laying a wet sheet across the openings of many tiny passages. The sheet covers them, but the passages beneath it still contain air. A spreading raindrop creates a similarly temporary seal—except that its water is also beginning to enter those passages.

The result depends on a race between two motions:

• Sideways spreading: the drop flattens and spreads across the ground.
• Downward soaking: water enters the soil’s pores.

If the water disappears into the soil too quickly, there may be little opportunity to form bubbles. If the drop spreads over air-filled pores first, the conditions can be right.

From One Raindrop to an Airborne Mist

High-speed imaging by researchers Young Soo Joung and Cullen Buie revealed the sequence behind this mechanism. It happens too quickly for you to follow with the unaided eye, but it can be broken into four steps.

1. The drop flattens and covers pore openings

On impact, the falling drop spreads into a wider, shallower shape. Under suitable conditions, that sideways motion is faster than the soil’s initial absorption of water.

Water therefore covers pore openings while air remains beneath it.

2. Entering water displaces pore air

As water penetrates the soil, it takes up space previously occupied by air. Some of that displaced air moves upward into the overlying drop, forming bubbles.

The pores are not necessarily isolated little chambers. They can form connected pathways through the soil, so the outcome depends on where both water and air can travel.

3. Bubbles reach the drop’s exposed surface

The spreading drop becomes thinner. Bubbles within it can reach its upper surface, where a thin film of water separates the air inside the bubble from the atmosphere.

4. Bursting bubbles eject microscopic droplets

When a bubble bursts, it can launch tiny water droplets into the surrounding air. These airborne droplets are aerosols—small liquid droplets or solid particles suspended in air.

This is different from simply throwing visible splashes off the edge of an impacting drop. Here, bubble bursting provides a route from the soil–water boundary to the air above it.

The original experiments documented this process on suitable porous surfaces, including soils.

Why Soil Type Changes the Outcome

You might expect the most porous soil to produce the most bubbles. But porosity alone is not enough. What matters is the arrangement of the pores, how readily water enters them, and whether the surface stays firm during impact.

Pore structure sets the timing

Pore structure means the sizes, shapes, connections, and arrangement of the spaces within soil.

Large, well-connected pores can let water move into the ground readily. Finer pores and more complicated pathways can slow that movement. Those differences affect whether a drop remains on the surface long enough to seal air, receive bubbles, and allow them to burst.

The familiar labels sand, clay, and loam offer clues, but they do not fully describe that structure.

These are tendencies, not rules for every patch of ground. A cracked clay surface, for example, offers very different pathways from a compact, uncracked one. Two soils with similar particle sizes may therefore handle the same drop differently.

Wettability determines how water enters

Wettability describes how readily water makes contact with and wets a surface.

It is distinct from pore size. A soil can have spaces available for water yet resist being wetted. On a water-repellent surface, a drop may bead up rather than readily enter the pore network.

The bubble mechanism needs a particular balance:

• Water must enter the soil to displace air.
• It must not enter so quickly that the surface drop disappears before bubbles can grow and burst.

So neither “absorbs water fastest” nor “resists water most strongly” is a reliable recipe for maximum aerosol production.

A firm surface matters, too

The experiments did not show the same bubble formation on loose powders or granular layers that deformed under impact.

That observation adds an important qualification: a collection of pores is not enough by itself. If grains shift as the drop lands, the boundary between water, air, and soil changes during the impact. The researchers’ proposed explanation for the difference was tentative, so it is best treated as an observed limitation rather than a fully settled rule.

Why Faster Drops Do Not Always Make More Aerosols

A harder impact might seem certain to launch more material into the air. For this bubble-driven process, however, speed creates competing effects.

Faster spreading can trap more air

A faster-moving drop initially spreads farther across the surface. That can cover more pore openings and create more opportunities for air to become trapped beneath the water.

Up to a point, greater impact speed can therefore encourage bubble formation.

Too much spreading can leave too little depth

A more forceful impact also flattens the drop into a thinner layer. That can restrict bubble growth and reduce the number of bubbles that go on to produce airborne droplets.

This distinction is easy to miss:

More bubbles initially does not necessarily mean more aerosols eventually.

You need to consider not only whether bubbles form, but also whether the remaining water layer allows them to grow and burst in a way that ejects droplets.

In the tested conditions, light-to-moderate impacts could produce more aerosols through this mechanism than harder impacts. That does not mean heavy rain produces no spray. It means this particular route—pore air forming bubbles inside an impacting drop—does not simply become stronger as impact speed increases.

Does Clay Smell Stronger Than Sand After Rain?

The physics suggests a tempting prediction: if some clay-containing soils produce aerosols efficiently, perhaps they always give off the strongest earthy smell.

The evidence does not support such a simple ranking.

Petrichor is the characteristic smell associated with rain falling on dry ground. One contributor is geosmin, an earthy-smelling compound produced by certain microbes. Getting an odor from soil to your nose involves more than generating droplets.

You need to separate two questions:

1. How effectively does the impact put material into the air?
2. How much odor-producing material is available to be carried or released?

The raindrop experiments measured aerosol production, not how strongly people perceived the smell.

Under the tested conditions, clay and sandy-clay soils often fell within a range favorable for vigorous aerosol production. The researchers also filmed aerosol formation on clay loam. Related experiments found favorable wetting behavior in sandy-clay soil, while sand absorbed drops too quickly to produce the same effect.

That makes a firm, dry, clay-containing soil a reasonable candidate for a pronounced first-rain burst—but not a universal winner among common soils.

Two clay loams could have similar pore structures yet smell quite different because they contain different amounts of odor-producing compounds. Conversely, a soil rich in earthy-smelling material may not generate many aerosols through this particular mechanism.

The Useful Mental Model: A Race With a Narrow Window

To understand how a drop traps air, picture a brief window of opportunity rather than a simple splash.

The drop must:

1. Spread across air-filled pores before all its water soaks away.
2. Let some water enter, displacing pore air into bubbles.
3. Remain thick enough for those bubbles to develop.
4. Allow bubbles to burst at its surface, ejecting microscopic droplets.

Soil structure and wettability control the downward movement of water and the escape routes for air. Impact speed controls how quickly and how thinly the drop spreads.

The central idea is simple: a raindrop can temporarily put a watery lid over air already in the ground. Whether that lid produces a burst of aerosols depends on getting the timing—and the thickness—just right.