Clue Found in Mystery of How Raindrops Form

How clouds let loose the summer showers that drench your clothes and cool you off has remained somewhat of a mystery.

Now with a new instrument, scientists have effectively reached into the clouds, revealing that a process they call cloud-stirring leads to the rainfall.

Knowing how clouds make rain is critical for predicting the effects of global warming, the scientists said. The amount of rain regulates how much water vapor, a potent greenhouse gas, stays lofted in the air. Plus, when clouds dump out their water droplets, they die out. Since clouds reflect sunlight, they can impact global temperatures by regulating how much solar energy reaches the Earth's surface.

"Until now, no one's been able to look at that process of how drops form—it's a missing link that has been in contention for 50 years," said Jennifer Small of the University of California, Santa Cruz, who will present the work at a meeting next week of the American Geophysical Union.

Floating droplets

As water vapor rises from the surfaces of the oceans and other bodies of water, it begins to cool off. This cooling causes condensation, which transforms the vapor into tiny water droplets—that form clouds—with diameters smaller than the width of a human hair. These droplets are far too small to fall as raindrops, which are typically at least a millimeter across and a million times heavier.

To balloon to raindrop size, the droplets need to collide and stick together.

Past studies have relied on computer simulations of the rain-making process. These showed that once the itsy-bitsy droplets materialized inside the clouds, they turned into raindrops in one or two hours. In the real world, however, rain can start to fall within 15 minutes of cloud formation. Something must be speeding up the mergers of tiny droplets, which are so light they float around in clouds and often easily avoid the collisions necessary to make rain.

One way to speed the coalescence of droplets is to stir them up. "If you stir up the droplets, they will more readily collide with one another," said study co-author Patrick Chuang of the University of California, Santa Cruz. "There are many ideas as to what the mixing mechanism is, but no one knows for sure how it comes about."

Let it rain

For decades, scientists have been debating over two main mixing mechanisms. One idea proposes that chaotic swirls of turbulent air work on tiny scales and mix up the droplets. The other describes a process called entrainment that happens when dry air mixes with moist air at the edges of clouds.

With no instruments to directly measure the cloud droplets, scientists have been unable to find absolute evidence for either theory. To find the heavenly mixer, Small and Chuang attached a newly-created device to the wing of an airplane. As the plane flew through the clouds above the Caribbean island of Antigua, the instrument beamed lasers out to measure droplet sizes.

They found droplets boasting larger-than-raindrop sizes in sinking pockets of air at the tops of clouds. This implies mixing occurred at the cloud edges, a support for the process of entrainment. 

But that doesn't completely rule out turbulence, they said. Some of the droplets could get a kick from turbulence, but this physical process, they explained, is tricky to study.