NASA answers : Why does it drizzle instead of rain?

foggy drizzle


Why do some rain clouds produce drizzle — more mist than rain — instead of fully formed raindrops?

New research by NASA scientists suggests updrafts play a more important role in curtailing drizzle than previously estimated. Scientists hope their findings will improve weather modeling and forecasting.

Water droplets form as water condenses on airborne particles or aerosols. Higher concentrations of aerosols encourage clouds with more small water droplets, increasing the chance of drizzle. Because aerosols are more likely to be found over land than over the ocean, models predict drizzle is more likely over land.

But the latest analysis of precipitation models suggests aerosols alone can’t account for the likelihood of drizzle.

Previous studies have shown warm air updrafts to play an important role in the formation of thunderclouds and strong storm systems. However, updrafts in lower-lying clouds are generally weaker, but their influence on precipitation dynamics hasn’t been well-studied.

“There was a previous hypothesis that updrafts could be important,” Hanii Takahashi, researcher at NASA’s Jet Propulsion Laboratory, said in a news release. “But the hypothesis had never been tested, and I wasn’t sure if updrafts were strong enough to affect the size of rain droplets.”

Researchers analyzed data from NASA’s CloudSat and Aqua satellites to better estimate updraft velocities in different types of clouds. They found updrafts in low-lying clouds were stronger than previously thought — strong enough to prevent drizzle droplets from falling.

When updrafts push drizzle back up into the upper portions of clouds, they remain airborne and coalesce to form larger droplets, eventually dropping as fully formed raindrops.

Researchers found updrafts were stronger among low-lying clouds over land than those over the ocean. Low-lying marine clouds, all else being equal, are more likely to yield drizzle.

Weather scientists suggest their findings can be used to update precipitation prediction models.

“If we make updraft velocities more realistic in the models, we might get both more realistic drizzle and more realistic surface temperature projections as a result,” Takahashi said.

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