What’s the News:
For a mosquito, venturing out during a heavy rainstorm means risking collisions with droplets 50 times its weight—but this doesn’t deter it from living in humid, rainy climes. In fact, researchers have discovered that the mosquito’s low mass, along with a sturdy exoskeleton, helps it weather (so to speak) the impacts of raindrops without much trouble.
How the Heck:
Thank god for air friction. Without it, falling rain would smack into our heads at hundreds of miles per hour. But friction works both ways—falling raindrops also slow down the movement of air molecules in the atmosphere. A new paper in Science calculated that raindrops dissipate as much kinetic energy from the atmosphere as air turbulence. Granted, at 1.8 watts per square meter and 0.75% of the atmosphere’s total kinetic energy, that’s not very much. What’s surprising is that rain drops are pulling more than their weight, as they make up only 0.01% of the atmosphere’s mass.
Researchers calculated the kinetic energy dissipated by a single raindrop and put it together with precipitation rates around the world. Since satellite precipitation data also show the height from which rain started falling, the researchers could plug how far raindrops fell into their energy calculations. It all adds up across the whole globe: the researchers estimate the total rate of energy dissipation from rainfall to be 1015 Watts. That’s a lot of energy, but still unlikely to affect major weather phenomena like hurricanes or tornados.
[via Nature News]
Image via Shutterstock
What’s the News: Bats have to use twice as much energy to fly when they’re wet as when they’re dry, a new study in Biology Letters found, which may help explain why many bats refrain from flying in heavy rain.
Schemes to hack the planet and save us from global warming have two layers of obstacles to overcome. First, is it technologically and physically possible to do what’s proposed? And then there’s the second: Is it politically possible to tinker with the planet?
Those who would argue “absolutely not” to the latter got a boost by a new study out in Nature Geoscience. Katharine Ricke and her team modeled the effects of one of the most popular geoengineering plans: seeding the atmosphere with aerosols to reflect away some of the sun’s rays, mimicking the way a massive volcanic eruption can cool the Earth. Ricke found that the effects on rainfall and temperature could vary wildly by region—and that what’s best for one country could spell disaster for another.
For example, Ricke says, her study found that levels of sulphate that kept China closest to its baseline climate were so high that they made India cold and wet. Those that were best for India caused China to overheat. She notes, however, that both countries fared better either way than under a no-geoengineering policy [Nature].
The rainmakers of the 21st century may be armed with powerful lasers. New research suggests that zapping clouds with laser beams could trigger the formation of condensation droplets that would fall to the ground as rain. But while the study, published in Nature Photonics, raises the tantalizing hope of bringing on-demand rain to parched regions, some experts argue the technique is unlikely to ever be practical.
For more than 50 years, efforts to try to artificially induce rain have concentrated on ‘cloud seeding’ — scattering small particles of silver iodide into the air to act as ‘condensation nuclei’, or centres around which rain droplets can grow. “The problem is, it’s still not clear that cloud seeding works efficiently,” says optical physicist Jérôme Kasparian at the University of Geneva, Switzerland. “There are also worries about how safe adding silver iodide particles into the air is for the environment” [Nature News].
We silly humans tend to think of rain just in our own terms, the falling water tainted with various toxins that draws out our umbrellas and cancels our baseball games. But across the solar system, it rains on other worlds with thick atmospheres–it’s just not rain we would recognize. On Saturn’s moon Titan, for instance, it rains methane. And now, a group of scientists says in Physical Review Letters, computer simulations have confirmed that it rains helium on Jupiter.
The term “rain” applies loosely here, because the hellfire precipitation happening on Jupiter isn’t much like a pleasant afternoon shower here on Earth. Droplets of helium form thousands of miles below the tops of hydrogen clouds, at temperatures around 9,000 degrees Fahrenheit–the helium stays in fluid form because of the planet’s high atmospheric pressure. Pressures and temperatures on Jupiter are so high that the droplets of liquid helium are falling through a fluid of metallic hydrogen [Space.com].
The pitter-patter of raindrops on your umbrella is caused by raindrops of all different sizes, and now physicists have a new explanation for how those raindrops form. A pair of researchers used a high-speed camera (video below the jump) to watch a single drop of water fall and change shape over the course of six-hundredths of a second, and found that the shattering of single raindrops after they leave clouds is enough to explain the wide variety of drop sizes [Science News].
Previously, the leading theory to explain the diversity of raindrops had been that raindrops grow as they gently bump into each other and coalesce. Meanwhile, more forceful collisions break other drops apart into a scattering of smaller droplets. All this action would explain the wide distribution of shapes and sizes [ScienceNOW Daily News]. But lead researcher Emmanuel Villermaux says he questioned that theory, with its supposition of frequent collisions. Real raindrops are so sparse, he said, that it is likely a drop would “fall on its own and never see its neighbours” [BBC News].
Who knew this spring’s soggy weather fell under the umbrella of physics research? Scientists found that when raindrops fall faster than physics predicts, the drops have actually broken into smaller droplets, according to a study in the journal Geophysical Research Letters. And because weather services gauge rainfall based on the velocity at which droplets fall–conventional wisdom holds that large drops should hit the ground at a higher speed than do smaller droplets–these results could improve the way we predict weather.
All falling objects have a so-called terminal velocity, a speed they can’t surpass due to air resistance. Therefore, larger drops generally should fall faster because their heftier size helps them power through air resistance more easily than little drops. (In the extreme case, think of fog: water droplets so small they don’t fall at all.) But data showing small drops sometimes impact the ground at the same speed as larger ones showed this conventional wisdom was wrong, and has puzzled scientists for years. To solve the mystery, the researchers collected a shower of data using optical equipment over a period of several years. The team clocked about 64,000 raindrops falling in Mexico City. The researchers measured their sizes and velocities only in extremely calm conditions, so the wind that often accompanies rain could not skew the data. They found that some drops plummeted faster than the so-called terminal velocity for their size [ScienceNOW Daily News].