Laser could be used to make rain on demand

Being able to create enough droplets is a key challenge

Ultra-fast pulses from a powerful laser can create droplets of water out of thin air, according to a new study. With the right conditions and large enough droplets, the researchers say, the technique could be used to make rain on demand.

Rain forms when water condenses around tiny particles in the atmosphere. Most of the time, dust or pollen do the job, but humans have long attempted to speed the process by seeding clouds with chemicals like silver iodide. Those chemicals provide the so-called "condensation nuclei" that trigger the consolidation of water into raindrops.

Unfortunately, such methods are difficult and could have environmental side effects, said Jérôme Kasparian, an optical physicist at the University of Geneva, Switzerland who was on the team that demonstrated the laser-triggered condensation. The study was published online May 2 in the journal Nature Photonics. 

"The potential advantage of laser is that it can work continuously," Kasparian said. If lasers can trigger rain on a large scale, he said, it would also be more efficient and cheaper than spraying silver iodide out of airplanes or shooting it into the sky from rockets.

Researches have long known that short, strong laser pulses can ionize air molecules, creating pathways of ionized gas called plasma channels. Kasparian and his team wanted to find out whether those plasma channels could be of use to wannabe rainmakers.

"Our idea is to use the laser to ionize the air, and the ions that are produced can then serve as the condensation nuclei," Kasparian said.

To test the idea, the researchers first used an atmospheric cloud chamber, a box that enabled them to vary temperature and humidity. After saturating the air in the chamber, the team flipped on a several-terawatt laser (one terawatt is a trillion watts) and watched with surprise as visible water droplets formed. Three seconds after the laser pulsed, the droplets swelled to diameters of 80 micrometers, smaller than a raindrop but larger than expected.

"What was very amazing was the fact that the cloud was very well visible to the [naked] eye, so the effect was quite strong," Kasparian said.

The next step was to take the laser outside. Using a weaker laser to monitor the formation of foggy air, the team blasted their multiple-terawatt laser into the sky of Berlin in Autumn 2008. Again, they saw heartening evidence: Particles coalescing in the atmosphere.

"This means that the laser can trigger the formation of droplets inside a cloud chamber, [but also] in the real atmosphere," Kasparian said. "Now the challenge is to find conditions that will allow the droplets to grow further into the size where they will fall and get turned into rain."

 

When it rains, buffalo have more sons

Findings may be evidence of so-called sex-ratio genes in large mammals

Something strange happens when it rains in Kruger National Park in South Africa: Buffalo mothers give birth to more sons. Now, scientists think they know why — it's in their genes.

The results from a new study suggest that certain genes in male buffalos distort the genetic material in their sperm so that they father only sons. And these genes are "turned on" when there's plenty of grass to chow down on, as happens during the rainy season.

However, there appears to be a few systems in play so that overall, the proportion of males to females tends to even out. For one thing, the males with these sex-distorting genes are usually less fertile than other males. Another possibility is that a different set of genes, triggered by the dry season, influence the resulting brood so that there's slightly more females. 

If true, the findings would be the first evidence of so-called sex-ratio genes in large mammals. These genes have so far only been observed in insects, such as files, and in rodents.

The findings suggest that sex-ratio genes might be present in more animals than scientists have suspected, and perhaps even in humans, according to study researcher Pim van Hooft of Wageningen University in The Netherlands.

"I think it's not unique for the buffalo," van Hooft said.

Sex genes
Sex-ratio genes are located on the chromosomes that determine an animal's gender. Chromosomes are packets of DNA inside a cell's nucleus. In buffalo and people, one pair of chromosomes, called the X and Y chromosomes, are considered sex chromosomes. 

Each parent provides one sex chromosome to the offspring, with males giving either an X or Y and females giving only X chromosomes. Offspring with XX become female and those with XY become males.

Sex-ratio genes are malicious, however. They work by either killing or disabling the opposite sex chromosome. So if they are lurking on the Y chromosome, they kill off the X, meaning the father's sperm will contain only Y chromosomes, so he'll only produce male offspring. And if they're on the X chromosome, the opposite happens. Sex-ratio genes are thought to carry out their attack during spermatogenesis, or the formation of male sperm.

"Because the males have a distorted sex-ratio [gene] in their sperm, you get a distorted sex ratio at birth," van Hooft said.

Too much diversity
Van Hooft examined blood samples collected from buffalo during 1998. Because the samples included the age of the animal, he was able to go back in time, looking at the genetics of buffalo that varied in ages from 0 to 15 years.

What he found was quite strange. The amount of genetic variation on the Y chromosome was surprisingly different between buffalo of different ages, even if they were only a year apart. It was as if there were two different types of Y chromosomes, and the ratio of these Y-chromosome types fluctuated from season to season, and also from year to year.  

Normally, you would expect buffalo that were so close together in age to have very similar Y chromosomes, van Hooft said. After all, it's the same parent generation producing the kids for quite a few years, and shifts in the population genetics just don't happen that fast.

He also saw that the number of males and females born during each season was slightly skewed. During the rainy season, the ratio of males to females was about 53-to-48, so if there were 100 buffalo babies born, 53 would be male and 48 female on average. The reverse was seen in the dry season, with relatively more females born.

Van Hooft reasoned that sex ratios were at play, and that they were triggered by environmental conditions, which in turn affected the sperm.

"Apparently, these sex-ratio genes are triggered by the body condition, or the semen quality," van Hooft told LiveScience.

Here's what could be happening:Some sex-ratio genes are turned on by the wet season, causing some male buffalo in the population to produce only sons. This increases the proportion of males in the population, and the Y chromosomes from those males would be overrepresented.

During the dry season, a different set of sex-ratio genes could be turned on, ones that had the opposite effect. The final result is an increase in females, along with a fluctuation in Y chromosome types.