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Four Puzzling Questions Answered: From 'Why Does it Smell After Rain?' to 'Are All Snowflakes Unique?'

While we experience weather events on a daily basis, sometimes certain phenomena can produce unusual results that leave us searching for answers.

Recently scientists at the Massachusetts Institute of Technology have discovered a process that might tell us why there is an earthy scent that wafts through the air after it rains.

In addition to why it smells after rainfall, there are several other intriguing weather questions that have piqued curiosity over time.

What is the Aroma That Lingers After it Rains?

Petrichor is the word used to describe the smell that occurs after a fresh rain. But how does that scent occur and why?

The MIT research team used a high-speed camera to observe that when raindrops hit a porous surface, tiny air bubbles are trapped at the point of contact. Subsequently, the bubbles will then shoot directly up into the air and burst from the drop in a fizz of aerosols, according to an MIT press release.

Researchers Cullen R. Buie, an assistant professor of mechanical engineering at MIT, and Youngsoo Joung, a postdoctoral student in Buie's lab, had their findings published in the journal Nature Communications in mid-January.

"The researchers suspect that in natural environments, aerosols may carry aromatic elements, along with bacteria and viruses stored in soil," the release states. "These aerosols may be released during light or moderate rainfall, and then spread via gusts of wind."

While conducting their experiments, Buie and Joung found that more aerosols were produced in light and moderate rain and far fewer aerosols were released during heavy rain.

"Until now, people didn't know that aerosols could be generated from raindrops on soil," Joung said. "This finding should be a good reference for future work, illuminating microbes and chemicals existing inside soil and other natural materials, and how they can be delivered in the environment, and possibly to humans."

Previous research done by Australian researchers led to the creation of the term petrichor. However, while the Australian team identified oils emitted by plants and chemicals from bacteria as possibilities for the scent, they did not identify a specific mechanism, Buie said.

"One hypothesis we have is that [petrichor] comes from this mechanism we've discovered," he said.

Are All Snowflakes Unique?

All snowflakes tend to look unique because each flake can experience different atmospheric conditions as it falls to Earth, according to the National Oceanic and Atmospheric Administration.

The shape and size of the of the snowflake usually depends on the temperature at which it freezes. For example, long "needle-like" crystals can form around 23 F, while very flat plate-like crystals will form at 5 F, NOAA said.

Larger, more complex snowflakes that form at lower temperatures will vary in design as the vast amount of water molecules in each flake can freeze in a various amounts of ways.

The likelihood of finding identical complex snowflakes is unlikely because the number of ways to make such a snowflake is "staggeringly large," according to Kenneth G. Libbrecht, professor of physics at the California Institute of Technology.

"It's so extremely unlikely, in fact, that even if you looked at every one ever made you would not find any exact duplicates," Libbrecht said on his website.

Alexey Kljatov, a photographer based out of Moscow, Russia, photographed a collection of high-resolution magnified flakes. Kljatov, who regularly photographs snowflakes as part of his focus in macro photography utilized temperatures and relative humidity, taken from weather sites when he snapped the images.

Since snowflakes can form into a variety of shapes depending on temperature and humidity, Kljatov tried to take his pictures in a variety of different conditions.

Why Can Sleet, Freezing Rain and Snow Occur at 32 F?

These different types of precipitation can occur due to differences in temperature of the atmospheric air column above Earth's surface.

As long as the atmosphere remains consistently below 32 F between the clouds and Earth's surface, then snow will fall. When there is a warm layer in the atmosphere, snow can partially melt, then turn into sleet when it renters a cold layer before hitting the ground.

Freezing rain forms after it melts in a warm layer of the atmosphere, and then freezes on contact with the ground, when the surface temperature is at or below 32 F.

Sleet differs from freezing rain because it takes the form of small ice pellets that bounce off the Earth. Yet, this is not as dangerous as freezing rain, according to Meteorologist Brett Anderson.

"Freezing rain is by far the most dangerous because it forms a solid sheet of ice, as opposed to sleet that just has small ice pellets that quickly bounce off of the surface," Anderson said. Sleet can actually provide traction for drivers, unlike freezing rain, he added.

What Happens When Lightning Strikes Earth?

Lightning bolts regularly strike the Earth and its bodies of water, and can indeed strike the same location more than once.

When lightning strikes the ground it can often times lead to the formation of a rock called a fulgurite, NOAA said. The rock is found throughout the world, but is said to be relatively rare.

According to the National Weather Service, before a lightning strikes water, a charge builds up along the water's surface. When lightning strikes the water, most of the discharge occurs near the surface. Most fish swim below the water surface, so the lightning does not impact them, the NWS said. However, it's unknown how deep the lightning's charge can reach.