If it is raining fishes, frogs, spiders and even algae, call a scientist to explain things!

A blanket of millions of baby spiders was spotted falling from the Southern Tablelands( Australia) sky recently, alarming residents whose properties were suddenly covered with not only the creepy critters, but also mounds of their silky threads.
Ian Watson, who lives in the region affected by the spooky shower, took to Facebook to describe what this strange "weather" looks like.

"Anyone else experiencing this "Angel Hair" or maybe aka millions of spiders falling from the sky right now? I'm 10 minutes out of town, and you can clearly see hundreds of little spiders floating along with their webs and my home is covered in them. Someone call a scientist!" Watson wrote on the Goulburn Community Forum Facebook page.
And here is what scientists told these scared residents to soothe their nerves (1):
Rick Vetter, a retired arachnologist at the University of California, Riverside, said Watson and his neighbors likely saw a form of spider transportation known as ballooning.

"Ballooning is a not-uncommon behavior of many spiders. They climb some high area and stick their butts up in the air and release silk. Then they just take off. This is going on all around us all the time. We just don't notice it."

The reason people don't usually notice this ingenious spider behavior is that it's not common for millions of spiders to do this at the same time, and then land in the same place, said Todd Blackledge, a biology professor at the University of Akron in Ohio. "In these kinds of events [spider rains], what's thought to be going on is that there's a whole cohort of spiders that's ready to do this ballooning dispersal behavior, but for whatever reason, the weather conditions haven't been optimal and allowed them to do that. But then the weather changes, and they have the proper conditions to balloon, and they all start to do it".

This is likely what happened in New South Wales, where certain species of small spiders — as well as the tiny hatchlings of larger spider species— are known to balloon around the Outback during late autumn (May) and early spring (August). But, as Blackledge explained, an abrupt change in the weather or wind pattern may have carried these migrating spiders up and away and then back down to earth en mass — not the orderly dispersal that they (or the residents of the Southern Tablelands region) were expecting.

And why do other animals like frogs and fish fall from the sky?
According to scientists, the most likely explanation for how small frogs get up into the sky in the first place is meteorological: a whirlwind, tornado or other natural phenomenon. Frogs and fish do not live in the sky, nor do they suddenly and mysteriously appear there; in fact they share a common habitat: ponds and streams. It's certain that they gained altitude in a natural way.S o, tornadic waterspouts may be responsible for frog and fish rainfalls. A tornadic waterspout is merely a tornado that forms over land and travels over the water. An especially strong kind of waterspout, they are not as strong as land based tornadoes, which can reach up to 310 miles per hour. But tornadic waterspouts can reach 100 miles per hour, which can still be quite destructive.

A popular misconception is that waterspouts “rise out of the sea.” In reality, they begin in the air and descend toward the water’s surface. The first visible sign of a tornadic waterspout is usually a dark spot on the water’s surface, which is caused by a spinning column of low-pressure air stirring up the water from overhead. As the spinning column of air, or vortex, gains momentum, the surrounding water is pulled into a spiral pattern of light and dark bands. Eventually a ring of spraying water, called the cascade,forms around the base. The characteristic funnel extending from the sky toward the water’s surface becomes visible in the fourth stage of the waterspout’s development. At this point, it is considered a mature storm.

Like a tornado, a mature waterspout consists of a low-pressure central vortex surrounded by a rotating funnel of updrafts. The vortex at the center of these storms is strong enough to “suck up” surrounding air, water, and small objects like a vacuum. These accumulated objects are deposited back to earth as “rain” when the waterspout loses its energy. Most of the water seen in the funnel of a waterspout is actually condensate —moisture in the air resulting from the condensation of water vapor.

Professor Ernest Agee from Purdue University says, “I’ve seen small ponds literally emptied of their water by a passing tornado. So, it wouldn’t be unreasonable for frogs (or other living things) to ‘rain’ from the skies”. Most scientists agree that salt, algae, stones, fish, or frogs can be pulled into a waterspout’s swirling updrafts and deposited once the waterspout hits land and loses its energy. When red algae came down, the rain looked red in colour in some instances.

Although waterspouts are the most commonly offered explanation for animal rainfalls, some scientists, such as Doc Horsley from Southern Illinois University, theorize that any unusually powerful updraft could lift small organisms or organic material into the sky during a storm . An updraft is a wind current caused by warm air from high pressure areas near the earth rising into cooler, low-pressure areas in the atmosphere. Because the cooling causes water in the air to condense, updrafts play an important role in cloud formation and storm development. During thunderstorms, updrafts can reach speeds of more than 60 miles per hour— comparable to the winds of moderate-intensity waterspouts.

Although no one has actually witnessed an updraft lifting frogs off the ground, the theory is scientifically plausible since updrafts regularly pick up lightweight debris and carry it considerable distances.

What is unusual in reports of animal rainfalls is the uniformity of the deposition. When it rains frogs or fishes, witnesses report only fish or only frogs falling. According to William Hayden Smith of Washington University, this makes sense since objects of similar size and weight would naturally be deposited together. As winds lose their energy, the heavier objects fall first and smaller objects drop later.

Despite the numerous reports of raining animals, scientists still approach the area with skepticism. Many historical reports are provided by second or third-hand accounts, making their reliability questionable. Also, because of the popularity and mystery surrounding stories about raining animals, some people falsely report an animal rainfall after seeing large numbers of worms, frogs, or birds on the ground after a storm. However, these animals did not fall from the sky. Instead, storms fill in worm burrows, knock birds from trees and roofs, wash fish onto the shores of rivers and ponds, and drive frogs and other small animals from their habitats. People who live in suburban or urban environments tend to underestimate the number of organisms living around their homes. Therefore, they may suspect that animals came from the sky rather than their natural habitat.

Despite the cautious skepticism of the scientific community, a number of eyewitness reports strongly suggest rainfalls of frogs, fish, and other materials on occasion (2).

References:

1. http://www.livescience.com/50856-spider-rain-explained.html
2. http://www.loc.gov/rr/scitech/mysteries/rainingfrogs.html