It's Raining Animals!
Historically, there have been numerous animal rains. These include, fish, frogs, toads, worms, jellyfish, etc. Most are believed to occur when small animals are lifted and dropped by tornadic waterspouts viz a tornado that forms inland and travels over water.
If you thought that it could only rain cats and dogs, you are in for a surprise! In some places around the world, people have actually reported witnessing a rain of spiders. Particularly, some regions of Australia and South America have seen a series of such events in recent years.
While thousands of spiders suddenly falling out of the clear blue skies may seem like a scene from the X-Files - inspiring the possibility that it may be a supernatural occurrence, in reality, the reason behind this happening is pretty straightforward and actually completely natural.
Arachnologists explain that, in some species of spiders, the hatchlings are known to make use of a technique known as ballooning for relocation. When the spider eggs (several to a thousand in number) hatch, the baby spiders tend to move away from their siblings, and spin their webs in far-off locales in order to avoid competing for food.
The baby spiders climb up on higher places, such as a tree branch, fence, etc., and spin out strands of silk into the air. When a breeze blows, these strands gets caught in the draft and begins to rise.
Since the spiders themselves are very small and lightweight, they too are carried along, and thus get transported from one locale to another. Depending upon the strength of the draft, the spiders can rise to very great heights, and are known to travel several miles.
This process used by baby spiders to migrate is known as ballooning. Ballooning is also observed among the adult members of some species of small spiders. It is considered to be one of the prime reasons why spiders have been able to spread across the world in diverse locations, including even remote ones such as Antarctica (though they don't survive there).
In the first, several baby spiders balloon together at the same time. This event, though not very frequent, takes place in the months of May and August.
In the Outback, many young spiders throw up their webs at the same time, and rise up to the sky traveling on the blowing winds. At the end of their journey, when they descend in large agglomeration, it appears like snow fall (due to the webs), coupled with a rain of baby spiders.
The second event that leads to spider rain is what was seen in Wagga Wagga Australia, that left vast areas covered in silk strands. It is believed to have been the result of spiders fleeing their homes after heavy rains caused eastern regions of Australia to get flooded. Around 13,000 people, with several resident spiders migrated in search of drier grounds.
As the flood waters covered the land, several spiders living on ground or in burrows had to move up into the foliage to avoid drowning. Like the migrating baby spiders, these too threw silk strands into the air to balloon themselves out of area that was hit. When millions of these spiders subsequently fell from the sky, it appeared as raining spiders.
Spider Rain in Brazil
The spider rains commonly seen in Sao Paulo and neighboring areas are a result of a phenomenon that can be best described as unintentional ballooning.
Scientists note that a small spider species, the Anelosimus eximius, found in this region, swarm together like ants or bees forming extensive colonies. The total number of members in these colonies can sometimes even exceed 50,000.
As each spider contributes to the building process, enormous webs are frequently seen. There have even been reports of some of the webs of this species extending to heights of more than 60 feet.
When a strong wind blows, these gigantic web formations detach and rise up in the sky, carrying the spiders along. Depending upon the wind, these flying webs can carry the spiders for several miles. When, at the end of this unexpected migration, the wind strength reduces sufficiently, the webs and the spiders, 'rain' down from above onto the new location.
Thus, spider rains aren't supernatural events, and as described, they do in fact have scientific explanations. It is important to note that, in all such events that have been reported, the spiders that fell from the sky were completely harmless to humans, and crawled away as soon as they landed on the ground.
Hence, even though spider rains might be an arachnophobe's worst nightmare, they are a safe, natural phenomenon, which is not very common.