The Library Of Congress website offers quite an insight into the subject of frogs, fish and other creatures falling from the sky like rain. Heres what they have to say:
"Many scientists believe tornadic waterspouts may be responsible for frog and fish rainfalls. According to Complete Weather Resource (1997), “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” (Chandler, 2004). Most scientists agree that salt, stones, fish, or frogs can be pulled into a waterspout’s swirling updrafts and deposited once the waterspout hits land and loses its energy.
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 (Chandler, 2004). 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.
When it rained frogs in Kansas City in 1873, Scientific American concluded that it must have been caused by a tornado or other land-based storm, since there were no swamps or other bodies of water in the vicinity (Cerveny, 2006). Similarly, when it hailed frogs in Dubuque, Iowa on June 16, 1882, scientists speculated that small frogs were picked up by a powerful updraft and frozen into hail in the cold air above earth’s surface. 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. For instance:
On October 23, 1947, A.D. Bajkov, a biologist with the Louisiana Department of Wildlife, was eating breakfast at a restaurant in Marksville, Louisiana when the waitress told him and his wife that fish were falling from the sky. “There were spots on Main Street, in the vicinity of the bank (a half block from the restaurant) averaging one fish per square yard. Automobiles and trucks were running over them. Fish also fell on the roofs of houses…I personally collected from Main Street and several yards on Monroe Street, a large jar of perfect specimens and preserved them in Formalin, in order to distribute them among various museums.”
On June 7, 2005, thousands of frogs rained on Odzaci, a small town in northwestern Serbia. Climatologist Slavisa Ignjatovic described the phenomenon as “not very unusual” because the strong winds that accompanied the storm could have easily picked up the frogs.
At the end of February, 2010, residents of Lajamanu, a small Australian town, saw hundreds of spangled perch fall from the sky. Christine Balmer was walking home when the rain/fish started to fall. “These fish fell in their hundreds and hundreds all over the place. The locals were running around everywhere to pick them up,” she reported."
Original post with citations may be found here: https://www.loc.gov/everyday-mysteries/meteorology-climatology/item/can-it-rain-frogs-fish-and-other-objects/