There are some experiences that you never forget no matter how long you live. And some people that really make an impression.

Octogenarian Hope Whipple made an impression and had an unforgettable experience. A birder and nature lover, Hope described to me a swarm of white butterflies in her yard in numbers that she had never seen in her 88 years. She wondered if I had ever seen or heard of such a profusion of pollinators, and what would cause this fantastic flyby.

While seeing a handful of butterflies is quite common, observations of masses of them, while infrequent, do happen and are clearly memorable. When large numbers of butterflies get together, they are referred to as a kaleidoscope, swarm or rabble.

Butterflies gather in great numbers for a few reasons. The most common and well known is to migrate. Monarchs are migrators that the lucky among us might see in groups as they are moving north or south depending on season. Numbers can be impressive, and in 2014 their large migration mass showed up on weather radar. The red admiral is also a migrator that can be observed in massive assemblages.

Another reason butterflies come together is to make mass flights. These movements can be in response to poor weather conditions, such as drought or heavy rains, that reduce habitat quality and send butterflies packing for greener (and healthier) pastures. As one example, consider the American snout butterfly. In Texas in 1921, one million snouts per minute crossed a 250-mile wide area.

But to return to Hope Whipple’s sighting, what she observed was neither a migration nor a mass flight. My first clue was the description of the butterflies’ color. The white butterfly she described was most likely a cabbage white. This species is known for its occasional congregations and is sometimes referred to as summer snowflakes. Cabbage whites can appear in larger numbers during periods of hot weather when multiple broods are born in one summer or when they gather to lay eggs.

Large numbers of summer snowflakes are understandable when you take into account this butterfly’s reproductive potential. One female cabbage white lays 200 eggs each cycle of multiple broods. When predation is low and their survival rate high, a single female can have 20,000 descendants.

Reports of rabbles of cabbage whites are well documented. These butterflies have been observed crossing the English Channel in swarms of hundreds. Jan Ritzema Bos relates another cabbage white amassing in Holland in her book Agricultural Zoology: “Another swarm, about a league broad, was seen in the air....and this swarm, flying without cessation, took from twelve noon to seven in the evening to pass over. It must have therefore have contained many million butterflies.”

Beyond science, the spiritual comes into play when one sees white butterflies. According to Chinese mythology, a white butterfly is the soul of a departed loved one or angels watching over you. Native American lore suggests that those white butterflies carry your dreams and provide protection, good luck and a good life.

Life couldn’t be better when you are treated to the miraculous sight of those summer snowflakes or any other butterfly bounty. Let’s stay ‘Hope’-ful for many such summer sightings. American biologist and founder of the North American Butterfly Association Jeffrey Glassberg summed it up well: “Beautiful and graceful, varied and enchanting, small but approachable, butterflies lead you to the sunny side of life. And everyone deserves a little sunshine.”

Suzan Bellincampi is director of the Felix Neck Wildlife Sanctuary in Edgartown, and author of Martha’s Vineyard: A Field Guide to Island Nature.