Michael Raupp, professor of entomology at the University of Maryland, offers this answer.
Imagine a monarch butterfly searching for nectar or a mate in a meadow on a humid afternoon in July. Suddenly, a fast-moving thunderstorm approaches, bringing gusty winds and large raindrops. For the monarch and other butterflies this is not a trivial matter. An average monarch weighs roughly 500 milligrams; large raindrops have a mass of 70 milligrams or more. A raindrop this size striking a monarch would be equivalent to you or I being pelted by water balloons with twice the mass of bowling balls.
Amateur and professional lepidopterists tell tales of butterflies darting into protective vegetation and scrambling beneath leaves when dark skies, strong breezes and the first raindrops signal an imminent storm. During heavy rains and wind, butterflies are rarely seen. Not only does rain pose a direct threat of injury or death, but the cool air associated with storms may also reduce temperatures below the thermal threshold for butterfly flight. In preparation for flight, these aerial acrobats expose their wings to direct sunlight, which rapidly warms their flight muscles. Overcast skies limit their ability to gather the solar radiation needed to take wing. A butterfly knocked from the air by raindrops thus faces the double threat of crashing in an inhospitable habitat where predators lay in wait and being unable to warm its body sufficiently to regain flight. Little wonder, then, that when skies darken, butterflies seek shelter in their nighttime homes.
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Butterflies are quiescent when it is dark and take refuge in protected locations called roosts within one or two hours of sunset. Roosts may be tall grasses, perennial herbaceous plants, tangled thickets of woody shrubs, undersides of large leaves, caves or, in some cases, man-made objects such as fences or hanging baskets. Butterflies may also roost in the vegetation beneath overhanging trees. The leaves of the upper canopy intercept raindrops and reduce their impact on vegetation and butterflies below.
Several species of neotropical butterflies, such as the zebra butterfly, Heliconius charitonius, roost in the company of their peers. Perhaps as a result of the good company, Heliconius exhibits curious fidelity to roosts, often returning to the same location or individual plant for several nights. When rain threatens, zebra butterflies enter their nocturnal roosts much earlier than they would on clear days. And, like us humans, they demonstrate considerable lethargy on rainy mornings, delaying their usual early departure by as much as several hours. Unusually long stretches of rainy weather may even reduce the population of butterflies in a roosting group, because cool temperatures hinder their mobility and therefore their ability to escape from predators.
Ultimately, what butterflies do in the rain is avoid it. But with the return of sunshine following a summer shower, they often resume patrolling and courting within minutes. So the next time the sky darkens and thunder rumbles, take a cue from the butterflies. Find a safe roost out of the rain, but as soon as the sun returns, go out and enjoy.