The Effect of Cold, Wet Weather on Monarchs
Contributed by Dr. Bill Calvert

Cold weather--accompanied by snow, sleet, hail and high winds--is bad news for butterflies.

The physical actions of the wind, rain and the various forms of moisture cause the clusters to become unstable. Butterflies peel off singly or in groups and, since it is usually too cold for them to fly, they fall to the ground. Sometime whole branches are blown down laden with butterflies. Falling kills or injures only a few butterflies; most survive this insult.

Butterflies on the ground after a storm.
Dr. Lincoln Brower, Sweet Briar College
..but the real problem comes AFTER the storm...

The real problem for them comes when the skies clear after the storm.

As heat escapes into the night sky, the ground and forest cool rapidly. (This is called "radiational cooling.") If temperatures drop to the mid-to low 20's (F) the butterflies begin to freeze to death. Monarchs are essentially tropical butterflies and cannot tolerate sub-freezing temperatures for very long.

Why do you suppose it is worse for a butterfly to be on the ground during cold weather, instead of clustered in a forest? (Think about this before reading on!)

Even Dew Drops Can Be Dangerous
Cold, wet weather can be lethal for monarchs. Consider what happens:

  • Colder temperatures take the air further below its dew point. (That is because colder and colder air holds increasingly less moisture.) The colder it gets, the more moisture drops out of the air in the form of dew. As dew forms it coats the butterflies.
  • When dew freezes, the water turns into ice crystals. These cause physical damage to the butterflies. The ice crystals can puncture the butterfly's body through the exoskeleton.
  • Rain, of course, has the same effect when it freezes.
Dew becomes frost when it freezes. Frost has sharp ice crystals that can puncture a butterfly's body

There's another reason it's bad for the butterflies to be on the ground in the open: It's colder there. It is warmer in the forest amongst the dense clusters of butterflies. This is because heat radiates from the trees and from the butterfly masses themselves. (Dr. Lincoln Brower often says the forest acts like a blanket and an umbrella, protecting the butterflies from cold and moisture.)

National Science Education Standards

Physical Science
Materials can exist in different states?solid, liquid, and gas. Some common materials, such as water, can be changed from one state to another by heating or cooling.(K-4)

Life Science
Changes in environments can be natural or influenced by humans. Some changes are good, some are bad, and some are neither. (K-4)