How Cold Are the Overwintering Sites in Mexico?

The graph shows two important things: It shows the average temperature at the overwintering sites in Mexico during each month of the year and the graph shows when the monarchs arrive and when they leave.

During the months that the monarchs are in Mexico, overnight low temperatures hover just above freezing. The night-time temperatures are similar to those in your refrigerator!

The forest shelters monarchs by keeping a very delicate balance:

  • The forest protects the monarchs from extreme cold. If the butterflies get too cold, they will freeze to death
  • The forest protects the monarchs from getting too warm. If the butterflies get too warm, they will burn their stored fat too quickly.

With regard to temperature, "The really critical time for the monarchs is January and early February when it's the coldest time of the season," says Dr. Brower. "We know temperatures below -8 degrees C will kill about 50% of the monarchs. If the butterflies are WET, it will kill close to 80-90% of the monarchs."

Thinking More About the Graph
  • To get comfortable with the concept of "average low temperature," record your own low temperature each night for a few days, then calculate the average.
    • How many degrees did your temperatures fluctuate above and below the average?
    • Dr. Brower says "-8 degrees C will kill about 50% of the monarchs." Look back at the graph. Draw your temperature fluctations beside the values shown. Did your average temperatures fluctate enough to be of concern?
  • Write the temperature for each month on the graph in degrees Fahrenheit. Then edit the scale to Fahrenheit. (Here is a conversion scale to help you.)

This graph shows the average temperatures. This means the daily low temperatures can fluctuate above and below the temperatures shown.