This is the second segment of a three-part series on the monsoon. View the first part here.
Rain comes when water molecules in the atmosphere clump together to form ice crystals or water droplets that are heavy enough to fall to the ground. But what causes that clumping to happen?
It turns out rainfall, including the Southwest’s summer monsoon storms, depends on tiny particles. They’re what scientists call “ice-nucleating particles,” or tiny bits of matter that begin the formation of ice crystals high in the freezing upper atmosphere. Once those crystals form, they can grow rapidly as more water molecules glom on.
Atmospheric dust commonly promotes the growth of ice crystals in dry climates like ours. But in recent years, scientists have discovered that living things, too, can play a role. Bacteria, spores of fungus, and even tiny bits of plant matter can be lofted high into the atmosphere by strong winds and updrafts, and they can remain airborne for a long time.
A striking example of the role these materials play in the formation of ice particles came in a study conducted by the University of California, San Diego and the National Center for Atmospheric Research. Researchers collected samples in clouds high above Wyoming and analyzed them in flight, and they learned that the great majority of ice particles contained cores of either dust or biological material.
Climate scientists are examining how changes in atmospheric concentrations of such particles may alter future precipitation patterns…. And that might help determine how the Southwest monsoon may be affected by climate change.
This Earth Note was written by Peter Friederici and produced by KNAU and the Sustainable Communities Program at Northern Arizona University.
