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Rain Is Coming to Burning Los Angeles and Will Bring Its Own Risks
Rain is forecast to begin as soon as Saturday
afternoon and to continue as late as Monday evening, says
meteorologist Kristan Lund of the National Weather Service’s
Los Angeles office. The area desperately needs the
precipitation, but experts are warily monitoring the situation
because rain poses its own risks in recently burned areas—
most notably the potential occurrence of mudslides and similar
hazards. “Rain is good because we’ve been so dry,” Lund
says. “However, if we get heavier rain rates or we get the
thunderstorms, it’s actually a lot more dangerous because you
can get debris flows.”
Fires do a couple of different things to the landscape
that can increase the risk of burned material, soil and detritus
hurtling out of control. When fires burn hot or long enough, they
leave an invisible layer of waxy material just under the surface
of the ground. This develops from decomposing leaves and
other organic material, which contain naturally hydrophobic or
water-repellent compounds. Fire can vaporize this litter, and
the resulting gas seeps into the upper soil—where it quickly
cools and condenses, forming the slippery layer.
When rain falls on ground that has been affected by
this phenomenon, it can’t sink beyond the hydrophobic layer—
so the water flows away, often hauling debris with it. “All of the
trees, branches, everything that’s been burned—unfortunately,
if it rains, that stuff just floats,” Lund says. “It’s really
concerning.” Even a fire that isn’t severe enough to create a
hydrophobic layer can still cause debris flows, says Danielle
Touma, a climate scientist at the University of Texas at Austin.
Under normal conditions, trees and other plants usually trap
some rain above the surface, slowing the water’s downward
journey. But on freshly burned land there’s much less greenery
to interfere; all the rain immediately hits the ground. [...]
Fortunately, the rain should also help firefighters tame
the blazes that remain active. The largest, the Palisades Fire,
is currently 77 percent contained. The second largest, the
Eaton Fire, is 95 percent contained. The Hughes Fire is third
largest and only 56 percent contained. A fire can be fully
contained but still burning. The containment percentage refers
to the amount of the perimeter that has barriers that firefighters
expect will prevent further spread.
Scientific American. January 27th, 2025. Adaptado.