Snowpack

Snow, weighed

Snow water equivalent — SWE — is how much water is standing in that snowbank. We weigh it from satellites and from snow courses walked on skis.

How it's measured

courses    manual, monthly, since the 1940s
snotel     automated pillows, hourly
lidar      basin-wide depth, flown each spring
compare    against the 30-year normal

Why it matters

When the SWE is 40% of normal in March, the reservoir operators start planning for a dry summer. We give them the number early.