Last week the remnants of Hurricane Simon passed through Tucson. The city received just an inch of rain, but the story was different up in the mountains. The summit of Mt. Lemmon, rising almost 7000′ above the city, recorded five inches of precipitation in a single day. The reason is orographic lift – mountains force air to rise, where it cools and loses its water storage capacity, resulting in precipitation. It’s one of the reasons some of the wettest places in the world are on windward mountain slopes.
These mountains are primarily hard rock, with very little soil development. The result is that five inches of rain can’t be buffered underground, leading to surface flows, rockslides, and waterfalls.
One of the consequences of all this rain is a canyon ecosystem full of life, once the flash floods clear out.
Here is some milkweed (Apocynaceae) with a caterpillar on a branch – maybe a queen butterfly (Danaus sp.)?
In five years of living in the desert, I’ve never seen the mountain so dynamic, or shrouded in clouds. It was beautiful.