Adapted from Jim Conrad's online book A Birding Trip through Mexico, This excerpt from "Nevado de Toluca Volcano" in México state

In the afternoon storm clouds boil over the western ridge threatening to wash back and upward to engulf my slope so I hike back toward the hut. Arriving there with sleet bouncing off my shoulders and certain gusts of wind almost knocking me to the ground I open the hut's heavy wooden door to find a companion for the night, a young man named Horacio. He's ridden here on his mountain bike along off-road trails from Toluca. Handsome and muscular, he sits at the hut's wobbly wooden table writing a children's story by candlelight. Instantly we're friends, and we talk into the night.

Next morning I take off for the crater again but see nothing I hadn't seen the day before. This time I spend more time sightseeing. From Nevado's northern slope the view into the broad valley below is majestic. Through binoculars nearly every major building and street in Toluca can be recognized. At midday I return to the hut to find Horacio gazing into the valley. He draws my attention to the fact that from horizon to horizon the sky is clear, except exactly over Toluca.

There, floating like an enormous mushroom with a bulbous cap, a dazzlingly white, billowing cumulus cloud casts its shadow onto the city below. Horacio says he's been watching the cloud grow from a single wisp, and that during other visits he's seen something curious many times: Hot air from Toluca's pavement and buildings rise into the sky, cools, and as the moisture in this air condenses, substantial clouds like this one form.

"For the rest of the day, keep an eye on this cloud," Horacio suggests. "Sometimes the very same cloud stays visible until dusk."

A couple of hours later the cloud has drifted toward the northwest and it's developed a rounded thunderhead towering so high that it spreads into a broad anvil shape.  Above Toluca, a second cloud has formed just like the first.

By dusk the sky all around has grown moody with dark-bottomed clouds, but we can still pick out our midday cloud, now grown into a massive purple bank, an immense thunderhead looming over smaller storms all around it. It's too far away to guess at its distance. Between this first cloud and Toluca now there rise four big clouds just like the first, all in a row and  all formed over Toluca.

At nightfall an impressive display of lightning takes place beneath our distant cloud, though it's too far away to hear the thunder. Surely by now this storm is in the next state, in Querétaro or Guanajuato.

I have never seen such a plain example of humanity altering the weather.