Q&A: Anatomy of a Rainstorm

Question: I was texting with a friend yesterday and he said it was raining at his place and had to go inside. Weird thing was, it wasn’t raining at my place and he only lives like three block away! I didn’t get any rain at all, but he said it rained for about 30 minutes at his place. I mean, I know it doesn’t rain everywhere at the same time, but we are so close I was expecting at least a drizzle. How can that happen? — CL, Mesa, AZ

Answer: It’s really not that surprising when you think about it. When rain falls, it does so over a finite area. And sometimes the edge of that area will lie between you and your friend’s homes. Even if he lived next door it could still theoretically happen. You might want to read my post Rain is a Local Phenomenon. And when it’s raining on your friend, you may or may not get any drizzle depending on which way the wind is blowing.

The top photo shows a large mass of stratocumulus clouds (the dark grey ones) moving east to west over Phoenix. The view is to the south. Note how the rain is slanting to the west from the wind. The next photo is a close-up. Both can be clicked and enlarged:

What’s happening here is not the rainstorm you described, but it shows the same effect. This photo was taken years ago during the 2014 monsoon season. When stratocumulus move over Phoenix they encounter warm rising air. Phoenix is a well known example of an urban heat island. All the concrete and asphalt surfaces in the city heat up during the day, and release that heat only slowly after sunset. Rural areas with natural ground cover cool off much more rapidly.

That rising air drags humid air from the stratocumulus along with it. As this humid air expands and cools, additional water vapor condenses into liquid droplets. This is what forms the puffy white cumulonimbus clouds above the stratocumulus. As always, if the droplets grow large enough they will fall as rain. Note how the rain is visible mostly below these cumulonimbus clouds.

That largest concentration of rainfall below the highest cloud is probably covering most of downtown Phoenix. But you can see the east and west limits, and a gap of no rain before the next concentration to the east. You and your friend were simply on opposite sides of the weather, and the edge passed between your homes.

Next Week in Sky Lights ⇒ Myth of the Watering Hole

The Doomsday Glacier
Myth of the Watering Hole