CLOSEUP Water droplets, captured on a grassy field, look like crystal globes.
By Pierre Ratte

This weekend’s rain released earthy fragrances, falling gently over two days and lessening fire-season anxieties. Maybe the rain is ending this year’s fire season and beginning winter’s filling of reservoirs and the Sierra’s snowpack. Last winter, Northern California received epic rain and snow. Let’s see what this winter brings.

CLOSEUP Water droplets, captured on a grassy field, look like crystal globes.(Photo by Pierre Ratte)

Fun facts: Water droplets are perfectly round—sometimes. Principles of cohesion act to form water droplets’ roundness. A droplet falling through the air takes on a spherical shape trying to minimize surface area to volume for aerodynamic efficiency—round shapes being best at subsonic speed and pointy shapes best at supersonic speed. If air resistance is too great, which happens when water droplets become too large, the droplet flattens like a hamburger bun.

Water is the stickiest non-metallic liquid. It likes itself. It’s predisposed to liking itself because of its atomic properties. Two positively charged hydrogen atoms nestle on one side, and a lone negatively charged oxygen atom is on the opposite side. With positive and negative sides, water is self-attracted like two magnets with opposite polarity. Stickiness, known as cohesion or adhesion in physics, is “like sticking to like” vs. “like sticking to other.”

The cartoon version of water dropping as a “teardrop” is not accurate, but suggestive. When rain forms, the small droplets are round. They attract other droplets until they become heavy enough to fall out of the cloud. When the falling droplets get larger, approximately 3 millimeters, they flatten until they break and form little round spheres again, which then attract other water droplets to grow larger, and so it goes. One might call it the rain cycle of a single droplet of water.

Modern weather radar with two polarized signals picks out round shapes and flattened shapes of rain falling, thereby better estimating the rate of rainfall.

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