The winter holiday season in Western Oregon is also the height of the rainy season: historically, December is our wettest month of the year, logging in at about 8½ inches of precipitation for the month.
I’m actually looking forward to standing at the window, watching it bucket. While I don’t like worrying about floods and leaks and muddy floors, I do relish the sight and sound and smell of a good solid rain —and I enjoy the coziness of being indoors during a downpour.
Rain seems pretty simple.
Rain falls from clouds; clouds form when invisible water vapor in air condenses to form visible water droplets. Like warm breath steaming up a cold windowpane, condensation occurs when moist air is sufficiently cooled. (The temperature at which the water vapor condenses to liquid, called “dewpoint,” varies with air pressure and the amount of water vapor in the air.) In a cloud, minute air-borne particles, such as dust or pollen, serve as nuclei or foundations for the gathering droplets.
Clouds can naturally form when relatively warm air is forced up to cooler altitudes (by running into mountains, for example), when air masses of different temperatures collide or squeeze together, when air moves over cooler surfaces, or when temperatures fall at night.
Fog is the result of vapor condensation at the surface — in effect, a ground- or water-hugging cloud. In fog, the vapor droplets have not yet clumped and are still small enough to remain airborne. Mist is very thin fog, sometimes with droplets that have grown to nearly falling size.
As the air currents in the cloud mix and tumble the condensed vapor, tiny droplets bump into each other and coalesce into larger droplets.
The cloud’s environment may change before the droplets grow very large, evaporating the water back to vapor and dissipating the cloud. Or droplets may coalesce to a size too large for the internal air currents to hold aloft and the droplets will fall from the cloud.
We generally label this falling-droplets-event according to the size of the droplets. Drizzle occurs when the falling droplets are under 0.5mm (0.02 inches) in diameter; rain occurs when the falling droplets are between 0.5mm and 5mm (0.02 and 0.2 inches).
In cold air, cloud vapor freezes, creating snow, hail or other forms. In very cold clouds or at ever-chilly high elevations, all the vapor in clouds is frozen. If the air is cold enough all the way down to the earth’s surface, the cloud’s precipitation will be called snow, hail or graupel (soft hail); if the air between cloud and land iswarm enough, the frozen droplets will melt before landing. The opposite can occur, too: precipitation can begin as rain and freeze en-route or as it lands.
Why don’t raindrops get huge? Raindrops that grow larger than 5mm become unstable as they tumble through the cloud or fall through the air, breaking up to form several smaller drops.
And falling raindrops aren’t actually “raindrop” shaped: the familiar pointy-top, round-bottom shapes are really “rain drips.”
Small airborne water droplets are round, pulled into a sphere by the surface tension of the water when they fall freely. Large raindrops tend to become hamburger-bun-top-shaped as they fall and encounter resistance with the air, wobbling between spheres and bun-tops as they tumble. Very large raindrops thin out in the middle as they wobble to become doughnut-shaped before breaking apart into smaller drops. The resulting smaller drops continue to grow and wobble as they tumble and fall.
You can often witness changes in the general size and timing of falling drops as the cloud cover changes, the storm progresses, or the winds shift. Tracking the range from drizzle to “bucketing” can be a fascinating observation: you’ll find that far from being incessant or constant, our rain is typically intermittent and varied.
If you’re physically out in the rain, experiencing raindrops directly, it may help to recall the old saying, “There’s no such thing as ‘bad weather,’ only ‘inadequate clothing.’”
Visit Wavecrest Discoveries on Facebook for a link to Alistair McClymont’s video of a falling raindrop. For information on how you can arrange an exploration of our fascinating natural history, contact Marty at 541/267-4027, or e-mail firstname.lastname@example.org. Questions and comments about local natural history are welcome.