Weather: Getting Down to Basics

Getting Down to Basics

Although this introduction to weather forecasting may seem flip, it is meant to show the importance of understanding climate in the weather-prediction process. And when you look outside and observe the weather over a period of time, your skills sharpen even more. In my weather office, I would much rather consult with someone who works outside as a farmer or a construction worker than talk to a new post-doc in meteorology. Experience combined with climate becomes key to figuring out what might be next. Weather and people formed those analogs and passed them along from one generation to another. It's called weather lore, and much of it really makes sense. I have my favorites.

Red Skies at Night

Here's a nugget of weather wisdom that goes back to the beginning of time:

• Sky red in the morning
• Is a sailor's sure warning.
• Sky red at night
• Is the sailor's delight.

References to this useful forecasting tip can even be found in some versions of St. Matthew's Gospel—and it should be the gospel for anyone who needs a quick idea of what the weather's about to do. There's a good reason why this saying has stood the test of time: It works.

Normally the weather moves from west to east. If clouds and storms are approaching, the first sign will be an increasingly threatening western sky. Sunlight consists of all the colors of the rainbow, including red, which represents the longest wavelength of the spectrum. When the sun is low on the horizon, all the other colors can be scattered and diffused away by atmospheric elements including dust and water vapor—but the longest wavelength, red, lingers. Shorter wavelengths scatter the most. So at sunrise or sunset, the sky tends to be red.

The red sky at night is a delight because it can only appear if skies are clear in the west—or at least if clouds have broken in the west while the sun goes down. Because the weather moves from west to east, that clear or clearing sky is coming this way. Just the opposite is true in the morning, when an overcast western sky and a clear or partly cloudy eastern horizon will cause the sky to turn red as the sun comes up. If the sky were overcast in the east, those red rays of sunlight wouldn't appear. Again, because the weather moves toward the east, the heavier western horizon clouds are on the way and sailors should take warning.

Blue Skies, Too

• When the sky is blue,
  It will be true.
  When the sky turns white,
  Get ready for a fight.

I just made that one up, because who wants to read a book about weather and never know why the sky is blue?

Sunlight contains all the colors of the rainbow, with each color having a different wavelength. The shortest wavelength, blue, is preferentially scattered the most by microscopic molecules and particles suspended in the air. When the air is really dry and clean, that scattering is most effective. As that light is scattered, the sky takes on a brilliant shade of blue. If the particles are too large—if the air is packed with pollution and moisture—there is no preferential scattering, and the sky looks hazy and murky. So when the weather is going to really shine, often with a crisp, clean breeze out of the northwest, that deep blue sky is there for all to see. But frequently, when the wind comes from the moist southwest direction, the sky turns hazy or cloudy, the weather becomes less comfortable, the air quality diminishes, and storms often begin to appear.

Know Which Way the Wind Blows

• When the wind is from the west,
  The weather is at its best.

Or the flip side:

• When the wind is from the east,
  It is good for neither man nor beast.

Now this does not hold universally true. Except for the west coasts of continents, the west wind comes from land areas and it is a dry wind. The weather is generally settled. But when the wind is from the east, the weather turns overcast and stormy. Because of the counterclockwise circulation around storms as they approach, the wind comes out of the east. That east wind gathers moisture from the ocean and skies eventually open up. Few ancients had anything good to say about east winds. Aristotle and his student Theophrastus warned about it. Even in Exodus it was written, "The east wind brought the locusts." Still an east wind can be very exciting as a storm gathers and makes its approach. Storm lovers and cyclopaths look forward to those days with an east wind. When the rain blows in from the east, "It is four and twenty hours at least." In other words, a day of rain is on the way.

Literature is rich with references to halos around the Moon or Sun being predictors of approaching storms. Those halos form when high, thin clouds are present. (We will be flying through all types of clouds in later chapters.) These particular clouds are so high that they always consist entirely of ice crystals. They are called "cirrus" clouds, a Latin word meaning thin, wispy, or feathery. When light passes through the ice crystals, the light bends, as if going through a prism. All types of optical effects are possible: rainbows, or double or triple moons and suns. The most common is a ring or halo.

Because those high, thin clouds frequently precede a storm by about 24 hours or so, those halos are solid indicators that the weather will take a stormy turn. Not all storms are preceded by cirrus clouds, but those that stay around for a day or so seem to always telegraph their approach by first sending in those clouds. A major change in weather is on the way.

Storm Signals

Now that we have observed the color of the sky, watched for halos, and kept an eye on the wind, it is time to stop and listen.

• Sound traveling far and wide,
  A stormy day will betide.

The next time you go for a walk listen to the different sounds. There are days when distant sounds can be heard sharply and distinctly. Often as a storm approaches warmer air pushes overhead while colder air clings to the surface. The air actually warms the higher up you go. That's called a temperature inversion because the normal temperature profile is inverted. It usually gets colder in the upper layers of the atmosphere.

At any rate, the speed of sound increases with increasing temperature and the sound waves themselves bend when going from a layer of one temperature to a layer of another temperature. That behavior is called refraction and is similar to what happens to light when it goes through ice crystals, as in cirrus clouds. When warmer air overlies colder air, upward directed sound waves will bend back to the ground. The sound is focused on where you are standing. Thus a distant train whistle or the hoot of an owl will seem much closer.

Excerpted from The Complete Idiot's Guide to Weather © 2002 by Mel Goldstein, Ph.D.. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.