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.
Weather forecasting has become big business on the evening news. Surveys of viewer interest show that local weather is the most popular portion of the news. Whenever the weather acts up and makes headlines, local newscasts as much as double their ratings. Along those lines, weathercasters are the most recognized personalities in a local television market.
Braving the Elements
The most famous weather day is February 2, known as Candlemas Day as well as Groundhog Day. For centuries, Candlemas Day was thought to mark an important point in winter, possibly a turning point. "If Candlemas Day be fair and bright, winter will have another flight. But if Candlemas brings clouds and rain, winter is gone and won't come again." Later, bears, badgers, and woodchucks were brought into it, and if they saw their shadows, six more weeks of winter would follow. Supposedly, the critters would be afraid of their shadows, and would return to their dens. Of course, those shadows could be seen only if Candlemas Day were bright and clear. Does this work? Not really, except that you can't trust a quiet spell during the normally harshest part of the season. A quiet February can lead to a very rough March.
Red Skies at Night
Here's a nugget of weather wisdom that goes back to the beginning of time:
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
I just made that one up, because who wants to read a book about weather and never know why the sky is blue?
Braving the Elements
Are you dreaming of a White Christmas? For most of the Northern Hemisphere, a White Christmas really is a dream, with the odds of snow cover for December 25 being far less than 50-50. But, according to weather lore, "A green Christmas makes a fat churchyard." The thought is that a warm Christmas and winter will eventually lead to sickness and disease. Because the weather is always busy balancing its extremes, a warm winter will eventually be balanced by colder weather—perhaps during the spring and growing season. Resulting crop failures will cause food shortages and possibly contribute to that fatter churchyard.
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
Or the flip side:
Here is some wisdom of the ages that applies to the timing for hurricanes, the greatest storms on Earth: "June—too soon. July—standby. August—look out you must. September—remember. October—all over." These storms develop in the tropics and feed off the warm oceans. During June, the waters are just warming up, and often it is too soon for storms in the Atlantic, although they can form then, especially in the Gulf of Mexico. By August and early September, the oceans reach their highest temperatures, and the hurricane season peaks.
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.
The month of March is named after Mars, the Roman god of war, and for good reason. It's one of the most belligerent months on the calendar. Sharp temperature contrasts create strong winds and often the biggest storm of the winter appears in early March.
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.
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.
A temperature inversion occurs when the air near the ground is much colder than the air above. Instead of falling with elevation, the temperature rises with elevation. This activity takes place most often during a calm, dry, and cloudless night. Refraction is the bending of light as it passes from one medium into another.
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.