Basic Illustrated Weather Forecasting

Introduction

Tonight’s weather is dark, followed by widely scattered light in the morning.…

—GEORGE CARLIN,

from his Hippi Dippi Weatherman routine

A number of years ago, actually more years than I care to remember, two friends and I were backpacking along the Appalachian Trail just north of Great Smoky Mountains National Park. We were on a week-long backpack to celebrate the independence of our sixteen-year-old spirits.

Just after dawn on the second day of our adventure, the clouds, which had been stacking ominously for hours, released a deluge of water lasting for several hours. The deluge reduced to a steady drizzle that continued until evening. Soaked and somewhat dispirited, we trudged into camp, happening upon a crusty old traveler slouched against a pack that appeared as if it may have been a prototype for later cruiser-frame models.

I glanced pensively up at the lowering ceiling, which would soon turn to a dense mist, and then over to the old-timer who had either been ignoring our dripping entrance or was unaware of our presence—I sensed the former. Shuffling closer, not wanting to offend, I managed a somewhat unsure, Mister, is this weather going to keep up, or do you think it’ll be sunny tomorrow?

With a cough the old-timer lifted the brim of his felt hat, gazed intently at me and my friends for what seemed an eternity, and then, without smiling, drawled, Waell boys, I kin guarantee ya’ll one thing fer sher. Come mornin’ there’s bound to be weather of some sort or the other. Wet or dry, ya’ll don’t got much choice in the matter, so why waste yer time frettin’ over what ya cain’t control. With those words of wisdom, the old-timer pulled himself upright, swung his pack onto one shoulder, tipped his hat respectfully in our direction, and then disappeared into the dusk and mist.

I’ll never forget those words, and though they are for the most part filled with truth, there is one important element missing. While one cannot do much about the weather, by learning to read and understand changes in weather patterns and what those changes mean, one can experience the vast difference between blind reaction and reliable preparedness. Often, that difference alone may determine the margin of comfort and safety that separates disaster from adventure.

Continually practice keeping a weathered eye turned upward toward the sky. The more you are aware and the more you learn about what causes weather, the more perceptive will be your observations and the more accurate your guesses as to the weather’s outcome. But never forget—predictions relative to weather are only educated guesses, never statements of fact. Always be prepared for the worst.

In the Northern Hemisphere warm (tropical) air moves north and cold (polar) air south, generally speaking. With that in mind one can expect warm fronts to be generated from the southern reaches and cold fronts from the northern. On weather charts and some maps, three types of air masses (figure 1) are often noted—maritime polar (mP), cold polar air that formed over the ocean; maritime tropical (mT), warm tropical air that formed over the ocean; continental polar (cP), cold polar air that formed over land.

With polar air masses the weather is apt to change abruptly, and as the air warms over land, it becomes turbulent, with associated cumulus clouds and often heavy precipitation. Tropical air is more stable since it is already quite warm, and while it often brings precipitation, the weather associated with a tropical air mass is apt to stay around for a while.

Both maritime and continental polar air influence local weather conditions around North America. In San Francisco, the cold maritime polar air that causes coastal fog during summer causes heavy rains in winter. Sometimes thundershowers form in the Sierra in summer, dropping precipitation on the western slopes. Continental polar air causes turbulent weather conditions and rains in the Great Lakes region during summer and heavy snows in the southeastern reaches of the lakes in winter.

Maritime tropical air brings with it humidity and extreme heat to the East during summer months. In winter it brings heavy rains.

FIGURE 1.

Movement of air masses: maritime polar (mP), maritime tropical (mT), and continental polar (cP).

As an air mass moves around the earth’s surface, contact with other air masses is inevitable. These points of contact are called fronts, and understanding how fronts interact is critically important to understanding how weather happens.

Fronts don’t just suddenly appear out of nowhere. Forces, called pressure systems, are at work, pushing and pulling the various cold and warm air masses at will. High-pressure cells of