Understanding and Predicting Summer Weather While Bicycle Touring

Three Stories about Rain

When I was about twelve, I received a Boy's Life magazine with an article on predicting the weather by looking at the clouds. Sitting on my front porch, I announced to the boy next door that it was going to rain in less than four hours, so he suggested that we check the barometer in his home, which indicated clear weather. So, he stubbornly maintained that it wasn't going to rain, and I just as stubbornly maintained that it was, and we went around the neighborhood, telling whatever neighbors we could find that it was and was not going to rain. By the time we had finished telling a dozen neighbors and receiving their opinions in return, the rain had begun to fall. My neighbor raced back to his barometer only to discover that the barometer was now saying rain.

One morning, on a bicycle trip in Alabama, I woke up in the morning to discover falling rain. I had two choices: to get ready and leave in the rain, thus getting wet in the process or to wait for the rain to let up, provided that it would let up and not just get worse. The overcast sky gave me no clue, but I did have a radio, so I decided to depend on the weatherman. For a large part of an hour, I tuned from music station to music station, hunting a weather forecast but finding none. I finally decided that the early morning rain was going to let up as no one was predicting a storm. Then, out of the calm, the wind arose and violent and heavy rains began to fall. Within fifteen minutes, my campsite was several inches deep in water, and my sleeping bag, rather than getting sprinkled on, was thoroughly waterlogged. When I reached the nearest store to await the end of the storm, I learned that the rain had been so heavy in a town where the storm had hit half an hour before it reached me that a pile of lumber had washed into the street. Yet the radio had said nothing.

On an earlier occasion, I was listening to the radio in the same area when the announcer reported that there was zero percent chance of rain for the day. After the next song, he reported that someone had called him and asked him to look out the window, and there he had seen what the rest of us already knew: the sky was black from horizon to horizon, and rain was falling heavily.

The Limitations of Weather Reports

These three stories illustrate some of the limitations of depending on instruments and weather reports. There several major weaknesses to weather reports:

1) Weather reports are made for several days at a time, but the accuracy of the prediction drops off rapidly with time. A weatherman wrote that if Monday's weather report is 90% accurate, Tuesday's will be 81%, Wednesday's 72%, Thursday's 63%, and Friday's only 54%. Friday's report is so close to chance (50%) that it is meaningless. Yet every Monday, my weatherman says more about Friday's weather than he says about Monday's, and he is usually wrong about Monday's. I can give a weather forecast for Alabama which is more accurate than the typical weekend report: in the spring, it will be cool and windy with a 50% chance of rain; in the summer, it will be hot and humid with a 20% chance of heavy thundershowers; in the fall, it will be cool and dry with rain unlikely; and in the winter, it will be either windy, cool, and rainy or cold and dry.

2) Weather reports are based on what is happening somewhere else, not on what is happening here, and they often ignore the affects of local climate and of present conditions. As an example, cold air frequently moves from Canada down to Alabama, dropping temperatures along the way. Based on these trends, weathermen predict how cold it will be tomorrow, and I often correctly predict that they have exaggerated. How do I know? If the soil is dry, then it is easy for the temperatures to drop a great deal, but if the soil is wet, that moisture will warm the air. This is especially true in winter, when the cold air has to freeze all that moisture before it can significantly lower temperatures.

3) Weather reports are usually too vague and indefinite to be useful. What does a 50% chance of rain really mean? Does it mean that half of the places will get wet or that it will rain half of the day or that it will only rain 50% as hard as it could or that it might rain and might not, all four, or what? Really, it would be a lot more useful to give the weather the old way and say, "Showers likely" because I at least know it's going to be showers rather than a steady drizzle.

4) Weather reports describe the weather over broad areas, but they don't report on the specific weather you will be having to endure. This is especially true during the summer, when the main source of rain is thundershowers. The TV weather channel shows clearly that definite areas get lots of rain while surrounding areas get none. But I have never heard a radio forecast say, "A dense band of heavy rain is moving from north of Huntsville to north of Atlanta."

Using Observation Rather than Weather Reports

Therefore, I depend to a great deal on my own ability to forecast weather, a sensitivity, almost an instinct that I've picked up over the years. Of course, I'm not always right, as I reported in the second story. But my ability to predict weather from observation is generally more useful to me than the weather reports I hear. My method of predicting weather is completely different from that of the weatherman; he watches the fronts, reads weather maps, and gets reports from other areas. I have no weather maps, and I seldom see a front, and I depend on my observations of the sky. Nonetheless, my weather sense has saved me time and again from getting soaked. I could tell the storm was coming before it hit, and I found shelter until it was over.

Why Not Ride in the Rain?

You may ask me, since I have a Gore-Tex rainsuit, why don't I just ignore the weather and ride? Well, the rainsuit is fine on days when the sky is overcast and light rain is falling, but it is dangerous to be riding in heavy rain as the motorists can't see as well or stop as well. In addition, in a heavy downpour, no rainsuit is waterproof; condensation will soak me if nothing else. Finally, why ride while it's miserable when I can rest for a short while and then start after the rain is over? Most summer rain storms are short; some last less than twenty minutes.

Predicting Weather from Observation

I am going to attempt to pass along what I have learned about summer weather, even though much of it is instinct rather than clear knowledge. Remember that there are three weaknesses to my observation method of weather forecasting: First, weather forecasting based on observation alone is unaware of weather changes elsewhere which can soon arrive. Second, weather is never completely predictable anyway; a prediction is an educated guess and no more, no matter what method is used. Third, characteristics that are fairly dependable during one season or in one part of the world may have a completely different meaning in another season or another part of the world. Most of my weather sense is based on my experience in Southeast USA, but I know first hand that weather signs have a different meaning in the cool climate of Canada and the dry climate of Colorado, and I'm sure weather patterns are quite different in other climates which I have never visited. Therefore, bike travelers in other regions of the world must develop their own local and quite different weather sense.

What Weather Information is Important

In looking at the sky for weather indicators, there are several different questions that we need answers to, both immediately and for tomorrow: 1) Is it going to rain, and what kind of rain will it be? 2) What direction will the wind be blowing and how hard will it blow? 3) Is the temperature going to get colder or warmer? To answer these questions, we are going to have to look at clouds and how they grow, and at wind, and where it comes from.

The Characteristics of Clouds

Clouds come in many sizes and shapes, and the different varieties are significant, as they indicate how the cloud is being formed (or being destroyed). On a calm day, it's very instructive to lean back against a tree and observe nearby clouds for some time. A pair of binoculars can speed the process. The ones that are billowy looking are doing just that; as you watch them, they are billowing higher and higher as they gain moisture from upflowing warm, moist air currents. As the warm air rises, its expansion causes it to cool, and its invisible water vapor turns into visible mist. Eventually, these clouds turn black on the bottom and begin to rain. The blackness is not a color but just the sun's rays being blocked by the heavy vapor. One day, when the wind was not blowing, I watched a tiny cloud appear in a clear sky after a heavy rain and slowly grow until it filled the sky and then rained again. The thin and wispy looking clouds are melting away like ice in hot water, indicating that the water vapor is being absorbed into a dry sky; thus these clouds indicate clear weather. Unfortunately, as cyclists, we have the disadvantage of usually riding in one direction while the clouds are blowing in another. But as cyclists, we also have the advantage of an excellent view of the sky and an ever-present opportunity to observe changes.

The different cloud types have names, and I have noticed that the names have gotten to be rather complicated. Basically, we can say that clouds fall into four groups, according to height: The highest clouds look like feathers, flakes, or webs and usually have "cirro," or "cirrus" in their names. They usually have little effect on weather and indicate calm or cool conditions. Lower, but still from one to several miles high are clouds with "alto" in their names. These clouds often look like sheep or twists of dough, but they can also turn into thick black curtains that bring rain. At elevations of three thousand to eight thousand feet are the puffy, friendly clouds with "cumulus" or "cumulo" in their names. In spite of their lovely appearance ("Flows and bows of angel hair, and ice-cream castles in the air"), these clouds can soon turn black on the bottom and produce rain. They can also grow miles high into the atmosphere and create violent thunderstorms. Finally, the lowest clouds have "nimbus" or "nimbo" in their names; these clouds are dark, sky-covering, and a certain sign of rain. These clouds, like the thunderstorms, are the naughty children of cumulus clouds.

The Thunderstorm

The most important cloud to pay attention to in the summer sky is the friendly cumulus cloud, as it can rapidly transform itself into a rainshower or a storm. The signs of an approaching thunderstorm are the billowing mushroom in the air (which is visible from a distance but not from close up), a black sky (when the storm is near), thunder and lightning, and violent gusts of wind. A thunderstorm grows from powerful updrafts which carry hot moist air high into the atmosphere (the updrafts are created by the hot air which is lighter than the colder air overhead). To compensate for the air going up, downdrafts of cold, dry air must also occur. A cyclist can use these currents of air to help run away from the storm, but if she can't see the storm, how does she know if she is running away or towards the rain? The answer is that she can just remember that warm, humid air rushes into the storm and cooler, drier air rushes away (it has warmed up in the descent, but it still feels cooler). In my experience, if I get onto the road in front of a thunderstorm, I can race in front of it, pushed forward by the storm. There is nothing I love better than the high-speed chase when a thunderstorm is after me; I have clocked 30 miles per hour, yet the fluffs of cotton floating in the air were passing me.

Less Obvious Indications of Rain

Sometimes, the storm is invisible in the haze, and there is no thunder, so its only sign is when the wind starts blowing on a hot, calm day. When the wind starts to rise, I start thinking about where that storm is, even if I can't see it. One day while a construction worker, I walked around a building and noticed low clouds moving through the overcast sky in two different directions. I immediately tried to warn everyone, but they paid no attention to me (I have always been a Cassandra). Very shortly, the skies opened up and the deluge fell, with violent winds. No one commented about my being right.

Another way to spot a rainshower can be used on a summer day with patches of clouds, some of them raining. Looking towards the horizon, it is possible to see the descending water from miles away. By occasionally looking at such a rainshower, it is possible to determine whether it is getting closer or farther away and even to tell how hard it is raining.

A final method depends on looking at motor vehicles rather than at the sky. If it's raining just ahead, the oncoming cars will have their headlights on and windshield wipers working. You can tell you're very close when the vehicles are still wet. Once, I stopped due to such indications, and after I started again, I was astonished to find water pouring off of a roof in a flood with not a drop of rain falling. I had missed the storm by seconds, yet not a drop had hit me.

In Canada, an odd rain cloud occurs in the summer. It is a very small, thin white cloud that doesn't look like a rain cloud at all, yet when it has almost passed me, it will dump a great deal of rain so quickly that it's impossible to find shelter or to put on a raincoat rapidly enough. The rain stops just as abruptly.

On a fairly calm and overcast day, I frequently have a premonition as to whether it will rain or not. It seems to me that changes in the brightness of the sky might be connected to these premonitions, but I notice that it rains sometimes while the sky is quite bright and doesn't rain sometime when the sky is darker. Nonetheless, I detect something; for instance, just yesterday it rained hard until about nine o'clock and then stopped. I immediate decided that the rain was over, got my bike ready, and headed into town. The rain had indeed stopped for the day, but how did I know that? It had rained off and on all day the day before. In Canada on an overcast day, two cyclists were ready to head out in the opposite direction, and I warned them that it was about to rain, but they left anyway, getting caught in a terrible downpour. If I only could have told them how I knew it was going to rain, they might have stayed dry.

Even after the first drops start to fall all around, it's possible to gain further information. Some rains are slow and gentle, and there is little reason to stop; others are hard and heavy, making stopping a necessity. Usually, the second kind of shower will be preceded by several little bursts of rain, warnings that a deluge is sure to follow. It seems to me that the size of the drops is significant along with the intensity. Very fine drops indicate light rain while large, heavy drops indicate a thunderstorm. In Colorado one day, I saw some astonishing drops. They were huge in size, could be spotted high in the air, shimmered and wobbling as they fell, and made spots on the pavement two inches in diameter. These must have been large hailstones that melted before they reached the ground.

Which Way the Wind Blows

There's another source of winds besides thunderstorms, and I have used these winds to help carry me across the great plains. The old expression is "nobody knows which way the wind blows," but we do know, and passages in the Bible indicate that they knew back then. Although wind patterns are partially chaotic, we can observe and predict their general behavior. Basically, winds from the north are cool and dry, winds from the Gulf of Mexico are hot and damp, and winds from the Southwest are hot and dry. The sun's heat creates the force that drives warm air northward, but northern air must return southward to replace that air. Another factor enters in: because the earth is spinning rapidly, the winds get twisted to the east when going northward and to the west when going southward; this is called the Coriolis effect, and this is the reason why prevailing winds are stronger towards the east in the summer, as the hot winds from the south drive north.

The wind is always moving from areas supplying heat to areas absorbing heat. In coastal regions, where I seldom ride, there is a daily onshore and offshore breeze which blows from the hot land to the cool sea on warm afternoons and from the warm sea to the cool land when the conditions are the reverse. The Great Lakes are large enough to influence the wind the same way, and they they also contribute significantly to rainfall and snowfall, as the dry winds passing over them pick up moisture.

In determining the direction of the wind, it's important to recognize that wind directions are slued when riding a bicycle. If the wind and you are traveling at the same speed, then the wind will seem to disappear when it is behind you, blow twice as hard when it is in front, and to seem to blow from the 45° when it is actually blowing from 90°. Therefore, to recognize the true wind direction, you must either get off of the bike or observe some object being blown by the wind, such as a dead leaf, some grass bending over, or a flag flapping in the breeze.

How Strong Will the Wind Blow?

The strength of the wind is largely determined by the forces driving it. Air is pushed out of high-pressure areas, that is, areas of bright sunlight and into low-pressure areas, that is, areas with overcast skies. It's all very complicated, but nonetheless, we can predict that the very hottest days will be producing the strongest wind. In Texas, it may be well over 100°, but there is no sign of a breeze. But a hundred miles farther north, that hot air is noticeably moving northward. Across Kansas, it is howling. By the time it gets to Canada, on the other hand, it has lost most of its force.

The Beauford Scale

The Beauford Scale was devised to make it very easy to recognize and communicate windspeed:

How Winds Affect Temperatures

Of course, in any one place, on some days the wind will blow from the south and on others from the north. These shifts in direction affect the weather, especially temperatures. While the wind is blowing northward, the days get warmer and thunderstorms are more likely; after the wind turns south, temperatures start to fall, and the chances for thunderstorms diminish while the chances of a drizzling day increase. Thus, if we want to predict the highest temperature today, we need to pay attention to the direction of the wind. On the plains, these winds are at ground level, but in the East, we may have to watch the clouds to tell their direction.

How Clouds and Water Vapor Affect Temperatures

Clouds and water vapor also affect ground temperatures. It's obvious that clouds can lower temperatures during the day, but relatively few recognize that clouds maintain warm temperatures during the night, as clouds block heat from both reaching and leaving the earth. During the day clouds keep out the sun's heat, and at night clouds keep in the earth's heat. Since the heat leaving the earth is not in the visible spectrum, it can also be blocked by invisible water vapor (high humidity). This water vapor is not entirely invisible, as the skies are hazy (stars aren't as clear at night). Nights are very cool in the West, even after a hot day, because there is little water vapor in the air to block the outflow of heat. In the East, clouds and moisture prevent the heat from leaving, and thus the morning begins almost as hot as the day before.

Conclusion

There is undoubtedly much I don't know about weather, and much that you can learn from your own observations. Developing a weather sense while bicycle touring can never hurt and only help, so I recommend paying attention to all the weather signs.