Read an Excerpt

1

What Will the Weather Be Like?

Every TV or radio news bulletin is always followed by the weather report, and it is often better than it's reputed to be. Forecasts of up to a week in advance are about 70 percent likely to be true, while there is a 90 percent success rate for those covering twenty-four hours ahead. Looking at it the other way, this means every tenth weather forecast misses the mark. The reason for this is chaotic weather conditions that simply cannot be predicted. I find it very irritating that presenters never admit to this with a statement like, "Because of the current situation, today's data is very uncertain." You simply never hear that. Nevertheless, it can't hurt to look outside and to read the signs for yourself if you wish to check what the clouds are up to. Over the years, you stand to develop a strong sense of what is going to unfold in the next few hours.

Cloud towers and rosy sunsets

The evening sun is a much-loved prophet. If it sets with a warm, rosy glow, it is taken as a sign of sunshine the following morning, as in the rhyme, "Red sky at night: shepherd's delight." This happens because the sunbeams stream in low through the atmosphere from the clear skies in the west and light up the clouds slowly drifting off to the east. And since, in western Europe, the weather usually comes from the west, a broadly cloudless western horizon means clear skies for the following few hours.

Things are the other way around with rosy dawn skies. The saying goes, "Red sky in the morning: shepherd's warning." This is also usually right. For the sun rises in the east, where the sky is still clear, and shines onto the clouds gathering in the west, which will rapidly spread and fill the sky.

Every rule has its exception, of course: when the wind blows not from the west, but from the south or the east, red skies at sunset or sunrise bear no prophetic significance.

The wind direction can itself be used as a forecasting instrument. The west wind carries moist sea air from the Atlantic, which form clouds and often rainfall. As clouds insulate the Earth like a blanket, they influence the temperature. Dense cloud cover in the winter prevents it from dropping as severely as when there are blue skies, by reducing heat loss at night. However, there is a greater chance of rain with a westerly wind. In summer, meanwhile, cloud cover prevents hotter spells, as it keeps the Earth's surface in the shade.

South winds bring warmth from the Mediterranean or even the Sahara. In summer, these southerly winds can trigger a heat wave and in winter they often carry storms in their luggage. This is because on their way across Central Europe they meet polar air masses that flow to us from the north, bringing about a violent exchange as the cold air mingles with the hot air. This can, of course, also happen with cold north winds as they come into contact with unusually warm winter air.

The east wind promises stable conditions and a clear sky. In summer, it is very warm and in winter bitterly cold. Without protective cloud cover, every season shows its extreme side.

To determine the wind direction, you can't beat the classic weathercock. The cockerel spins about on his seat of a cross, whose four arms each bear a letter for the four points of the compass. Why not install a weather vane like this in your garden or on the roof of your house? As the cock always looks in the direction from which the wind blows, assuming it is correctly installed, it shows the wind direction and thus allows you to predict the coming weather.

The key players in determining the weather, however, are the clouds. Whether the weather will be fine or poor-whatever our criteria-depends on the presence of clouds and their cargo, raindrops. If a low-pressure area emerges, the air literally becomes thinner (as in a tire if you let some air out). The water vapor cannot dissolve completely in this thinner air and becomes visible in the form of clouds.

An early harbinger of a bad weather front is the appearance of artificial clouds, i.e. the condensation trails of airplanes (contrails). If these don't dissolve, that means humidity is on its way, and with it a low-pressure area. The sky will soon cloud over.

We can also generally rely on the following rule of thumb: the weather always changes when the clouds approach from a different direction to that of the wind at ground level, which can lead to the appearance of beautiful, small, fluffy clouds.

We can tell how dense the layer of cloud above us is from its coloring: thinner clouds appear white, because some sunlight is still able to travel through them. Thicker, taller clouds, on the other hand, appear gray or even black, because barely a single beam of light can battle through these immense towers of water droplets. The higher these structures, the sooner it will rain.

Buzzards on the rise       

When the sun's rays heat the Earth, the layer of air nearest the ground also warms up. This results in a temperature gradient moving upwards. Warm air has the tendency to rise as it is less dense than cold air. It doesn't do this in a uniform way, however, but it forms invisible, tubular structures with a diameter of anything between a few feet and a few hundred feet. Warm gas rises high into the atmosphere, and in turn, cold air sinks to the ground at the edges of these tubes. What we're talking about here, of course, is thermals. There is an indirect way that you can observe this fascinating phenomenon. On a fine day, you will see individual fluffy cumulus clouds forming at the top of a column of rising warm air, where it cools and condenses into water droplets.

In terms of animal behavior, you'll know you're looking at a thermal when you see circling birds of prey. They use the upward lift to soar for hours without a single wing stroke. They can only maintain this, however, for as long as they remain within the thermal column. And as these shift (which you can see from the drift of the clouds), buzzards and kites also gradually move along with them. Migratory birds use the warm air to gain height without expending too much effort. You will often see crows suddenly start to circle for about fifteen minutes until they leave the area of lift, a story higher than they started, and continue on their way.

During longer periods of bad weather, the whole thing stops working. No sun-no lift. One exception is on mountain slopes, beaten by rainy winds, as here again the air masses move upwards. And here you'll also find the birds that want to soar high.

Precipitation is formed by two processes. One way is that water droplets collide and form ever larger drops. This is a very slow process and the result is a long-lasting drizzle, more typical of flatter clouds. Larger rain drops can only form in higher towers of cloud, because for this process ice comes into play. At the top of the cloud it is very cold, and here the water freezes. In no time at all, more water clings to the ice crystals, immediately freezing on contact. These ice crystals become too heavy to remain airborne and fall to the ground. On the way down, they thaw as the air gets warmer and the result is very large raindrops. From this you can conclude that the larger the droplets, the thicker the clouds must be, and the greater the quantity of rainfall per minute.

Every heavy raindrop was once an ice crystal or a large snowflake. If the flake doesn't melt on its way down to Earth, it will be snow. Strictly speaking, it can also snow in summer; it's just that the snow melts high up, long before it reaches us.

Speaking of snow, there is something else we can tell from a snowflake's size and consistency. Basically, the smaller the flakes, the colder the air and the greater the chance of it settling. This is because cold air can hold almost no liquid water, so the flakes don't take on more water, which is what makes them grow in size.

Meanwhile, larger snowflakes indicate warmer weather. They keep accumulating water vapor and growing bigger and bigger until shortly before they reach us. Sometimes it snows great clumps of snowflakes, but their splendor is only short-lived. And because these thick flakes usually contain a lot of moisture, this seemingly harmless kind of snowfall in fact brings with it considerable hazards. Landing on branches or power lines, the snow forms into a thick layer that gathers for some time without falling. The accumulated weight of this "damp snow" can cause branches and pylons to collapse, as well as the entire roofs of buildings.

Snowmen can also be used for weather forecasting. It is only in relatively mild weather that snow has the right consistency to be rolled into balls. Therefore, building a snowman can also mean spring is around the corner, unless another cold spell follows, of course.

But back to the clouds. If you see tall, towering clouds on the horizon, it means rain (or snow) is likely soon. If they seem to puff out at the top, or form an anvil shape (where the cloud tower is being pulled apart at the top), then a thunderstorm is on the way. Shortly before the storm front vents its fury, the wind grows brisk and strong, perhaps even reaching hurricane levels. It is only when the heavens open that the wind drops again, almost instantaneously.

After the rain front passes, it usually gets colder. This is because a low-pressure area (which brings the rain with it) is drawn across the country with a warm front, and what follows is a cold front. Both fronts spell rain, but in between it often clears up briefly. Until the cold front passes by completely, however, the sunny intervals do not signify a shift to better weather. Short showers will continue until the low pressure finally moves on.

A special case is fog and its by-products: dew and hoarfrost. It becomes foggy when the water vapor can no longer disperse into the air because the air is already saturated. Cold air can't hold much water; unlike warm air, which can hold a lot. This is why foggy weather is particularly common in the colder half of the year, while in summer there is usually good visibility. Incidentally, a hair dryer works according to precisely this principle: the air around the hair is heated so that it can absorb more water, and the hair dries.

If the temperature drops sharply at night, the air can no longer hold the water and "sweats" it out. Small droplets accumulate on the ground as dew or, if the air temperature falls below freezing, as hoarfrost. When you see this phenomenon, which is combined with a drop in temperature, in the garden in the morning, or on the roof-tiles of the house next door, you can generally bet that the weather that day will be fine. Such a sharp drop in temperature is caused by relatively dry air, with little excess water to form clouds. Without the cozy blanket of cloud cover, the landscape cools down sharply.

Plants as weather prophets

When a high-pressure fine spell subsides and a low-pressure system sits threateningly on the doorstep, the air humidity gradually rises. And many plants don't like this, because the coming rain plays havoc with their offspring. Many species send their seeds off on their way, borne on small fluffy hairs, which are carried away by even the gentlest breeze. But when they're wet, these little hairs are effectively grounded; a rain shower flushes all the splendor from the blossom down to the ground beneath the mother plant. The opportunity to conquer new territories is lost, squandered.

The same applies to the pollen in fresh flowers: if knocked to the ground by rain, it can't be couriered away by bees and used for fertilization. When the air gets more humid, suggesting rain is on the way, certain flowers react with a precautionary measure, closing their petals protectively over their interior. One example is the silver thistle, now a protected species. Its large flowers are particularly decorative and the way it folds up is no less striking. It's not for nothing that their common name in German is "weather thistles." The forecasting works even with dried plants, since it is based on a purely mechanical process. The outer petals swell with a rise in air humidity and stand up on end. In the past, people used to hang these flowers by their front door to give early warning of impending rain.

There are indeed other plants whose flowers react to changes in the weather, such as the gentian or the water lily. In the case of aquatic plants, the ability to react to a change in moisture makes little sense: water lilies, for example, sit in water the whole time anyway. And yet their blossoms are nevertheless a reliable indicator of a coming change of weather. It is not yet clear whether the trigger is the pressure difference (high or low pressure) or only the diminishing brightness of a cloudy sky. But it certainly seems to be a reliable forecaster. The flowers close when they sense rain, often hours before it comes.

I would like to highlight one more example: the daisy. It grows practically everywhere, and if you don't already have some in your garden, I would certainly reserve a corner for them. One glance at the white and yellow flowers is enough to tell if you should hang your laundry out in the garden or if inside would be safer. If rain is on the way, or a storm, the petals close up. Some also droop downwards, to avoid letting a single drop in. When the weather is fine, the blossom remains open. The entire response mechanism only functions during the day, however, because daisies always close up shop in the evenings, like many other flowers.

In the case of daisies, this opening and closing mechanism is well understood; it's a matter of thermonastic motion. This term refers to the difference in growth between the upper and lower sides of a petal. The upper side grows faster at higher temperatures than the lower side. In the warm sunshine, the flower therefore opens up, while dark rain clouds cause cooling temperatures, encouraging the underside to grow faster, and making the petals close up. This process explains why they close up at night, when it's cooler. For the daisy to be able to react at any moment, the petals need to be constantly growing, and so they grow longer day by day, bit by bit. This means you can also distinguish younger flowers from older ones.

(Continues…)

---

Excerpted from "The Weather Detective"
by .
Copyright © 2018 Peter Wohlleben.
Excerpted by permission of Penguin Publishing Group.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.

---