As soon as we step out the door, nature surrounds us. Thousands of small and large processes are taking place, details that are often fascinating and beautiful. But we've long forgotten how to recognise them.
Peter Wohlleben, bestselling author of The Hidden Life of Trees, invites us to become an expert, to take a closer look and interpret the signs that clouds, wind, plants and animals convey. Chaffinches become weather prophets, bees are live thermometers, courgettes tell us the time.
The Weather Detective combines scientific research with charming anecdotes to explain the extraordinary cycles of life, death and regeneration that are evolving on our doorstep, bringing us closer to nature than ever before. A walk in the park will never be the same again.
Peter Wohlleben spent over twenty years working for the forestry commission in Germany before leaving to put his ideas about ecology into practice. He now runs an environmentally-friendly woodland as well as caring for both wild and domestic animals.
He is celebrated for his distinctive ability to bring together ground-breaking scientific research with his own observations of nature. The author of many books, they include The Inner Life of Animals and the international bestseller The Hidden Life of Trees.
Also by Peter Wohlleben
The Hidden Life of Trees
The Inner Life of Animals
THE MOMENT WE step out of the door and stroll through the garden or a nearby park, we are surrounded by nature. Thousands of processes, from the minute to the gargantuan, are unfolding all around us, and they are fascinating and beautiful to behold – if only we open up our senses and take notice of them.
In the past, it was vital that everyone could recognise and interpret these signs. People were dependent on nature and intimately familiar with it. Nowadays, fully stocked supermarket shelves, constant energy supplies and measures in place to insure us against any conceivable act of nature all trick us into thinking that we no longer rely on our ancient bond with the natural world. Our distance from nature is particularly obvious during hot, dry summers. While farmers and foresters are desperate for rain, most of the urban population is delighted to hear forecasts predicting ongoing dry weather, oblivious to the impact of a prolonged drought. And yet, in the face of climate change and damage to the environment, it is more urgent than ever that we recognise and understand the signs of nature. Only then will we appreciate what we stand to lose.
Television, radio and the internet all make gazing out of the window to find out what the weather is like rather redundant. We have countless specialised services at our fingertips to let us know what is going on outside in the garden. There are regular updates for everything we could possibly imagine wanting to know about – from whether we’re faced with rain or shine, to when birds will migrate or aphids hatch – and such information is readily available for anyone interested to look it up. If you want even more precise prognostic data, you can simply install an electronic weather station outside that sends a live feed to you in the comfort of your living room.
But if you enjoy gardening and spending time in nature, you can manage perfectly well without these bulletins updating you constantly about the weather. We can glean most of the same information from clues around the garden, from the animals and plants in our local area, in fact even from the inanimate environment. Whether it’s forecasting what’s ahead or assessing current weather events, whether it’s insect infestations or when it’s safe to say a season has started or ended, you can read all of this data from your garden much more accurately than any newsreader from an autocue. There can be a huge difference, after all, between your garden and another location just a few miles away in terms of how a natural event unfolds and the impact it has. And that is ultimately why we look to media forecasts: to assess the situation on our doorstep.
This guide will help you to decipher the vast quantities of information you can glean from your local environment and especially your garden. You can become your own nature expert. It will address many everyday questions that in the future you’ll be able to answer for yourself; and many phenomena will suddenly be easier to understand when you know the background.
The most important motivation for writing this book was the prospect of encouraging more people to take pleasure in time spent outdoors and relaxing outside. How wonderful it is to experience things consciously that you had until now passed by obliviously. How exciting it is to foresee changes in the weather, and in flora and fauna, before they happen. When we are out and about, experiencing our surroundings with all our senses, nature is closer to us than ever before. And the ancient bond between us and our environment can be renewed.
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 per cent likely to be true, while there is a 90 per cent success rate for those covering 24 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 which 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.
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 which 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 tyre if you let some air out). The water vapour 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 aeroplanes (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 colouring: thinner clouds appear white, because some sunlight is still able to travel through them. Thicker, taller clouds, on the other hand, appear grey 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 metres and a few hundred metres. 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 behaviour, 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 15 minutes until they leave the area of lift, a storey 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 rain drops. 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 vapour and growing bigger and bigger until shortly before they reach us. Sometimes it snows great clumps of snowflakes, but their splendour 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 vapour 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 cosy blanket of cloud cover, the landscape cools down sharply.
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 splendour 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 fertilisation. 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.
However, even among the colourful weather prophets, not every bloom takes part in this to and fro process. There are flowers that leave their pollen and nectar open to takers even in the rain. Some cultivated varieties might perhaps have lost this ability to react. Or maybe some of these lone wolves want to make themselves attractive to less rain-shy insects and to gain an advantage in the pollination stakes. There remain so many mysteries to be explained.
Besides animals that react to the rain itself, there are those that demonstrate behavioural changes in advance of a rainstorm’s arrival. One species is particularly renowned for this meteorological quality: the minuscule thrip, an insect just one to two millimetres in length, which is also known variously by the common names thunderfly, thunderbug and storm fly. Thrips have fringed wings, although these are really more like paddles, which the tiny insects use to propel themselves through the air. For creatures of this size, air has the same resistance as water does for us humans, giving them a certain buoyancy. The result is that these diminutive beasts don’t fly in the true sense of the word. Their motion is more like swimming through the air, and it’s therefore a rather slow action. The conditions they love best are when it’s hot and sticky and when there’s good air movement; with a warm breeze, they can travel from plant to plant much more efficiently. It is precisely these conditions (sultry air and rising winds) that emerge in the run-up to a storm, so that is when you’ll see the air swarm with these microscopic pests. Seeing them can therefore be an early warning of an impending storm.
Swallows, on the other hand, are less reliable as weather prophets than convention dictates. It is said that low-flying swallows mean the heavens are about to open. The reason is the abundance of insects flying low over the grass. But researchers have discovered that, if anything, it is the other way round: as the wind picks up before a storm, swallows are likely to fly higher than usual. The saying ‘when swallows fly high, the weather will be dry’ could well lead you astray.
The chaffinch has its own idiosyncratic way of warning us of a change in the weather: it modifies its song when the weather is set to turn. The males usually trill a melody that sounds a little like ‘chi-chip-chirichirichiri-chip-cheweeoo’. (At my forestry college in Germany, we were taught a word pattern to remember their call – Bin bin bin ich nicht ein schöner Feldmarschall? (‘Aren’t I, aren’t I, aren’t I a lovely field marshal?’) – and their song does usually fit this mnemonic phrase, while the Sun is shining, at least.) But this sunny call is reserved for fine weather. If storm clouds loom or it starts to rain, the chaffinch switches to a rather monosyllabic chant. His so-called ‘rain call’ is a simple ‘raaatch’. Here too experts disagree on whether or not the male chaffinch’s behaviour can be relied on for forecasting. He clearly reserves his ‘raaatch’ call for disturbances of various kinds, not just heavy rain. I spend a lot of time in old deciduous forests which are teeming with chaffinches. When I show up, my presence is a (minor) disturbance, and yet they continue to chirp away undisturbed, trilling their ‘sunshine call’. It is only when the weather changes that I hear them adopt their rain call. But judge for yourself how reliably your native finches moderate their song according to the changing circumstances.
It is not at all unusual for people too to sense physical changes in the weather. After all, high and low pressure areas are so called precisely because the air pressure differs considerably in them. If a high is followed by a low, it’s like letting some air out of a tyre. The device that measures this pressure is called a barometer. It functions in the same way as the tyre pressure gauge at a petrol station. And for us, living in the Earth’s atmosphere, it’s just like sitting inside a gigantic car tyre.
Some people have a kind of built-in barometer, suffering pain or discomfort when the air pressure drops. The term for this is ‘weather sensitivity’ (or meteorosensitivity), but there is not yet consensus about it in the scientific community. One theory suggests there’s a change in the conductivity of cell membranes in the body. The sensitivity threshold of the nervous system is lowered, so that pain occurs more easily. People who have an acute medical condition seem to be particularly affected.
Other experts attribute the symptoms to changes in the air mass; that is, to the rapid change from warm, dry air to cold, damp air. Much remains unexplained, but one thing is clear: for some people, inclement weather makes itself known through physical complaints. Pay attention to what happens to you next time the barometer shows a particularly strong drop in pressure. Perhaps you’ll find you can dispense with this measuring device, after all.
OUR EARTH IS surrounded by a delicate layer of gases: the atmosphere. This separates us from the universe and is, depending on the definition, about 100 km thick. Or should we say thin? Because the density of the atmosphere decreases rapidly with altitude, making the air too thin for us to breathe at an altitude of just a few kilometres.
This delicate structure shelters us from cosmic rays. Distant stars and other celestial bodies, but above all our Sun, pelt a ceaseless shower of protons and atomic nuclei down at the Earth. We wouldn’t last long if fully exposed to it, but thankfully the atmosphere filters out most of this lethal radiation. Our air cushion also balances out the immense temperature differences between day and night. Our companion in the universe, the Moon, makes for a telling comparison: it has no atmosphere and therefore no buffer. Night temperatures on the barren crater landscape plummet to an icy –160 °C, while during the day the thermometer climbs to a staggering 130 °C.
The Earth’s air bubble contains 21 per cent oxygen, a very aggressive gas. We perhaps shouldn’t take it as a given that it is necessary for life: there was a time after all when there was no oxygen, only water vapour and carbon dioxide to ‘breathe’. Higher life forms did not yet exist, and the prevailing cyanobacteria got by just fine in this primordial atmosphere. At least they did until they had polluted all the air with the gas they exhaled: oxygen. Bad for the bacteria, but good for the other life forms, which now adapted to the new conditions and threw themselves into a steady uphill development. This is all 2.4 billion years in the past, but in hidden corners around our globe, for example at the bottom of the oceans, certain bacterial species still survive that breathe hydrogen and sulphur instead of oxygen.
You can see evidence of oxygen’s aggressive nature every day in the garden. It attacks the metal parts of your hoes and spades, resulting in rust. And because iron is also found in many rocks, rust formation takes place there as well, and is responsible for the reddish colouring of stones and sand in certain areas.
The air is also an important transport route for animals and plants. Seeds fly with the wind to new locations, and for birds, our atmospheric bubble provides an ecological niche, as useful for long-distance journeys as it is for hunting insects. Some species spend almost their entire lives in the air and come to the ground only to nest. For example, the common swift sometimes flies for several months without interruption, even sleeping on the wing, albeit for a mere few seconds at a time.
The air plays a fundamental role with respect to something else: the weather. Due to the temperature differences between the poles and the equator, there is a constant exchange of warm and cold air around the planet. The rotation of the Earth causes further deflection and acceleration, so that the air masses are constantly on the move. These masses carry water vapour with them, which rises up from the seas and forests and is offloaded again many thousands of miles away in the form of precipitation. In Europe, we owe our favourable climate and constant supply of water to the oceans. Areas of rain migrate to us from the Atlantic in the west. The evaporated seawater brings lush life to fields and forests, replenishing rivers and lakes. This transport of water vapour only works with moving air masses, i.e. the winds.