Reading the weather without an app: clouds, animals and plants

You are standing in the bed at half past five with the watering can, your phone says a 20 per cent chance of rain, and forty minutes later you are watching from the shed while it pours. The app was not wrong. It simply spoke about an area in which your garden is roughly the size of a pinhead.

That is exactly where the gap sits. The models behind the forecast compute in grid cells of around two kilometres across. Whether the shower cell drifts over your plot or over the next village is decided at a scale the model cannot resolve at all. For the next three days the app is unbeatable. For the next two hours above your own bed you are the better measuring station, because you can see what is actually up there.

One thing at a time: which clouds genuinely announce something, which two signals at ground level carry the most weight, what animals and plants indicate, and where the inherited signs are simply pretty.

What you are actually looking at up there

Clouds sit in storeys, and the storey tells you more than the shape does. Right at the top, between roughly 5 and 13 kilometres up, it is so cold that clouds there consist of nothing but ice crystals. They look fibrous, cast no hard shadows and seem feather light. The middle storey lies at 2 to 7 kilometres, the lower one reaches from the ground to about 2 kilometres.

The rule of thumb behind it is simple. The higher a cloud sits, the further into the future lies the weather it announces. And the more strongly a cloud grows vertically, the shorter the lead time becomes.

The same thing works with the moon. A ring around the moon has the same radius and the same cause; it becomes visible from around half moon onwards, once there is enough light. Do not confuse it with the corona: that one is far smaller, sits right against the moon and forms through diffraction on fine water droplets in thin mid level clouds, usually altocumulus. It is not a frontal sign the way the wide ring is, but it can equally belong to a cloud deck thickening overhead.

The thunderstorm tower above your own garden

Summer thunderstorms are the case where looking up genuinely beats the app. The cell that hits you often forms only two hours beforehand, and in a spot no model could name in advance. What you see is a real head start.

1. Watch the cumulus build during the morning

   If thick heap clouds are already standing there in the morning, long before the warming of the day has peaked, the air is moist and unstably layered. On days like that the shower arrives early and hard. If the sky stays empty until midday, the risk of a home grown heat thunderstorm drops. Storms that sweep in from the south west as an organised complex in the evening give you no morning warning at all. For those, the official warnings from the DWD, GeoSphere Austria or MeteoSwiss remain what counts.

2. Watch the height, not the number

   Twenty flat little puffs are harmless. One single cloud growing taller than it is wide is the signal. Compare it against a church spire or a tree on the horizon and you will see the growth within minutes.

3. Read the underside

   As long as the base is bright and fluffy, nothing is happening. Once it turns flat, dark grey and sharply defined, there is water hanging in it. Ragged streaks below the base are precipitation already falling.

4. Secure things now, while the tower is still growing

   As soon as the cloud grows taller than it is wide and the base darkens, you usually still have 30 to 60 minutes. That is your window: pots against the wall, climbing frames tied back in, fleece off the bed so it cannot sail away. Anyone who waits until the wind shifts is doing it too late.

5. Work out the distance to the lightning

   Count the seconds between the flash and the thunder and divide by three. The result is the distance in kilometres. Under 30 seconds, so roughly 10 kilometres, you are inside the danger zone and you go indoors. Stay inside for another 30 minutes after the last thunder.

6. Once the gust front arrives, your time is up

   Just before the cell reaches you, the wind shifts abruptly, turns cool and comes from the direction of the storm. From here nothing more gets sorted out outdoors. The strongest gusts land exactly with that shift, a free standing climbing frame will go over along with whoever is in front of it, and the cell is long since within lightning range. Get inside. Whatever falls over now is property damage.

Wind and pressure: the two signals that carry

Clouds show you what is already on the way. Wind and pressure show you which direction the situation is developing in, and they do it earlier.

The relationship is over 150 years old and is named after the Dutch meteorologist Buys Ballot, who published it in 1857. William Ferrel had derived it theoretically a year earlier. Stand with your back to the wind in the northern hemisphere: the low lies to your left, offset slightly forwards. The practical part follows from that.

If the wind turns anticlockwise over the course of the day, say from west through south to south east, while the pressure falls, a low is approaching and the weather will deteriorate. If it turns clockwise, from south through west to north west, and the pressure rises, the front has passed and things improve. Meteorologists call this backing and veering. It is one of the few old rules that still appears in every textbook today.

To put numbers on it: a drop of 1 to 2 hPa in an hour announces a vigorous change, often with wind. If the pressure falls by more than 10 hPa in six hours, things are heading towards a gale, and that is when climbing frames, runner beans and heavy pots become the issue. A slow rise across two days, by contrast, is the friendliest trend there is.

Red sky at night and in the morning, read meteorologically

The saying is old and travels through half of Europe in similar forms: red sky at night, shepherd's delight; red sky in the morning, shepherd's warning. It works, but only because our weather predominantly arrives from the west.

At sunset the sun stands in the west, which is exactly where tomorrow's weather is coming from. For its light to reach you red and unobstructed, the air in that direction has to be largely clear. A strong red sunset therefore means there is no dense wall of cloud sitting in the west.

At sunrise it is the other way round. The sun stands in the east and lights up from below the clouds that have already moved in above you and to your west. If half the sky is glowing in the morning, high cloud has moved in overnight, and that usually belongs to a front.

Two caveats come with it. First, this only holds for westerly weather patterns. In summer thunderstorms often arrive from the south west, and in a Vb pattern the rain comes from the south east. Second, dust, Saharan air and smoke also colour the sky red, with no weather meaning whatsoever. The rule is a tendency, not a promise.

Reading animals: what swallows actually show

“When the swallows fly low” is the best known of all the animal rules, and it does have a genuine core. The chain is simply longer than the saying suggests.

Swallows hunt insects. In a stable high pressure situation warm air rises and carries aphids, midges and flies up with it, sometimes several hundred metres. The swallows follow. When a low approaches, the air turns moister and cooler, the thermals collapse, and as the temperature drops the insects' flight activity falls away with it. The prey stays low, and so do the hunters.

So the bird predicts nothing. It measures the layering of the air for you, and does it rather precisely, because its dinner depends on it. That is also exactly where the limit lies. Swallows also fly low in the early morning, when the thermals have not got going yet, and in warm, humid air ahead of a thunderstorm they sometimes stay higher up.

With the other animal rules things get thinner. That ants block up their entrances before rain has been observed often and is plausible, because a flooded nest is a real problem; solid research on it is scarce. And for the lying cows there is not a single piece of evidence. Cows lie down when they are full and chewing the cud. They do that in any weather, for roughly half the day.

Plants that respond to light and moisture

The plant rules are the most honest of the lot, because you can see the mechanics directly. Two groups belong here, and they measure different things: flowers that follow light and temperature, and dead tissue that swells with humidity.

The two genuine farmhouse barometers work far more mechanically. The silver thistle (Carlina acaulis) carries a ring of silvery bracts made of dead, papery tissue. The underside of these bracts takes up more water than the upper side as humidity rises. The tissue swells unevenly, the bract curls upwards, and the flower head closes. When it dries out, the head opens again. This works on a cut, long dead flower exactly as it does on the living plant. That is why it used to hang on farmhouse doors and is still called the weather thistle.

The cone of the Norway spruce (Picea abies) does the same thing on a larger scale. In dry conditions the scales spread apart so the seeds can fly out and be carried far. In damp conditions the fibre layers swell at different rates, the scales fold in and protect the seeds from a pointless launch into the rain. Hang a cone on a thread under the eaves and you have a measuring instrument that needs no battery. Use an old cone from the forest floor that has already opened. A fresh green cone is still too resinous and will not budge, and even a hung cone gradually resinifies over the years and turns sluggish.

Morning dew, and what it says about the day

Dew does not form in damp weather, it forms in clear weather. On a cloudless, windless night the ground radiates its warmth out into the sky unimpeded and cools below the dew point of the air above it. The water vapour settles out as droplets.

Dew is therefore a conclusion, not a herald. It proves the night was clear and still, and that as a rule means high pressure influence. This is why the old rule about dew in the morning meaning a day without worries genuinely holds, at least for the first half of the day.

The reverse holds just as well. A morning on which the lawn stays dry despite the cool suggests cloud or wind overnight, so an air mass on the move. The same goes for ground fog in a valley: it sits under an inversion and burns off by midday on a high pressure day.

What holds up and what does not

After all of that, an honest sorting is worth the effort. The signs differ not in whether they are “true” but in what they are able to say anything about at all.

Physically solid and with lead time are the air pressure, its tendency and the change in wind direction, plus the sequence of cloud storeys from cirrus through cirrostratus to a closed grey deck. Those are the tools for twelve to twenty four hours.

Solid but with no lead time are the indicators of the present. The only true hygrometers are the silver thistle and the spruce cone, because both consist of dead, swelling tissue. Closed daisies indicate light and temperature, low hunting swallows the layering of the air, the smell of rain on dry ground the drops that have already fallen. They measure the present very accurately and the future not at all. Their value lies in showing you a change you would not have noticed yourself.

And then there is the group you can stay friendly towards without believing it. Lying cows, thick onion skins as a winter forecast, calendar days meant to decide an entire year. For concrete weather the theoretical horizon of a forecast sits at around two weeks; beyond that all information about the individual case dissolves.

One special case often ends up in that last group unfairly: the Seven Sleepers rule. It says nothing about the single date of 27 June but about the weather of the first ten days of July, and there is real physics behind it. Around the turn of the month the position of the jet stream settles for several weeks. The statistical persistence of a large scale weather pattern is something different from a forecast for one particular day, and that is why, according to the DWD, the rule reaches a hit rate of 60 to 70 per cent in southern Germany, Austria and Switzerland. Towards the north it falls to chance level. The piece on old weather lore put to the test works through the numbers in detail.

What follows from this for the garden is unspectacular and has worked for generations. The app handles the weekly planning; your look upwards decides the next two hours: whether you water again this evening, whether the fleece stays on the bed, whether you tie the climbing frame back in, when you bring in the last damsons. Nobody needs a model for that, just two minutes at the garden gate.

Because reading the weather is only learned through repetition, writing it down pays off. Note in your garden diary what you saw in the sky and what came of it. After two or three years you will recognise the patterns that always run the same way over your plot: the cell that regularly passes to the left of the slope, the hollow where the dew settles first, the wind that always shifts a quarter of an hour before the rain where you are. None of that is in any app, because it only applies to your garden. It is in your own records, more precisely with every passing year.

Common questions

How far in advance do clouds announce rain?

That depends on the storey. High feather clouds that are thickening run roughly 12 to 24 hours ahead of a warm front. If they turn into a milky veil with a halo and after that into a grey, uniform deck, there are usually another 6 to 12 hours before the steady rain. With summer thunderstorms the lead time is far shorter: between the first cauliflower shaped cumulus and the first thunder there are often only 30 to 90 minutes.

Is it true that swallows fly low before rain?

Partly, and for a different reason than most people assume. Swallows follow their prey. In stable high pressure weather the thermals carry insects upwards; when a low moves in the thermals collapse and the falling temperature reduces flight activity, so the insects stay low. The bird therefore measures the layering of the air, it predicts nothing. Take care with the interpretation: in the early morning swallows fly low even in the finest weather, because the thermals are not there yet.

What does a ring around the moon mean?

A wide ring with a clear gap to the moon is a halo. It forms when moonlight is refracted through hexagonal ice crystals in high veil clouds, and thereby proves a sheet of ice cloud above you. In central Europe that usually belongs to an approaching warm front, so precipitation follows within the next 12 to 24 hours more often than not. Do not confuse it with the corona: that one sits tight against the moon, forms through diffraction on fine water droplets in thin mid level clouds and is not a frontal sign.

How do I tell that a cloud is turning into a thunderstorm?

By three changes in the same cloud. It grows taller than it is wide, its bulges start to look like a cauliflower, and the underside turns flat and dark grey instead of bright and fluffy. Once the head at the top loses its sharp outline and spreads sideways into an anvil, the cell is mature. At that point at the latest, work out the distance: seconds between flash and thunder divided by three gives the distance in kilometres.

Do I need a barometer in the garden?

A dedicated instrument is nice but not necessary. Most smartphones have a pressure sensor built in, and a simple barometer app draws the curve of the last 24 hours from it. The phone has to stay in the same place for that, because 8 metres of altitude already shift the reading by 1 hPa. What matters is not the absolute value but the direction. A drop of more than 1 hPa per hour means a vigorous change; more than 10 hPa in six hours is heading towards a gale. An aneroid barometer in the hallway with the marker needle reset each evening does the same job without electricity.

Can I rely on a red sunset?

As a tendency yes, as a promise no. The rule works because our weather predominantly comes from the west: a strong red sunset means the air to the west is largely clear. It fails with thunderstorm patterns out of the south west, with lows moving in from the south east, and when Saharan dust or smoke colours the sky. Treat the red sunset as one sign among several and look at the wind and the pressure as well.