What’s weather anyway?

Sunlight peering through overcast skies. A burst of rain. Lightning that splits the heavens in two. A great gust of wind. They're all ingredients that make up weather. Yet while the existence of weather maybe a constant, the weather itself is constantly changing. The same is true of our ideas about weather. People have always marvelled at the atmosphere, but our concepts of weather have changed enormously throughout the centuries.

Magic and superstition ruled many people's experience of the weather for millennia. As recently as the eighteenth century, scientists had no idea that local weather was controlled by great swirls of air – at greater or lesser pressure – moving from place to place. Even by 1910, cold fronts and warm fronts had yet to be recognized and named as we now know them. Two great twentieth-century observation tools, satellite and radar, have shown us how weather looks on a large scale. Computers now tell us where important weather features should be three, five, or even ten days in advance. Even though these projections aren't always correct, we now acknowledge that weather is a sub-set of the known physical world, something that can be understood and predicted – in principle, if not always in practice.

Weather ВС

The earliest humans – foragers, hunters and eventually farmers – were utterly vulnerable to weather and climate. Some of the earliest aspects of civilization, such as clothing and shelter, were adaptations to the weather. As cultures slowly coalesced, they began to accumulate rules and sayings about the weather's behaviour. Such weather lore persists to this day, especially among indigenous people. Weather stones called “mourners” in Gaelic folk culture grew darker as a result of temperature and moisture, revealing the state of the current atmosphere and, it was believed, the weather to follow. When seasonal rains failed in eastern Australia, the Yarralin people – noting that rain there usually moved from west to east – believed that other Aboriginals living to the west had somehow blocked the rains.

On a larger scale, climate and its long-term cycles affected peoples choice of habitat, and this had much to do with how ancient cultures evolved. Some groups gathered where the climate was pleasing, while others may have been forced by competition into less favourable climes, where they learned to adapt. In his survey of civilizations growth, Guns, Germs, and Steel, Jared Diamond asserts that the spread of people and plants occurred far more readily across east-west than north-south belts.

Weather conditions aren't always identical across a given latitude, but the length of day and the intensity of sunlight are.

During the last glaciation (ending some 10,000 years ago), Earth's sea level was more than 100m/330ft lower than today. Great Britain was joined to France by a land bridge that traversed what is now the English Channel. This gave England a climate more like that of modern-day Germany: hotter in the summer, colder in the winter. Similar land bridges joined east Asia to Alaska and Australia to Tasmania and New Guinea In the window between the retreat of the ice and the inundation of the Bering Strait bridge, people migrated across the Strait, spreading themselves (by roughly 11,000 BC) over present-day North America As the climate warmed and the glaciers melted, humans occupied more and more parts of the globe, although the going wasn't always smooth. For instance, a break in the warm-up (from about 6200 to 5600 BC) caused the level of the Black Sea to fall more than 100m/330ft. People had settled around its shores during this 600-year interval, but they suffered a rude awakening when the post-glacial warm-up resumed. Geologic records show that the rising Mediterranean Sea poured into the Black Sea in a cataclysmic torrent that may have been the key source of the Bible's flood myth.

With the rise of organized religions came a more systematized view of how weather worked: in most cases, the gods were assumed to be in charge. Rainmaking rituals were part of Egyptian culture by 3500 BC, if not earlier. Weather elements often personified the deities of the time, including Zeus, the Greek god of lightning; Daz Bog, the Slavic creator of lightning and the Sun; and Indra the Indian god of thunderstorms. Priests were the chief scientists of the day, deciding how and when to supplicate the gods for particular weather. Even so, people observed that weather patterns followed a certain prevailing logic when the gods were otherwise occupied. The annual cycle of heat and cold was recognized in Chinese and Greek calendars and often tied to astronomical events such as the appearance of constellations. One of the Bible's oldest books, Job, notes that “Fair weather cometh out of the north” – still a decent rule of thumb, at least for the Northern Hemisphere, since north winds often herald the arrival of high pressure and calm weather.

In his Meteorologica (written around 340 BC) Aristotle described hail, thunder, winds and other weather features in far more detail than any of his predecessors. Many of his theories miss the mark in the light of contemporary knowledge: for instance, he believed that the Sun calmed winds by drying up “exhalations” from the ground. Nonetheless, for centuries, Meteorologica was considered the book on weather. A few other voices emerged in the centuries that followed. For example, in about 1000 AD, Ibn Al-Haitham used the length of twilight (the period between sunset and full darkness) to deduce the height of the atmosphere. Throughout the Middle Ages, however, most scholars simply interpreted Aristotle, or expanded his findings. Mariners and explorers devised weather “signs” that helped to explain local conditions, sometimes successfully. Many others simply watched the sky and relied on the legends and sayings of their ancestors.

The age of observing

It was technology – in particular, three instruments created in the seventeenth century – that led to our modem understanding of weather. Galileo experimented with a device that used the expansion of a liquid (water, initially) to show the temperature of the air. Other inventors later sealed mercury in glass and attached a scale to create a bona fide thermometer. Following fast on the thermometer s heels in the late-seventeenth century were the barometer (for measuring air pressure) and the hygrometer (for relative humidity).

Widespread by the eighteenth century, these tools led to a worldwide boom in weather observing. Several United States founders were part of the craze. Thomas Jefferson duly noted weather conditions at the signing of the Declaration of Independence on July 4, 1776. (For the record, it was 22.5°C/72.5°F in Philadelphia at 1pm.) Jefferson kept a continuous weather journal from that year until shortly before his death a half-century later. He also mused on how settlement patterns in the emerging States might have been altering climate and suggested a coordinated, twice-daily network of weather observers that could help decipher such a development The multitalented Benjamin Franklin made his own contributions. On October 21, 1743, Franklin was thwarted from seeing a lunar eclipse by clouds that rolled into Philadelphia. Later, corresponding with his Boston brother – who'd had a clear view of the eclipse before the sky became overcast hours later – Franklin realized that the weather system obscuring his own view must have shifted from Philadelphia northeast to Boston, even though the clouds he saw in Philadelphia had moved in from the northeast Franklin thus hit on the fact that midlatitude weather systems – regardless of the local wind direction – generally move from west to east (which is true in both Northern and Southern Hemispheres, as we'll see later). From 1732 to 1757, Franklin published Poor Richard's Almanac, a mix of weather lore and dubious long-range forecasts, based on astrological signals, similar to the almanacs that had swept Europe two centuries before.

True weather forecasting had to wait until there was a quick way to pool observations. The telegraph adequately filled this need By the 1850s, networks of observers scanned the skies and began sending reports to Washington DC, London, Paris, St Petersburg and other cities. Before long, it was possible for government agencies to combine these reports, see where weather systems were going, warn the public of storms, and issue “indications” that appeared in the rapidly growing daily press.

Announcer Noelle Middleton and meteorologistT H Clifton collaborated on an early BBC weathercast in 1954.

High-tech as this was, the new forecasts had their limitations. Weather maps tracked high and low pressure centres, and forecasters learned how to connect these to certain kinds of weather, but there was no unifying concept of how the highs and lows related to each other and what made them develop and subside. Forecasters were also unable to tell what was happening above ground level, where so much weather is shaped.

The twentieth century

As it turned out, we needed to see the atmosphere for what it was: a giant pool of fluid sloshing across the surface of the planet. A small group of pioneering meteorologists refined this vision around the time of World War I. Based in Bergen, Norway, the Bergen School, led by the father-and-son team of Vilhelm and Jacob Bjerknes, began to explore ideas based on fluid dynamics, and thereby discovered the importance of the boundaries where cold and warm air masses bumped into each other. They labelled these boundaries “fronts' using the wartime analogy of armies facing off. Suddenly a world of weather features came into focus, and the daily weather map was transformed.

But what made fronts move? The Bergen School believed that winds at upper levels pushed the surface fronts forward, and they developed a three-dimensional theory of fronts to strengthen their case. Their concepts were bolstered by a growing set of upper-level observations gathered from kites, balloons and high-altitude weather stations. Starting in the 1930s, weather services around the world began launching weather instruments by balloon each day. The packages radioed data back to Earth from more than 20km/12 miles high. Finally, forecasters could track the upper-level features that choreographed the weather below on a day-to-day basis.

How to deal with all this information also needed to be addressed. Digital computers afforded the answer. A team of scientists in Princeton, New Jersey, carried out the first computerized weather forecast in 1950. It was useless in itself the 24-hour outlook took about to complete! – but invaluable as a sign of what would follow. Things became far more sophisticated in the succeeding years. By 1960, computers around the globe were predicting major weather features a day or two in advance. Human experts filled in the local blanks. Scientists were also putting another new instrument, radar, to work in locating water droplets and ice crystals, thus allowing the location of rains and snows to be tracked from a distance every few minutes.

One of the classic photographic icons of the 1960s was the appearance of Earth from space as a lustrous, cloud-laced, blue-green marble. Eye-popping at first, such photos quickly became routine tools in weather forecasting. In pointing out the raw isolation of our planet, satellite photos helped make us far more aware of our environment. Chemists and meteorologists teamed up to study the local effects that triggered and trapped air pollution. In big cities, ozone alerts became part of the daily weather outlooks.

Today's weather is more effectively predicted and more closely scrutinized than ever. Three-day forecasts in the US are now more accurate than the two-day outlooks of the 1980s. A torrent of data is available to all on the Internet Colourful, information-packed TV weathercasts have left their dowdy predecessors in the dust. However, we haven't yet reached meteorological perfection. Small-scale weather features such as thunderstorms cannot be nailed down much more than a few minutes ahead of time. The chaotic nature of weather may well prohibit local forecasts of more than about two weeks in advance.

On top of all of this, woven through the ups and downs of daily weather, is the ominous counterpoint of climate change. Although the greenhouse effect was identified a century ago, it gained relatively little attention until the late 1980s, when a spike in global temperatures coincided with ominous projections from global climate models. The 1990s were likely the warmest decade of the last millennium, and the first decade of the twenty-first century may end up even warmer. Aside from a very few outspoken dissidents, the worlds scientists now agree that emissions of greenhouse gases (primarily carbon dioxide) bear a substantial share of the blame for the global temperature rise.

In the light of climate change, the quicksilver shifts of our daily weather have taken on a whole new cast. For very different reasons than before, science has once again put us in the role of our superstitious ancestors: we suspect that how we behave affects the clouds and wind and the warmth we experience. As writer Jay Rosen observed in 1989, just as the greenhouse effect was entering popular awareness, “The happy atmosphere of the weather report will be difficult to maintain, for the weather can no longer serve as a haven from history”.