68 / Basics of Environmental Science
Frontal systems of this general type, interspersed with periods of settled weather as the cold and warm
sectors pass, typify mid-latitude climates. They can also bring severe weather, especially in the warm
sector ahead of a cold front. Where warm surface air is lofted by an advancing cold front and winds at high
altitude flow from a different direction, a moving line of thunderstorms, called a squall line, may occur.
Water vapour, condensing in the rising air, releases heat, so the air continues to rise and water vapour
is constantly evaporating and condensing. In a fair-weather thunderstorm, caused by locally strong
heating of the ground, the upward development is limited by precipitation falling from the top of the
cloud, which slows the updraughts, but in a squall line the high-level wind shifts the top of the cloud
to one side. This removes the limiting factor, because the precipitation falls to the side of the cloud
rather than through its centre. The updraughts carry water droplets, which freeze near the cloud top,
partly melt as they fall, then freeze as they rise again, growing the ‘onion-skin’ layers of ice that
make hailstones until they are heavy enough to fall from the cloud, sometimes melting again in the
warmer air between cloud and ground.
The vertical motion leads to a separation of electrical charge. Positively charged ice crystals accumulate
near the top of the cloud, negatively charged particles near the bottom, and a positive charge is
induced in the ground beneath the cloud. Lightning discharges these differences as soon as they are
big enough to overcome the electrical resistance of the air, the energy released by the lightning
causing an explosion of the air adjacent to the flash, which we hear as thunder.
There are many variations to this general scheme. It is not only a cold front that may force warm air
to rise, for example. Air also rises as it crosses high ground and may lose moisture as a consequence.
The western side of Britain has a wetter climate than the eastern side, because air from the Atlantic
loses moisture as it crosses the hills, especially the Pennines and the mountains of Wales and Scotland.
Air forced over a mountain may cool adiabatically to below the temperature of surrounding air. On
the lee side of the mountain it descends again, warming adiabatically as it does so, to form a warm,
dry wind, called a Föhn wind in Europe and a Chinook in North America. Cold air may also flow
down a hillside, to produce ‘frost hollows’. Valleys may funnel the wind, intensifying it. The cold
mistral of southern Europe is caused by funnelling along the Rhône valley.
Such local and regional effects mark the difference between the weather people living in a particular
place experience and the climate, which is the average of the weather conditions over an entire
region. Aided by satellite observations of weather systems and their direction and rate of movement,
a wide network of surface reporting stations on land and at sea, and immense computing power, it is
possible nowadays to forecast weather with a fair degree of accuracy for a few days ahead. Weather
systems are extremely complex, however. Two meteorological situations that appear identical can
develop in radically different ways over a matter of days or weeks. This is because although they
appeared identical, in fact they differed in ways too small to be measured and these differences
directed their development. This extreme sensitivity to minute variations is characteristic of ‘chaotic’
behaviour, the most famous analogy being ‘the butterfly effect’, in which the flapping of its wings by
a butterfly in Beijing alters the development of storm systems a month later in New York. This makes
it impossible to forecast weather further ahead than a few days; indeed, such long-range forecasting
may be inherently impossible (GLEICK, 1988, pp. 9–31).
18. Glacials, interglacials, and interstadials
Jean Louis Rodolphe Agassiz (1807–73) was a zoologist with a particular interest in fishes, both
living and extinct. He was appointed professor of natural history at the University of Neuchâtel in