Environmental Engineering Reference
In-Depth Information
clouds] in the sky, not long wet, not long dry', goes another. Clearly these sayings
sometimes contain a grain of truth - that is presumably why they have survived - but not
enough for them to be reliable.
During the course of this century, as we have started to understand atmospheric
processes in more detail, methods of forecasting have become more sophisticated. The
main approach used today involves understanding the basic processes of weather
formation and using these physical laws to predict events. Unfortunately, although we
know many of the basic laws, and can express them mathematically, the equations that
result are difficult to solve. Only recently, with the development of super-computers, has
it been possible to tackle this mind-stretching task; the first attempt to forecast weather in
this way, without computers, in 1921, took several months!
To forecast values of pressure and wind, the globe is subdivided into a grid consisting
of about 60,000 squares, each with 217 points from one pole to the other and 288 around
most latitude circles. For each point, the upper atmosphere is subdivided into twenty
levels and the values of the critical atmospheric properties are determined, mainly from
satellite information. The physical equations of motion, continuity and thermodynamics
are applied to each grid point at each level to predict the new value a short time period
ahead. The new data set then provides the starting point for the next set of predictions,
and so on every fifteen minutes until the twenty-four-hour or forty-eight-hour forecast is
produced. Clearly a vast amount of calculation is required, though, with the speed of
modern computers, six-day forecasts take only about fifteen minutes of computing time.
Realistic results are produced in this way, and most meteorological services now use
computer methods to predict future weather patterns. For local forecasts up to fifteen
hours ahead the UK Meteorological Office has developed a meso-scale model using a 15
km horizontal grid covering the British Isles. It is particularly good at allowing for hills
and the land and sea surface contrasts.
We might get the impression from this technique that we can forecast the weather for
the distant future, but that is not true. It appears that small deviations can seriously affect
the development of weather-forming systems. New predictions have to be made on a
daily basis to incorporate small-scale changes which could become very important. The
problem is that we do not have enough information (or large enough computers) to solve
the equations accurately.
Efforts are being made to improve our techniques of long-period forecasting but
success
has
been
limited.
In
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