The variable

The rate of evaporation from the ground's surface and from plants is controlled by the relative humidity and temperature of the air, the amount of net radiation, the wind speed at the surface, the amount of water available and the nature of the surface (for example its roughness). The type of vegetation and the state of the plant's stomata are also involved since transpiration is generally lumped in together with evaporation. To allow for this, the ungainly word evapotranspiration has been coined; many do not like it, but it comes in useful and acts as a reminder. Open water presents another situation, as do ice and snow. The net incoming solar energy is apportioned to the three fluxes — sensible, latent and soil heat, and to photosynthesis — according to the infinite variety and combination of circumstances.

The variable

A history of raingauging

The first written reference to rainfall measurement was made by Kautilya in India in his book Arthasastra in the fourth century BC (Shamasastry 1915). The next reference comes from the first century AD in The Mishnah, which records 400 years of Jewish cultural and religious activities in Palestine (Danby 1933). But neither the Indian nor Palestinian measurements continued for long. They were just isolated events, doomed to be ignored and discontinued. There were to be no more quantitative hydrological or meteorological measurements for another 1000 years — a period in which scholars believed, or were forced to believe, that one turned to the sacred scriptures for answers to questions such as ‘Where do springs arise from?’.

The variable

Integrated over the whole of its radiation spectrum, the sun emits about 74 million watts of electromagnetic energy per square metre. At the mean distance of the earth from the sun, the energy received from the sun at the outer limits of the earth's atmosphere, at right angles to the solar beam, is about 1353 watts per square metre (W m–2) and is known as the solar constant. In fact the energy received is not quite constant but varies over the year by about 3%, because the earth is in an orbit around the sun that is elliptical, not circular. The actual output of the sun itself also varies with time, the most familiar regular rhythm being the 11-year sunspot cycle, although the variations due to this are less than 0.1%. There are other, longer, cycles such as the 22-year double sunspot cycle, and the 80- to 90-year cycles (Burroughs 1994). It is useful to define some terms.

The oceans: a history of their measurement

The entire heat capacity of the Earth's atmosphere is equivalent to about the top 3 m of the ocean's water (Houghton 1997). And below these 3 m lies another 2000—5000 m, with trenches down to 11000 m, with an average depth of 2000 m. In addition to their great depth and thermal mass, the oceans also cover 71% of the Earth's surface. A glance at a globe demonstrates that the Pacific Ocean alone covers almost half of the planet, and, viewed from the south, the 81% ocean-cover of the southern hemisphere is clearly evident. Given this dominance by the seas, it seems a reasonable expectation that the weather, and changes in climate, may well be driven as much, if not more, by the oceans than by the atmosphere and the continents. At the very least we should pay as much attention to measuring the weather above the oceans and the conditions down into their depths as we have done to measuring that over the land and up into the atmosphere.

The variable and its history

The Torricelli experiment did not come about by accident, but had its origins early in seventeenth century Italy in a question first asked 2300 years earlier by Aristotle as to whether a vacuum could be made or could exist naturally. Aristotle's view was that it could not, because he believed that there would be no dimensions in a vacuum – no up, down, north, south, east or west, and that light could not pass through it. This was still a common view even in the seventeenth century. There was also uncertainty over whether air had weight or exerted pressure on objects submerged in it. The same doubts existed concerning water.

The variable and its history

Observations of the extent, height and type of cloud-cover are important for many purposes, including meteorology and aviation, and also now for climatology, since clouds have a considerable influence on the energy budget of the Earth. At present, climate modellers have a problem in predicting change in part because of the difficulty of representing clouds in the models. Low, liquid-water clouds tend to cool the climate, the higher ice clouds to warm it, but there is the complication of supercooled cloud when the drops remain liquid well below the freezing point, for they reflect almost half the incoming solar radiation and so lead to cooling while the models might assume from their temperature that they are made of ice. At present all the effects appear to be nearly in balance, but with a slight net cooling. To improve the models it is important to compare what they foretell with what actually happens. The measurement of clouds thus takes on a new important role.

The need for measurements

Whether it be for meteorological, hydrological, oceanographie or climatological studies or for any other activity relating to the natural environment, measurements are vital. Knowledge of what has happened in the past and of the present situation, and an understanding of the processes involved, can only be arrived at if measurements are made. Such knowledge is also a prerequisite of any attempt to predict what might happen in the future and subsequently to check whether the predictions are correct. Without data, none of these activities is possible. Measurements are the cornerstone of them all. This book is an investigation into how the natural world is measured.

The things that need to be measured are best described as variables. Sometimes the word parameter is used but variable describes them more succinctly. The most commonly measured variables of the natural environment include: solar and terrestrial radiation, air and ground temperatures, atmospheric humidity, evaporation and transpiration, wind speed and direction, rainfall and snowfall, barometric pressure, soil moisture and soil tension, groundwater, river level and flow, water quality, sea level, sea surface temperature, ocean currents and waves, properties of the upper atmosphere and the concentration of trace gases, clouds and lightning, visibility and the ice of polar regions.

The variable and its history

Lightning was probably the cause of the first fires seen by humans, but before Benjamin Franklin performed his experiments with a kite, it was just a mysterious and terrifying natural phenomenon, although others had been debating the matter before this. Born in 1706, Franklin was one of those rare creative people who encompassed great areas of knowledge and action — as printer, moralist, essayist, civic leader, statesman, diplomat, philosopher and, of course, as scientist and inventor. He lived in Great Britain from 1757 to 1762 and was long opposed to the American colonies separating from the British Empire. But after the Boston Tea Party, and Britain's harsh response, his efforts were doomed and in 1775 he became convinced that ‘more mischief than benefit’ would come from closer union. He helped draft the Declaration of Independence, and signed it at the age of 70.

Subsurface water processes

Soil moisture

Soil moisture (or soil water) refers to the water that occupies the spaces between soil particles. It is at its maximum when the soil is saturated, that is when all the air between the particles is replaced by water but, if the soil can drain, the spaces will normally also contain air, the water then forming a thin film on and between the soil particles, held by capillary attraction. As the soil dries out this film becomes thinner and progressively less easy for plant roots to extract. The water is free to move through the soil, up or down, by gravity and by capillary attraction; it is taken up by plant roots, evaporates at the surface or recharges the groundwater.

Reasons for telemetering data

Telemetry is the transmission of data from one point to another. If data are needed in real time they must be telemetered, for example for weather forecasting and flood warning. Telemetry also has two significant advantages over in situ data logging, even if the measurements are not required in real time: the cost of visiting field sites to collect data is saved and the failure of field stations can be detected — months of data could be lost if a logging station failed soon after a visit. Logging is best suited to applications where stations are within relatively easy access or where the loss of some data is not a serious problem.

The general process of telemetering data is sometimes referred to generically as system control and data acquisition (SCADA), although the term applies more strictly to management applications — where not only are data acquired from a remote location but remote control is also exercised back. A dam managed from a distant control-room, for example, is a more appropriate use of the term SCADA than is the one-way collection of environmental data.

The structure of a telemetry system

Figure 12.1 is a schematic of a telemetry system, showing its main subdivisions into sensors, logger, modem, communications link and a PC at the base station. This basic arrangement is similar for all telemetry systems although it will differ in detail, mostly depending on the communications link used. A telemetry system is in effect a logging station with a communications link appended and with a remote base station to receive the transmitted data. The front end of such a field station is composed of sensors, identical to those used at a logging station, and a unit that performs the same functions as a logger, even if it is not called such – interfacing, multiplexing, analogue-to-digital conversion and memory. Previous chapters have been concerned with the sensors and with logging; all that needs to be addressed in this chapter is the communications link.

Sensors for measuring the quality and quantity of surface water, including the oceans, and groundwater are similar in principle, and so it makes for greater clarity if this chapter is organised by sensor type rather than by application.

Measuring water level

Staff gauges

Graduated staff gauges are widely used for the manual measurement of rivers, lakes and sea level. They are usually installed vertically in the river bed or fixed to a weir (Fig. 10.1(a)), bridge or harbour wall. Boards are made in 1-m and 2-m lengths and are about 15 cm wide, fixed one above the other to cover greater depths, and marked to span up to 12 m, or more.

Most advances in measuring the natural environment that have taken place over the last three to four decades have come about through developments in microelectronic chip technology. This has led to the now ubiquitous PC and to an abundance of other devices including (relevant to us) data loggers and satellites able to both generate images and relay data from ground stations to a distant base.

Developments in sensor technology for in situ measurements of the environment over the same period have been relatively modest in comparison; when they have occurred, they have usually been driven by the same microelectronic technology. Much of the actual ‘front-end’ of the sensor technology has, however, changed remarkably little. We are still using cups and vanes to measure wind, but with electrical sensors attached (although sonic anemometers may slowly replace them if costs can be brought down further). Temperature is still measured by electrical resistance thermometers in small screens. Despite being able to measure solar radiation with precision, sunshine duration measurements are still in demand, although sensed electronically. A most useful microelectronic development has been the introduction of general purpose photo diodes which can also be used as cheap, but slightly less precise, solar radiation sensors. Thermal solarimeters, the most accurate, have remained virtually unchanged since 1965, as have net radiometers. Humidity is still widely measured by the wet and dry method although thin-film capacitive sensors are now replacing it (developed originally for radiosondes).

The variable

Only about 17% of solar radiation is absorbed directly by the atmosphere as it passes through it (Lockwood 1974). In fact the atmosphere is heated primarily as follows: solar radiation heats the ground and the ground’s heat is transferred to the air, firstly by molecular diffusion across the laminar boundary layer (a layer only a millimetre or so thick, which clings to most surfaces); beyond this, in the turbulent boundary layer, transfer is by turbulence, which is much more effective at transferring heat than is diffusion. Heat is also, thereafter, transferred by convection, bubbles of warmer air rising into the cooler air above. This transfer of warmed air away from the surface is the sensible heat flux. While it is difficult to measure the rate of energy transfer (see The eddy correlation method; Chapter 7), the resultant changes in air temperature are important and more easily measured.

Just as the oceans cover a large fraction of the Earth's surface, so too do the cold regions, and they are even more poorly monitored. In the past, the main reason for little data being collected here was simply that few people lived in these regions or travelled to them, data coming in the last 150 years mostly from the occasional expedition to Antarctica or from a special laboratory such as on Ben Nevis in Scotland. But modern automatic instrument systems do not require frequent attention and it is now possible to site them anywhere. The main difficulty now in cold regions is not the need for operators but the problems of the environment itself.

Observation problems at low temperatures

Low temperature in itself is not a major problem for instruments. Electronic and mechanical components can operate in temperatures down to —40 °C or lower, and batteries continue to function at these extremes, although perhaps with a reduced capacity. The main problems arise from the effects of snow and ice adhering to (or filling) the sensors, often accompanied by strong winds. The question considered here is how to protect sensors from their damaging and disabling effects.

The variable

Just as air, warmed by contact with the ground, is transferred into the atmosphere by processes of diffusion, turbulence and convection, so too is the water vapour produced by evaporation. The ratio in which the net radiative energy is divided between heating the atmosphere, heating the ground and evaporating water is dependent on many factors, such as the amount of water actually available, the nature of the ground and the type of vegetation. Knowing the rate of evaporation of water is useful information in hydrology, meteorology and agriculture, but it is difficult to measure. However, the amount of water vapour in the air, i.e. the air’s humidity, is easier to measure and this chapter looks at how it is done; Chapter 7 addresses the more difficult problem of how evaporation rates are measured.

Working on the Cairngorm mountains in Northern Scotland we often experienced gale-force winds while installing experimental equipment (Fig. 5.1). In such places the power of natural forces strikes home. In the winter, on the mountain, there can be a feeling of considerable threat, which I have felt nowhere else. It is not just in a tropical hurricane that the wind's power can be felt.

The variable

Wind is caused by imbalances in the atmosphere due to temperature and pressure differences. The movement of the air is an attempt to attain equilibrium but, owing to solar heating, this is never achieved. Although air movement is three dimensional, the horizontal component is usually by far the greater and this is what is normally meant by the term ‘wind’. However, vertical motion also occurs, both at a small scale near to the ground as eddies caused by turbulent flow and convection, and on a large scale as a result of solar heating in the tropics, which powers the general circulation of the atmosphere.

The variables and their history

For use in numerical weather prediction models, for climate change and pollution studies, to predict radio propagation behaviour, for aviation and for the launch of space vehicles, it is necessary to know how conditions change with altitude, in particular temperature, humidity, pressure and wind.

Upper air measurements started around 1650 when Pascal and Perrier carried barometers up a mountain (see Chapter 6). But because of the lack of suitable platforms to carry instruments aloft, there was no further progress until about 1749 when Alexander Wilson made one of the first attempts using a kite, this being followed in 1784 by a balloon flight over Paris, giving the first observation of temperature lapse rate. Balloons and kites developed alongside into the nineteenth century when Gay-Lussac made a balloon ascent to 23 000 feet while in 1865 Glaisher flew to 11 km in a coal gas balloon, almost dying in the process.