Fundamentals of Environmental Sciences I: Definition, Principles and Scope, the Structure and Composition of the Four Spheres, Thermodynamics, Heat and Mass Transfer and Material Balance, and Meteorological Parameters, Lapse Rates and Wind Roses

The atmosphere is divided by the way temperature changes with height. In the troposphere, which reaches about 8 km over the poles and 16 to 18 km over the equator, temperature falls with height at an average of about 6.5 °C per km, and almost all weather and about three-quarters of the atmosphere's mass lie here. Above the tropopause the stratosphere extends to about 50 km; temperature rises with height because ozone, concentrated between roughly 15 and 35 km, absorbs ultraviolet radiation, and the resulting inversion makes the layer stable. The mesosphere, from about 50 to 85 km, cools with height to the coldest temperatures in the atmosphere, near −90 °C at the mesopause. The thermosphere above it heats strongly by absorbing short-wave solar radiation and contains the ionosphere; the exosphere merges into space.

The hydrosphere holds about 1.39 billion km³ of water. About 97.5% is saline, almost all of it in the oceans, and only about 2.5% is fresh. Of the fresh water, roughly 69% is locked in ice caps and glaciers and about 30% is groundwater, leaving little more than 1% in lakes, rivers, soil moisture, the atmosphere and living things. The lithosphere is the rigid outer shell, the crust together with the uppermost mantle. Continental crust is thicker (about 30 to 50 km, averaging near 35 km) and granitic in composition; oceanic crust is thinner (about 5 to 10 km), denser and basaltic. By mass the crust is dominated by eight elements: oxygen about 46.6%, silicon 27.7%, aluminium 8.1%, iron 5.0%, calcium 3.6%, sodium 2.8%, potassium 2.6% and magnesium 2.1%.

The biosphere is the thin zone where life exists, from the deep ocean floor and the deep subsurface to a few kilometres up in the atmosphere, and it is where the other three spheres meet. It is a system rather than a layer: it depends on the atmosphere for gases, on the hydrosphere for water and on the lithosphere for nutrients, and in turn it changes all three, having created the oxygen of the atmosphere and much of the soil. Most of the living biomass on Earth, measured as carbon, is in plants.

The zeroth law defines temperature: two bodies each in thermal equilibrium with a third are in equilibrium with each other. The first law is the conservation of energy, ΔU = q + w: energy changes form but is neither created nor destroyed, which is why an ecosystem's energy budget must balance. The second law states that the entropy of an isolated system never decreases, so no conversion of energy is 100% efficient and some energy is always degraded to heat. It explains why energy flows one way through a food chain and why only a small fraction, often taken as about 10%, passes from one trophic level to the next. The third law states that the entropy of a perfect crystal is zero at absolute zero.

Two radiation results are examined often. Because emission rises with the fourth power of absolute temperature (σ = 5.67 × 10⁻⁸ W m⁻² K⁻⁴), doubling the temperature multiplies the emitted energy by 2⁴ = 16. By Wien's law the Sun, at about 5,800 K, emits most strongly near 2898 ÷ 5800 ≈ 0.5 µm, in the visible, while the Earth, at about 288 K, peaks near 2898 ÷ 288 ≈ 10 µm, in the thermal infrared. That separation is why greenhouse gases can be transparent to incoming sunlight yet absorb outgoing terrestrial radiation.

Mass transfer moves a substance from a region of higher to lower concentration or chemical potential. Within one phase it follows Fick's first law of diffusion, J = −D (dC/dx). Across an interface, such as air and water, soil and air, or sediment and water, the direction and extent are set by the equilibrium partitioning between the phases (for a gas and water, Henry's law), and the rate is described by the two-film theory of Whitman (1923), in which resistance lies in thin stagnant films on either side of the interface. Oxygen entering a stream, volatile solvents escaping from a lagoon and pesticides moving from soil into groundwater are all interfacial mass transfer.

The material balance is the accounting rule of environmental engineering: for any defined control volume, accumulation = input − output + generation − consumption. At steady state accumulation is zero, and for a conservative substance (no reaction) input equals output. The commonest application is the mixing of an effluent with a river. If a river carrying 10 m³/s at 2 mg/L of a conservative pollutant receives an effluent of 0.5 m³/s at 50 mg/L, the fully mixed concentration is (10 × 2 + 0.5 × 50) ÷ (10 + 0.5) = 45 ÷ 10.5 ≈ 4.3 mg/L.

Atmospheric pressure is the weight of the air column above a point; the standard sea-level value is 1013.25 hPa, and it falls roughly exponentially with height. Temperature is measured in a shaded, ventilated screen; precipitation with a rain gauge. Humidity has several measures. Absolute humidity is the mass of water vapour per unit volume of air (g/m³); specific humidity is the mass of vapour per unit mass of moist air; the mixing ratio w is the mass of vapour per unit mass of dry air, w ≈ 0.622 e ÷ (p − e), where e is the vapour pressure and p the total pressure; the saturation mixing ratio ws is the same quantity when e equals the saturation vapour pressure es. Relative humidity is the ratio of the actual to the saturation value, RH ≈ (w ÷ ws) × 100 ≈ (e ÷ es) × 100. Radiation is measured with pyranometers, and wind velocity with an anemometer for speed and a vane for direction.

A parcel of air that rises expands because pressure falls, and if it exchanges no heat with its surroundings (an adiabatic process) it cools. For dry, unsaturated air the rate is the dry adiabatic lapse rate, Γd = g ÷ cp = 9.81 ÷ 1004 ≈ 9.8 °C per km. Once the parcel is saturated, condensation releases latent heat and it cools more slowly, at the saturated (moist) adiabatic lapse rate, which is not a constant but ranges from about 4 °C/km in warm, moist air to near the dry rate in very cold air, with about 6 °C/km a common working value. The environmental lapse rate is different in kind: it is the actual rate at which the temperature of the still air falls with height at a given place and time, measured by a radiosonde; the standard atmosphere uses 6.5 °C/km.

A worked case: a dry parcel leaving the ground at 30 °C and lifted 1.5 km cools by 9.8 × 1.5 = 14.7 °C, to 15.3 °C. If a radiosonde shows 25 °C at the surface and 14 °C at 1,000 m, the ELR is 11 °C/km, greater than 9.8 °C/km, so the layer is superadiabatic and absolutely unstable, which favours dispersion of pollutants. A wind rose summarises the wind at a station over a period. Spokes radiate in 8 or 16 compass directions; the length of each spoke is proportional to the percentage of time the wind blew from that direction, its bands show speed classes, and the percentage of calms is written in the central circle. It is used to site industries, airports and monitoring stations: a polluting plant belongs downwind of the town, that is, on the side towards which the prevailing wind blows, so that the town lies upwind of it. A pollution rose plots concentration against wind direction in the same way.