Our Environment - Mining and the Environment: From Ore to Metal - page 292

The atmosphere contains four different layers, but it is the first layer, called the troposphere,
where the majority of our weather, the climate, occurs. This layer, which varies in thickness
from about 8 to 16 kilometres, contains about 80% of the total mass of the atmosphere.

Temperature

The maximum air temperature occurs near the Earth's surface in the troposphere. Air
temperature declines uniformly with altitude at a rate of approximately 6.5 Celsius degree
per 1,000 metres, a phenomenon termed environmental lapse rate.
Much of the Earth's climatic variation results from uneven heating of its surface by solar
radiation or, respectively, by uneven lateral distribution of temperature, caused by the spheri-
cal shape of the Earth and the angle at which the Earth rotates on its axis as it orbits the sun.
About half of the energy received from the sun in the form of solar radiation is absorbed by
the land and the oceans. To maintain the Earth's long-term mean surface temperature of 16ºC,
however, the Earth must lose heat as well as gain it. The energy gained by the surface must
be transferred to the atmosphere, with the result that on average, our atmosphere is primarily
heated from below by heat given off from the Earth. This means that the atmosphere is set into
convective motion.
Measurements made over the Earth's surface also show that on average, more heat is
gained than lost at equatorial latitudes, while the opposite is observed at higher latitudes.
Winds and ocean currents remove accumulated excess heat in the tropics and release
it at higher latitudes, where a heat deficit exists. Closer inspection also shows that land
and ocean areas respond differently to solar radiation. Land has a lower heat capacity; it
changes temperature more rapidly than oceans as it gains or loses heat between day and
night, or summer and winter.
The long-term average distribution of gained and lost radiant energy with latitude does
not include the annual cycle of radiant changes related to the seasonal north-south migration
of the sun. The intensity of solar radiation and thus temperature remains fairly constant at
equatorial latitudes over the year, but seasonal changes increase with increasing latitudes.
The complex pattern of average solar radiation determines the Earth's average climate
pattern, but daily weather is influenced by countless others parameters, some better under-
stood than others. Understanding the climate at a specific location on the Earth, such as a
mine site, usually requires long-term meteorological measurements at the particular location.

Understanding the climate
at a specifi c location on the
Earth, such as a mine site,
usually requires long-term
meteorological measurements at
the particular location.

Fallen and Deposed Precipitation

Precipitation is a critical variable for establishing water balances and their variability.
Precipitation includes fallen (liquid or solid, i.e. rain, snow, and hail) as well as deposed
(dew or frost) forms. The observed variable is precipitation depth defined as the depth of
liquid water accumulated during a defined time interval on a horizontal surface. Hence,
precipitation is measured in mm per time interval. Precipitation drives the land sur-
face hydrology in the same way that incoming solar radiation drives the surface thermal
regimes. It is therefore the key variable in the terrestrial hydrological cycle (surface water
budget), and is essential for all vegetation growth. Reliable, high-resolution records of
precipitation are critical inputs for the design of mining projects, for understanding and
monitoring regional effects of project development on hydrology, and for estimating water
availability (or respectively water scarcity) for project consumption. For most applications
(water cycle and budget), area averaged information on precipitation is sufficient.

Precipitation drives the land
surface hydrology in the same
way that incoming solar radiation
drives the surface thermal
regimes.