Geoscience Reference
In-Depth Information
complex and cannot be measured easily, they are represented and treated in different
ways depending on who is measuring the two variables.
The contribution of energy mainly comes from the sun, and solar radiation is the
main element that is measured. It is measured as power, in Watts per meter square,
as being an instantaneous source of energy supply.
The radiation budget of the Earth's surface also takes other sources of energy
into consideration, coming from both the Earth and the atmosphere. The radiation
budget can be written as follows:
Rn = RG (1-a) - T + A
where:
- RG is the total solar radiation that is received by the Earth's surface. RG is the
sum of the direct solar radiation and diffuse solar radiation that reaches the Earth's
surface;
- a is the albedo, the reflection rate of sunlight which, in theory, ranges in value
from 0 to 1, yet in nature the range of values is actually from 0.05 to 0.80;
- T is the amount of radiation that is emitted by the Earth (in infra-red, given the
temperature of T);
- A is the amount of atmospheric radiation, which is also recorded in infra-red.
Therefore, it is possible to consider the amount of energy that is produced by the
Earth, by either studying the value of T, or by studying the value of the Earth's
surface ts, which is associated with the value of T thanks to the Stefan-Boltzmann
law. According to this law, T is a partial function of ts
4
. It is quite easy to measure
the value of ts using satellites, airplanes or even on the ground with the use of a
simple radiometer. But the heterogenity of the Earth's surface increases as and when
the area being studied increases.
Another way to consider the amount of energy emitted by the surface of the
Earth is to use the temperature of the air ta. The relationship that exists between the
variables of ta and ts is quite complex and depends strongly on how wet the surface
of the ground is: the drier the surface of the ground becomes, the more the
difference between ts and ta during the day increases. Statistics have shown that air
temperature is a factor that plays a role in the development and progression of forest
fires.
There is also another set of factors that plays an important role in the
development of fires, and these factors are linked to air humidity, which can be
divided into two subgroups: absolute and relative humidity.
- Absolute humidity (e) is the quantity of water vapor (an odorless and colorless
gas) in the air, and can be referred to in many ways: vapor pressure (hPa), ratio of
mass/volume (g/m
3
), ratio of gasoline to air (g/kg of dry air), etc. Absolute humidity
