Geoscience Reference
In-Depth Information
can also be expressed as a temperature: the dew point temperature td. The capacity
of the air to contain water vapor decreases with temperature. If td represents the
temperature of a mass of air that becomes saturated with water vapor and to which
td is linked, then the value of td is then expressed as a quantity of saturated water
vapor.
- Relative humidity (h) is a variable that is much easier to measure than absolute
humidity (e), and can be measured by the use of a hair hygrometer, by a
psychrometer, or by the use of an electronic sensor. Relative humidity explains the
relationship that exists between absolute humidity and the maximum amount of
water vapor (ew) that can be held in a mass of air for a specific temperature at a
given moment in time. For example, the air temperature (ta) 30°C, and the dew
point temperature of the air (td) is 18°C, this means that the absolute humidity of the
air (e), also known as its vapor pressure is 20.6 hPa. With the temperature of ta
being 30°C, the air is capable of containing more water vapor than the quantity e
suggests: the value of ew is 42.4 hPa. The relative humidity is 48% and is worked
out as follows (e/ew) x 100. If the air temperature decreases in the evening (in the
same location, without a change in pressure) and if we assume that the absolute
humidity (e) does not change, the following pattern occurs: the value of ta
decreases, as does the value of ew, and if the value of ew reaches the same value of
e then the ratio e/ew is equal to 1, and, therefore, relative humidity reaches a value
of 100%. For example, the value of ta fell from 30°C to 18°C, in other words it fell
to the same value of td. As a result condensation begins to appear because of the
excess water vapor that was left over and which was a result of the fall in
temperature. This consideration can be present in the form of dew or fog.
It seems that the value of h plays a more important role in the creation and
spread of fires than the value of e does. During the winter, the values of e are low, if
not extremely low, due to the low temperatures during this season and possibly due
to the continental origin of this air. Even though the value of td is very low, the
value of h can still remain quite high. For example in winter, an absolute humidity
of 6.1 hPa can become saturated at a temperature of 0°C (h = 100%, ta and td are
0°C). In summer, however, a temperature of 30°C for ta, and a relative humidity of
only 30%, means that the value of e is 12.7 hPa, in other words the value of the
summer vapor pressure is more than two times higher than the absolute humidity
recorded the previous winter.
It is indeed this lack of water in relation to how much water the air can contain
(which depends on the air's temperature) that favors the combustion process. This
phenomenon can also be represented by the saturation deficit ew - e, as well as
being represented by relative humidity (h).
This lack of water can also be explained by the water deficit ETP-ETR, where
ETP represents potential evapotranspiration and ETR represents actual
evapotranspiration. ETR is the amount of water vapor released by a complex
ground-vegetation system over a given period of time. It is generally not possible to
