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from the end of March in the low-mountain belt till the end of May and even the
beginning of June in places with an altitude above 1,500-1,700 m.
In addition to being a resource, climate is also of very high importance as a con-
dition (environment), in which all human activities proceed in the open - from the
numerous daily individual activities to economic activities such as forestry and agri-
culture, construction, recreation and tourism, collection of herbs and mushrooms,
hunting, fishing, etc. Here it has to be noted that with respect to recreation and
tourism, climate is an important factor not only as a resource but also as an envi-
ronment, in which the recreational-tourist activity takes place, including activities
using the above-mentioned specific elements of climate as a resource. The climatic
environment exerts various effects on man and his organism - physical (by means
of precipitation, winds, mists, etc., which restrict in a purely mechanical manner the
ability of people to stay in the open, as well as aerosols, noise, odor and pollen pol-
lution of the air environment, and UV radiation), biotropic (by exchange of the air
masses), psycho-emotional (most often related to the degree of lighting - the dura-
tion of sunlight and of the day, cloudiness, visibility, etc.), and thermo-physiological
(Mateeva et al., 2009 ). The latter occupies a special place among the climatic effects
of the environment, since it affects the thermo-physiological comfort of man and
hence his general biostatus. For this reason it is considered the most significant
climatic factor of the environment.
The thermal environment exerts an impact on the thermo-physiological status of
the human organism via thermal exchange, realized with it, which depends on the
combination between the parameters of air temperature, humidity, wind, solar and
thermal radiation, and the level of human activity/clothing/exposure. The assess-
ment of this process is realized by a method involving the heat balance of the
human body. In conformity with the spatial and temporal regularities, the thermal
balance parameters for Bulgaria show the following: during the warm half-year tran-
spirant exchange prevails, and in the cold half-year - turbulent thermal exchange
is dominant. This is because of the specificities of the wind regime, regardless of
the characteristics of the air temperature. With increasing altitude the structure of
thermal exchange is subjected to significant alteration - regardless of season, the
turbulent thermal exchange increases, while the transpirant one decreases. In the
summer season this process is so clearly manifested that on the high parts a type of
thermal exchange is formed, which is just the opposite of that in the low parts of the
country. Concerning the heat balance of the human body and respectively its gen-
eral physiological loading, occurring as a result of the impact of climatic conditions,
the warm half-year is most favorable in the high mountain parts and the cold one -
in the plains in the interior of the country and in the low-mountain belt (Mateeva,
2002 ).
The investigations of the mountain and fore-mountain regions in Bulgaria
(Bioclimatic potential of the mountains in Bulgaria, 1983 ) show that in a consid-
erable part of these localities, weather without restriction on the continuous stay of
people in the open predominates all the year round (Fig. 5.4 a). The frequent change
of the various types of weather as well as the degree of contrast between the single
changes, are also important criteria for human comfort. In this respect, the season
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