))
The fully exposed to natural radiation
here can be no question of review here the many books that deal with natural radiation as it does, from the sun and sky, the ground level, a station fully exposed. Many eminent authors have studied these issues, and published the results of their observations. In France, excellent developed were made ??by MAURAIN (1937) and PERRIN DE BRICHAMBAUT (1963) among others. The exact references of these works are given in the Bibliography which concludes this study.
However, it should provide some clarification on the character of this radiation, because they are essential to understand what happens when he enters the forest.
Day length and exposure time
Day length is obviously determined by the interval of time between sunrise and sunset. However, this time interval depends on the latitude of the proposed site, and the day of the year. To locate the problem, very briefly, in the northern hemisphere shows a diagram (Fig. 7) the duration of the longest day (summer solstice) and the shortest day (winter solstice) for latitudes from the equator near the Arctic Circle. Is how to present in a specific place in the world, alternating constantly varying the length of day and night which is the characteristic photoperiodic this place, and whose influence on vegetation will be explained later. It should be noted, however, that besides the "astronomical day" which has just been mentioned, it also defines a "calendar day", a "day boat" a little different durations.
FIG. 7 - Variation of the length of the longest day (L) and shortest
day (C), Ecuador at the Arctic Circle, near the Paris meridian.
But this element (day length) does a very imperfect idea of the real importance of the radiation. Indeed, a long day can be
cloudy and sunny day runs. We must therefore take into account also the average number of sunshine hours in the various stations considered. Comprehensive measures have been carried out, a few years ago, across the globe, and the results are summarized in the table below, which takes the classification used above (Fig. 7) for premises located near the meridian of Paris.
Sunshine hours
(approx) |
|||
North Latitude |
January |
July |
Total |
67° |
0 |
50/100 |
1200 |
60° |
50 |
100/150 |
1200/1400 |
50° |
50/100 |
150/200 |
1600/1800 |
40° |
100/150 |
350/400 |
2600/2800 |
30° |
250/300 |
350/400 |
3600/3800 |
20° |
300/350 |
250/300 |
3600/3800 |
10° |
250/300 |
150/200 |
2200/2400 |
0° |
200/250 |
150/200 |
1600/1800 |
We immediately notice the high number of hours of sunshine in the Sahara and North Africa, from the northern half of Europe, which is well known, and compared to the equatorial regions, which is less so. In any case, we can emphasize the deficit, often important, the number of actual hours of sunshine compared to the number of possible sunshine hours (4380).
In the north-eastern France, the sun shines only for 40% of the total hours of sunshine possible. The rest of the time, the natural radiation received at ground level from cloud surfaces.