Rila Monastery Nature Park Management Plan - part - usaid

February 2004
2.1.4 Solar radiation
At lower altitudes (1,000 m and below) there is an average of 2,200 hours of sunshine
per annum. At higher altitudes, due to a more substantial cloud cover, the average
annual duration of sunshine is 1,800-1,900 hours.
Winter solar radiation
As altitudes increase, the winter cloud cover becomes scarcer. During the winter
months (November through April), the average duration of sunshine at altitudes
above 1,900 m is between 1,000-1,100 hours on average. At lower altitudes the
average duration of sunshine for the same period is 800 hours.
Summer solar radiation
With the increase in altitude, the summer cloud cover becomes more abundant. For
that reason, during the summer months (May through October), the average duration
of sunshine at altitudes over 1,900m asl is ca. 1,000 hours.
The annual fluctuations of sunshine are typical of the mountain climate and the
geographic location of RMNP.
2.1.5 Snow cover
Physical description
The orientation of mountain sections and the exposure of slopes and the altitude are
such that a permanent snow cover can only form in mid-December at the earliest. The
duration for which the snow holds at altitudes up to 1,200 m is between 160-180 days
(the snow cover remains stable for 70-80 days), and in the higher sections, between 190
and 200 days. In the lower zone, the thickness of the snow cover is 60-70 cm (March),
while at higher altitudes (e.g. at Ribni Ezera Chalet) it can exceed 2 m.
In the higher reaches of the Park, spring thaw begins in the first third of April. No
estimation has been made of the snow stocks in the territory of RMNP.
Long-lasting snow spots and snow-drifts
The long-lasting presence of a thick layer of snow is a characteristic feature of the
deep bottoms of cirque hollows. The drifted and compacted snow holds until the
middle of June; once it melts, the snow-spot that remains in its place constitutes a
specific natural habitat with a characteristic population of the so-called chionobiones.
Rila Monastery Nature Park
Management Plan -Draft
2004 - 2013
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