Rila Monastery Nature Park Management Plan - part - usaid

February 2004
Temperature inversions and isotherms
Some clearly defined temperature inversions are manifest in the territory of RMNP,
having the following characteristics:
• The number of days with temperature inversions, whereby temperatures at higher
altitudes are higher or equal to those at lower altitudes, is 200-220 annually.
• Inversions are more common during the colder season and in the cooler hours of
the day, when an average of 8% of all days are temperature-inverted.
• The highest inverse temperature differences are observed about 7 in the morning.
Isothermy (temperatures remaining identical with changes of altitude) are also a
kind of inversion.
Such a high incidence of temperature inversions and isothermy in the territory of
RMNP is due to the penetration of relatively warm, oftentimes more humid masses of
air of Mediterranean origin from the southwest.
2.1.3 Precipitation and humidity
Precipitation
The average amount of precipitation in the foothill zone is 700-800 mm annually. In
the 1,000 to 2,200 m zone, the annual precipitation is between 1,050 and 1,200 mm,
and in the highest altitudes precipitation drops again, and is mostly of snow (between
November and May). February is the driest month year-round, while the maximum
amounts of precipitation have been recorded in May-June, with daily values ranging
between 40-50 mm. In the past decade, there has been a steady tendency towards
decrease in precipitation, by 3-4% annually.
Evaporation
In the lower parts (800-1,000m asl) evaporation ranges between 450-500 mm
annually, and in the higher portions of the mountain (1,000-2,200m asl), it is between
350-400 mm. As can be seen, annual rainfall still exceeds potential evaporation rates,
which determines the positive annual water balance.
Air humidity
In the higher portions of RMNP air humidity is 80-85%, but can drop to 30% on a
cold winter day. Air humidity changes with air temperature, the average annual value
being by 4-4.2 Hpa (steam pressure). The humidity deficit on north-facing slopes (36-
38 Hpa) is less than on south-facing ones (46-50 Hpa).
The increased air humidity at higher altitudes in the summer months is due to
ascending conventional air currents, which bring moisture from a long distance.
Rila Monastery Nature Park
Management Plan -Draft
2004 - 2013
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