ltivation of a wide
variety of field crops, fruits, and vegetables, including warm-weather
crops, such as cotton, tobacco, rice, sesame, and grapes. Frequent
summer droughts, however, lead to wide fluctuations in crop yields and
necessitate extensive irrigation.

The Stara Planina (literally, Old Mountain), or Balkan Mountains, divide
the country into several climatic and agricultural regions. The broad
Danubian tableland that lies north of these mountains has a continental
climate, except for a narrow strip along the Black Sea coast. Cold
winter winds sweep across the plateau from the Eurasian land mass,
causing prolonged periods of frost, which tend to damage orchards and
vineyards. There are 180 to 215 frost-free days in the year, and summers
are hot. A continental climate also prevails in the Sofia Basin and in
the region surrounding the headwaters of the Struma River.

In the Thracian Plain, south of the Stara Planina, the continental
climate is modified somewhat by the influence of the Mediterranean Sea.
Compared to the Danubian plateau, winters are less severe, and summers
are longer and warmer. The number of frost-free days per year ranges
from 198 to 206. A near-Mediterranean climate prevails in the valleys of
the lower Struma, Mesta, and Maritsa rivers; in the Arda basin; and on
the southern slopes of the Rodopi (or Rhodope Mountains) (see ch. 3).
The mountains protect the inland valleys and basins from strong winds;
summers there are hot, and winters are mild. Yet winters are not mild
enough for the cultivation of Mediterranean crops, such as olives and
citrus fruits.

The Black Sea coast is warmer than the interior of the country in winter
but cooler in summer; from 241 to 260 days in the year are frost free.
Frequent gale storms and hot winds resembling the African sirocco,
however, have an adverse influence on crops.

Although annual rainfall is reported to average about forty inches on
the higher mountain slopes and to reach seventy-five inches in the Rila
mountain range, precipitation in most farming areas averages only twenty
to twenty-five inches per year. Rainfall measures even less than twenty
inches in the Plovdiv area and in the coastal districts of the Dobrudzha
region in the northeast. Most of the rainfall occurs in the summer
months, but the amount and timing of precipitation are often unfavorable
for optimum crop growth. Drought conditions reached crisis proportions
in 1958 and 1963 and wer