Bulgaria is positioned in the moderate climate zone. In parts of the country the climate varies between moderate continental and moderate Mediterranean.
The country’s climate has a well defined 4 seasons cycle. Main influence over the weather is played by the continental flow of cold air masses from the NE and NW in summer and winter and Mediterranean flow from SW in spring and autumn.
Quick and dynamic weather changes under the influence of NW and NE transfer of air masses define the Spring in Bulgaria. It’s common to have consecutive cold and warm fronts which helps destabilize the atmosphere. Usually spring has good thermic conditions. Therefore, quite inconsistent air mass. Fairly strong conditions in the mountains are common for early Spring (March – April).
The flat lands often have powerful inversions suppressing the thermic activity. Very often with S winds – the Danube flat lands experience foehn winds with a combination of mountain wave over Stara Planina mountain range. Unfortunately spring instability doesn’t allow for consistent wave conditions very often. The amount of snowfall in central Europe during the winter determines the humidity and amount of rainfall in Spring. The cross country potential allows for distances up to 500 km – mostly in the mountain regions. Late spring (May – June) is usually more wet and rainy with thunderstorms being very common, specially in the mountain regions. Flat lands start working more reliably. Average cloud base height in Spring is between 1500 and 2500 m. Due to the dynamic conditions forecast reliability is usually limited to 1-2 max 3 days.
Continental air masses advancing from NW and NE dominate the Summer (June – September). The atmosphere becomes more stable and weather predictability goes up to 3-4 days. The northern flows brings instability mostly in Northern, Central and Eastern Bulgaria. Thermic conditions are really good all over the country. Best places to cover big distances are the Danube flat lands, Stara Planina mountain range and the Thracian flat lands.
Distances of up to 750 kilometers are possible. The Stara Planina and Sredna Gora mountain ranges offer good conditions for the formation of consistent convergence lines. Flights of 300 km are fairly easy to achieve. No mountain waves are observed in Summer time as a result of the high instability of the lower layers. NE flow of moderately-moist air mass creates the best instability. This usually produces best flying conditions in the whole country. Southerly flows are rare. Therefore, leading to stabilizing conditions as a result of the warm and wet air coming form the Mediterranean. Cloud base varies between 2000 – 3500 m
SW and NW transfers dominate Autumn conditions (September – November). The beginning of the season is again characterized by dynamically changing conditions. Instability brought by NW winds brings thunderstorms in the beginning of September. After the atmosphere stabilizes at the end of September conditions for consistent mountain waves improve. Flat lands and valleys fill up with consistent inversion layers which practically kills thermic activity. The mountain regions still offer good thermic conditions for flying till mid or even late October. Cloudbase varies between 1000 and 2000 meters. At the end of the season cloud base settles at about 1500 meters and thermal strength rarely goes above 2 – 2.5 m/s. In November NW transfers define the overall drop in temperatures. Layered clouds are common and it is possible to have fresh snowfall at the tops of the mountains.
Due to the natural drop in temperatures and the semi-mountainous terrain in the country, usually the valleys and flat lands are under constant strong inversion layers. Thermic activity is negligible between November till the end of February. At the start of the Winter time (November – December) mostly NW winds are dominating the country. These bring moist and cold air which produces very low clouds and snowfall usually in almost the whole country. The stable atmosphere creates good conditions for strong mountain waves especially in NW and SW winds. Wave heights often reaches up to 10 000 m and beyond. Most reliable waves form over the Stara Planina and Vitosha mountains. The inverted air in the valleys often causes the formation of thick could covers not allowing VFR flights. At the end of the season (February) thermic flights become possible with mostly S winds in the mountain ranges.