The weather in Bulgaria is influenced mostly by the North Altantic, European continental and Mediterranean climate zones resulting in four distinct seasons:
- white winter (January - February): Temperatures can drop up to -15° C (it is warmer and sunnier in high mountains than in flatlands and closed valleys); The snow cover is from the end of December to the second half of February (from December to April at northern slopes of higher mountains).
Sometimes the country is invaded by cold Siberian anticyclonic air masses with prevailing North and NorthEast winds and sunshine in high mountains above the strong ground inversions bellow. In the flatlands and especially on closed and bigger valleys the inversions may trap Stratus clouds and cause periods of 3-5 days without sunshine.
The flying is best in higher mountains where naked trees and rocks can produce thermals. Flatlands are also working and can be better choice when mountains are surrounded by low clouds. Around the seacoast the climate is influenced by the warmer sea - the snow cover is for very few days and pilots reported 20-30 km cross-country flights from 200-300 high hills. Some sites may become good for soaring in winter because of the more stable and dense wind. There are more regular cross-country flights after the second half of February. The dry air makes the cloudbases high and winter cross-country flying can be very charming.
- energetic spring (March-May): The weather is changeable because of fighting cold North and warm South air masses. The temperatures rise and can reach 20-30°C but still cold in the night. The snow is mostly in high mountains but in March short snow revenges can surprise lower valleys and flatlands.
Sometimes the country is visited by weak cyclones from Italian, Greek and Turkish Mediterranean which can regenerate in the area and cause 3-4 days of rainy weather. The humidity is increasing and combined with the night ground inversions can obscure the closed valleys with stratus clouds.
In March the snow covered mountains can still create local falling gravity winds.
The ground inversions are burning out in March and the snow covered mountains and naked forests provide an excellent temperature gradient with 5-6 m/s strong thermals (often blue or with 2500 - 3000 m amsl high cloud bases). April and first half of May may have days with stronger south winds which create Fion effect in north Bulgaria but the temperature gradient is good and there were several flights of over 100 km in these conditions.
From the second half of May to the first half of June the atmosphere becomes more unstable with bigger chances of Cumulo Nimbuses but still flyable before their development later in the afternoon. The most reliable cross-country flying periods are from March/April and April/May but generally the weather in spring is difficult to predict. At the beginning of May a pilot reached a record cloudbase of 4300 m amsl (it happened in Koniavo in south winds). The more unstable atmosphiere sometimes makes flatlands better choice for cross-country.
The colder Black Sea and the much warmer land are producing sea breeze winds and good seacoast soaring conditions. In spring were also registered good convergences (between humid and cold sea breeze and humid and cold NW winds and airmases ) along the seacoast when some coastal height gain records (1100 m amsl) were done.
- hot and dry summer (June-September) Most of the summer is influenced by the high pressure systems from Mediterranean and about every 2 weeks is refreshed by quick cold fronts from cyclones from NorthWest Europe or by small local cyclones (mostly around the beginnings of June and September). This and the plenty of northern winds minimize the stable inversion days. Temperatures reach 35°C and nights are warm in flatlands and cool in mountain areas. In the beginning of June there might be more Cumulo Nimbuses but later they become less, isolated (mostly in mountain areas), weaker and slow growing (which allows use them quite confidently).
High altitude flights (4000 m amsl) were made in slow growing Cu congestus clouds. Cloudbases are about 2500 m amsl but pilots registered 3600 m amsl in mountain areas. The typical NorthWest and NorthEast winds in July and August bring cooler air aloft (thus good gradient and 3-6 m/s strong thermals) and most big flights and records were set then (including few flights of more than 200 km). Everything works good - mountains, their north slopes, flatlands. Surprisingly sea breeze can be strong and persistent in August.
- autumn up and down (October - December) The weather in autumn is mostly influenced by big cyclones and fronts from NorthWest Europe and warm South airmasses preceding their coming. The first longer periods of bad weather (up to 1 week) usualy come in October but even later in November there could be long periods (up to 2 weeks) of gypsi summer. The second half of November and beginning of December may be foggy and rainy. Strong inversions start to fill up the flatlands and closed valleys and good thermal days become rarer. The typical South winds in the autumn produce Fion effect and excellent wave conditions in mountain areas. Ridge soaring becomes better because of the denser and stable wind, except around sea coast, rivers and lakes where fog is forming more often.
The climate of Bulgaria is one of the best in Europe and provides big variety of flying conditions (thermals, cross-country, waves, ridge and see breeze soaring). This combined with the various landscape (2-3000 m high mountains, hills, flatlands and Black Sea coast) makes Bulgaria an interesting location for all year round flying.