Sunflower is the leading oilseed crop in Ukraine. Substantial temperature increase, rainfalls decrease and its irregularity has recently become a typical characteristics of the climate of the Steppe zone of Ukraine. This caused the productive moisture stock decrease in the arable and meter layers of the soil, occurrence of prolonged hydrothermal stresses during critical phases of plant development, especially of the late spring crops including sunflower. High-efficient oilseed crop production which has been applied in Ukraine recently resulted in problems related to oversaturation of crop rotation system with sunflower. Sunflower yield depends on weather conditions during the entire production cycle, from sowing to harvesting. The weather risk factor, which significantly affects the crops yield, is an objective one as well as the least predictable. Weather risks are external by their genesis, not related directly to the activities of the enterprise. Therefore, the production of sunflower seeds, as well as other crops in many farms of the Steppe zone of Ukraine is distinguished by a decrease in yield and its stability and an increase in the production cost. That is why the aim of our research was to determine and to ground the features of sunflower yield formation in the conditions of the Southern Steppe of Ukraine depending on agrometeorological factors. Comparative, analytical, field, modeling and statistical and mathematical methods were used. Long term regime of agrometeorological conditions on the example of Melitopol district of Zaporizhzhya region is analyzed. Analysis of rainfall during the growing season reveals a steady moisture deficit during the majority of sunflower vegetation. The results were mathematically processed with the Student's t-test and the Agrostat software program. Studying the period of active sunflower growth (April-August), which covered 60 months during the period of 2005-2016, shows that the share of months (26 months) with typical rainfall amount was 43.3 %, the share of atypical rainfall amount (25 months) – 41.7 %, with rare conditions (9 months) – 15 %. The largest number of dry months was July and August. Average moisture coefficient was 0.27, that, according to the N.M. Ivanov classification refers southern Steppe zone to semidesert. Due to the moisture shortage increase and water consumption decrease in recent years, the sunflower yield is unstable, the coefficient of yield variation is C_v=18.7 %. A close positive correlation was established between agrometeorological parameters and unflower yield, which ranged from 0.97 to 1.55 t/ha for the years of research. Based on the value of the determination coefficient  a conclusion was made that research factors (rainfall (x₁, mm), the minimum relative air humidity during the flowering period (x₂, %), the sum of active temperatures during the vegetation period (x₃, ^oC) have a high impact on yield of sunflower in comparison with the effect of randomness. One of the determining factors in sunflower yield formation is minimal humidity during flowering, the share of the factor is 54 %. Features of sunflower yield formation are determined and grounded and a linear regression model proposed, that will allow to predict the yield of sunflower based on agrometeorological factors and to control the formation of the production technologies elements.

Key words: yield, sunflower, agrometeorological conditions, evaporation, moisture coefficient, regression model, productivity.