“I didn’t believe in the old omen: bird cherry blossoms for a cold snap”, - is sung in the song of the composer G. Ponomarenko. The inhabitants of central Russia have no reason not to trust this popular omen, because they observe its confirmation every year.
A cold snap in mid-May is common in temperate latitudes. The air temperature drops by 6-7 ° C, sometimes the cooling is accompanied by rain or even snow. People call such cold weather "bird cherry", because at the same time this shrub begins to bloom.
According to some researchers, there is indeed a connection between plant life and weather changes. Mid-May is the time when the leaves on all plants have fully blossomed. Due to this, less amount of sunlight reaches the earth's surface, the degree of their absorption is reduced, which means that the atmospheric air heats up less.
Cooling is also associated with a decrease in the content of carbon dioxide in the atmosphere, which is actively absorbed by the blossoming leaves of plants. This gas creates a greenhouse effect that increases the temperature of the air, respectively, a decrease in its content causes a decrease in temperature.
In autumn, the opposite effect takes place: the leaves fall off, the absorption of solar energy by the earth's surface increases, the balance between oxygen and carbon dioxide changes in favor of carbon dioxide, resulting in a short-term warming, popularly called "Indian summer".
Of course, in summer, leaves also cover the earth's surface from the sun and absorb carbon dioxide, but in summer the earth receives so much solar energy that these factors are no longer critical for the weather, and in May they are still critical. As for the "bird cherry cold weather", folk wisdom associated them with bird cherry because its flowering is the most noticeable event in the plant world in mid-May.
Other scientists do not agree with this point of view and associate the May cold exclusively with the instability of the atmosphere during this period. Bird cherry in the course of evolution adapted to bloom precisely during a cold snap. A decrease in air temperature prevents the activity of insect pests, whose vital activity puts the plant at risk. Plant protection during such an important period from the point of view of species survival as flowering has become a valuable evolutionary gain.
The relationship between flowering and cooling is characteristic not only of bird cherry, but also of oak, which blooms at the end of May. But this cooling is not so significant, and the flowering oak does not look as impressive as the flowering bird cherry, therefore the "oak cold" is less known than bird cherry.
