Question: How does the glider sailplane fly?
Answer: On nuclear power, and the power-plant is the sun.
Question: What does the distance of the flight depend on in thermal conditions?
Answer: On intensity and duration of heating the soil by solar irradiation.
The average earth distance from the sun is 150 million km, but varies from 147.1 in perihel (3. of January), to 152.1 million km in aphel (3. of July) every year (Solar irradiation).
Intensity of solar irradiation decreases with the square of distance (Physical geography, chapter 6, The nature of radiation) which can be seen in the diagram of the same document.
According to that, heating of the earth is 6.9% more intense at the beginning of January when the sun heats most intensive the southern hemisphere near southern solstice latitude, then at the beginning of July, when the sun intensively heats northern solstice latitude.
That is the possible explanation of the longer cross country distance sailing flights in thermal conditions in the southern hemisphere.
In the last years, there are some attempts of using solar energy for various purposes. Tests have shown that it would be possible to use much more of solar energy in height (Aerostat for Solar Power Generation) what can be seen in the table from the quoted document.
Intensity of solar irradiation decreases passing through lower levels of atmosphere. Irradiation is significantly more intense on higher altitudes. From the graphic we can assume that on 1300m altitude, irradiation is 150% comparing to the sea level, or 50% higher (calculated irradiation 1685KWh/m2 , at 1270m altitude).
That is the explanation of record cross country gliding flights being achieved in the regions with high ground altitude.
There are large regions in southern hemisphere with high ground altitudes where glider pilots from the whole world every year make dozens of flights of more than 1000km distance (OLC).