It is a basic law of nature in the temperate climate zone: thermals in the spring are strong, agitated and bumpy; they get weak and lazy after August. Every paraglider pilot with at least one year of experience knows this. But probably few have asked themselves why exactly this happens. We will try to find an answer to this question in this article as part of a more general attempt to inject some inquisitive and analytical thinking into this sport. Please note though that this will probably not make you a better pilot by tomorrow, but it may shape your general approach towards this sport to use a little more reasoning.
We all know that thermals are primarily influenced by air temperature and the intensity of solar radiation. Cold air and a strong sun add up to strong thermals. Let us therefore check the seasonal change of these two factors to understand their effect on thermals. The following graph of weather data measured at Fermilab shows solar radiation and air temperature for an entire year superimposed on each other.
Figure 1. Comparison of solar radiation and temperature. Data measured at the weather station of Fermilab. Source: https://www-esh.fnal.gov/Weather/SolarRadiation.htm
The graph shows that both air temperature and solar radiation are at their lowest at the end of December (winter solstice) and peak in June-July (summer solstice, when the sun is at its highest). There is a subtle but important difference between the two factors though: temperature lags behind radiation. More simply put: the sun grows stronger till the summer solstice and grows weaker till the winter solstice; air temperature does basically the same but with a delay of three to five weeks. In meteorology this is called the temperature lag. The phenomenon produces two different results at different times: there is a period of increased radiation but still cool air in the spring (-> good thermals) and a period of fading sun with heated air (->weak to no thermals in and after August).
Figure 2. Solar radiation increases in March and April and the air heats up slower. In August to November solar radiation gradually decreases with air cooling only later.
Temperature lag is also observed in the course of a single day as solar radiation grows till noon and gradually fades till sunset. We will consider the relevant factors to explain an even more important and disturbing phenomenon: the thermal lunch break. Stay tuned and enjoy your flights!