Same Sun, different effects: comparing Costa Rica and Italy
Zenith Sun: maximum energy, but not the whole story
These days, at the “Italy–Costa Rica” Biological and Meteorological Station, we are in the period of the zenith Sun, when at noon the Sun is almost directly overhead. This is a typical condition of tropical regions and allows solar radiation to reach the surface with maximum intensity, passing through a minimal thickness of the atmosphere.
One might therefore expect significantly higher solar energy values than at mid-latitudes, but the data tell a more interesting story.
Daily energy: a surprising balance between tropics and mid-latitudes
A comparison of real observations shows that, on clear days in mid-April, daily global radiation is surprisingly similar. At the Karen Mogensen station in Costa Rica, 21.7 MJ/m² were recorded, while at the Modena Campus station of the Geophysical Observatory (DIEF–UNIMORE), on a similarly clear day close in time, the value was 21.8 MJ/m².
This apparent similarity is explained by the different length of the day: in equatorial regions, daylight remains close to twelve hours throughout the year, while in Modena, in spring, it already approaches 14 hours. The tropical Sun is more intense but for a shorter duration; at mid-latitudes it is less intense but shines longer, leading to a comparable daily energy balance.
UV radiation: this is where the real differences emerge
The differences become clear when considering ultraviolet radiation. Under the same conditions, the daily UV dose was much higher in Costa Rica (26.6 MED) than in Modena (15.7 MED). In percentage terms, the UV-B component accounts for about 2.5% in the tropics, compared to just over 1% at mid-latitudes.
This is due to solar geometry and the path of radiation through the atmosphere, as well as the thickness of the stratospheric ozone layer. With the Sun high in the sky, as in the tropics—especially during zenith passage—solar rays travel through less air and undergo less absorption, particularly by ozone, the main UV filter. At our latitudes, the lower solar angle results in a longer atmospheric path and therefore greater attenuation of UV radiation.
Impacts on health and ecosystems
These differences have practical implications. In the tropics, tanning can occur rapidly, but the risk of sunburn and, in the long term, more serious skin damage such as melanoma also increases. Even with the same total energy, the biological effects of radiation can be very different.
Similarly, tropical ecosystems have developed specific adaptations: plants regulate pigmentation and leaf structure to protect themselves from UV radiation, while many animals adjust their behavior according to radiation intensity throughout the day.
Same energy, different effects
The comparison between Modena and Costa Rica highlights that it is not enough to consider the total amount of solar energy; understanding its spectral distribution and effects is essential. Two environments may receive similar daily energy, yet experience profoundly different climatic and biological conditions.
The key role of monitoring
For this reason, continuous monitoring of solar radiation and meteorological parameters is crucial. Measurements from the Modena Campus station of the Geophysical Observatory (DIEF–UNIMORE) and the “Italy–Costa Rica” Biological and Meteorological Station allow these differences to be directly analyzed and linked to atmospheric processes and ecosystem responses.
Reliable and comparable long-term datasets are essential to better understand climate, its impacts on life, and its implications for health and the environment.
