Zenithal sun, what is it?

At home in lockdown due to COVID-19, today we delve into an astronomical phenomenon typical of the tropics. What is the zenithal sun and what does it entail?

The days at home for the Coronavirus emergency are an opportunity to explore little-known, but important, aspects of meteorology. We have talked in previous articles about the curiosities of tropical meteorology and the meteorological and biological station Italy Costa Rica , which is located at the Karen Mogensen reserve. Today, thanks to the data coming from that distant meteorological station, we deepen a curious meteorological phenomenon, which indirectly affects the general circulation of the atmosphere, not only in the tropics.

The zenithal sun

On the day of the spring equinox the sun is on the equator on the vertical of the zenith at solar noon. In practice, thinking of being an observer who is, precisely, at the equator, in true noon the sun will be right on the vertical of the vertical imaginary line that passes through the point where the observer is located, until it intersects the celestial sphere .

Consequently, this is the day where the sun’s rays are more concentrated , and therefore more intense, per unit area. Think, as an example, to illuminate with a torch that you keep inclined, you will illuminate a large surface, but with less intense light than keeping the torch on the vertical of a point. In this way, energy is distributed in a smaller area, and therefore there is more heating of the air .

Returning to meteorology, in the tropics the days of the zenithal sun are usually the hottest of the year , while a few weeks later also the ICTZ, the intertropical convergence zone that starts from the rainy season, usually goes up again.

More specifically, in Costa Rica and generally in the range between 8 ° and 10 ° north latitude, these are the days when the phenomenon of the zenithal sun occurs, and in addition to the weather implications there is a curious consequence.

The shadow under your feet

On the day of the zenithal sun, if we place ourselves in the sun at true noon we will not see our shadow because, literally, it will be under our feet! This does not happen, actually, at 12 o’clock of our clock but, precisely, at solar noon, which differs from area to area and area and according to the day of the year with the 12 o’clock of the time zone where we are. For Costa Rica, in the area of ​​the Karen Mogensen Reserve and more generally of the Nicoya Peninsula, the true midday occurs at about 11:40 and the day of the zenithal sun is April 15th.

A recommendation, if you really do this experiment, protect yourself well from the sun , and stay there little, because the intensity of solar and especially ultraviolet radiation is really extreme in this situation.

Alternatively, you can use a stick, a wand, the pole of a weather station or observe the shadow of a tree.

Read also: solstice, welcome Summer: here are some curiosities

Does the zenithal sun exist in Italy?

The zenithal sun is a natural astronomical event that occurs twice a year exclusively in the intertropical region, that is, between the parallels of the Tropic of Cancer (in the northern hemisphere) and Capricorn (in the southern hemisphere). In Costa Rica it occurs in April and August.

In the middle latitudes the sun is never at 90 ° C on the zenith , with the shadow under your feet. The day when the sun is highest on the horizon at solar noon corresponds to the day of the summer solstice, this year in Italy will be June 20. On this day the maximum height of the sun will vary from 65 ° C in Aosta to 75 ° C in Palermo.

Curiously, the hottest days of the year in Italy and in general at medium latitudes are actually a few weeks after the maximum height of the sun , for reasons related to the complex energy balance between sun, land and seas that accumulate and still release heat for several days.


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