The Zenithal days, in Querétaro may 23th and Jul 20th are the days, the Zenith is the point where the sun is vertically above of the observer position. This picture shows the first of four sequence.



There are two holes, One in the roof, and another in the intermediate floor/ceiling, the sun beams passes trough the two holes, aligned vertically with a plumb.



This is a picture a few minutes before the noon on a zenithal day. This photo is taken on the second floor.



This is the sun passing trough the two holes ! The sun is exactly over the photograph camera, and made  some funny effects with the len.

These zenithal days marks the difference between a tropical or no tropical place, not the weather or temperature.