Warm wall in autumn: how to use the solar irradiation in full?( * )

We describe the relationship between changes in solar irradiation of vertical surfaces and the apparent movement of the Sun across the sky during a year. Those changes can substantially prolong the sense of warmth for people both in the countryside and in urban areas. The historical analysis shows that the vertical wall positive effect was noticed hundreds of years ago (the so-called Indian summer). Nevertheless, even the main details of the relevant astronomical and physical phenomena have not been presented for tutorial purposes. In this publication, we tried to fill this lacuna. Specifically, by means of a simple set of equations, we analyzed the contribution of geometrical and atmospheric factors in different seasons for the delivery of solar energy onto south-faced vertical plane and necessary conditions for getting local comfortable warmth. In addition to tackling purely scientific aspects of the problem, we emphasize the pragmatic goals of the solar energy utilization since this source of energy is one of the naturally replenishing resources. In particular, the considered phenomena can be used for development of ecological housing or comfortable alcoves in private gardens suitable for their use during sunny days in late autumn or early spring.