The paper analyses the response of the masonry arch, modelled according to the usual Heyman no tension model and hit temperature variations. This problem is considered basic for the study of the thermal response of more complex constructions like domes, vaults etc. under the daily or the annual ordinary temperature changes. All the problems inherent the resistance to fire is instead excluded. The paper shows that the minimum or the maximum thrust states, classical in the behavior of all the pushing no tension structures, are the key stones to explain the response of the arch to the action of uniform temperature changes. Special attention is given to the evaluation of the consequent displacements and the corresponding excursions of the crack widths.