A sky can change the mood of a landscape almost instantly.  For a painter they can be a useful element to resolve or even generate a composition in a painting.  Both these propositions are true for this painting.  Any darker and that cloud would be very oppressive.  As light as it is it still has a looming presence and the scribbled texture gives it a life of its own.  The yellow edges around the bottom, as if there is an obliterated sun low sun behind it somewhere, help link it to the yellowy green of the hill and field below.