Abstract: It is founded by several authors that: the total angular momentum of the atmosphere decreases from winter to summer, compensating tho increase of the angular momentum of the solid earth in the same poriod (principle of conservation of total angular momentum of an isolated system). However the average (over the globe) westerly circulation in the lower atmosphere(from 800 mb to surface) is stronger in summer than in winter. It is the authors, opinion that: it may be this increase from winter to summer of the westerly circulation near tho earth's surface, that speeds up the angular velocity of the earth's rotation from winter to summer. It may also be this increase that causes a higher rate of destruction of atmospheric angular momentum in summer, thus diminishing the total amount of the angular momentum from winter to summer.