In the southern hemisphere there is an uninterrupted belt of low pressure between latitudes 60° and 70″, where there is a vast expanse of the oceans. But in the northern hemisphere there are large land masses between these latitudes which are very cold. Therefore the pressures over these land masses are increased.
Thus, the continuity of sub-polar low pressure belt in the northern hemisphere is broken. However, there are well-defined low pressure cells over the northern oceans. The centers of these low pressure systems lie in: vicinity of Aleutian Islands in the Pacific Ocean and between Greenland and Iceland in the Atlantic.
During the winter season, there is a great contrast between the temperatures of the continents and adjacent oceans. This helps in reinforcing the Aleutian Low and the Icelandic Low.
In summer the temperature contrast between the continents and oceans is much reduced, so that% belt of sub-polar low is more developed and becomes more regular.