Winds are the result of a difference in uneven heating.
When you look at the earth as a whole, you see the sun heating it unevenly. This uneven heating creates winds from many different directions, which form different air masses that are separated by fronts. These fronts, the separation of the air masses are moved by upper level winds driven by an above ground river, the jet stream.
Wind flow is a very dynamic element of weather. It can be an extremely destructive force. When winds are funneled through the fjords, an increase in speed of 20 to 30 knots is normal. A Viking ship encountering these winds could find its sails torn or the ship uncontrollable, especially if the winds are unexpected and increase in speed within minutes which it is capable of doing.
With the approach and passage of an intense cyclone from the North Atlantic, gale force winds affect the coastal and exposed highland areas of Scandinavia. These winds blow over the relatively narrow and unsheltered Denmark and Swedish lowlands, and may cause damage a considerable distance inland.
In the Kattegat and the various channels into the Baltic Sea, winds along the coast are extremely variable, especially in April and May. Though winds from the west dominate in the summer, those from the southeast to the west (through south) predominate in late autumn and winter. The northwesterlies tend to be gusty and squally (quick, strong wind and rain over a brief period), and can present hazards to shipping near a lee shore. Farther east, Swedish coastal stations have strong winds about 1 day in 7 during the winter, but only 1 in 15 or greater during the summer.
In general, the southern parts of the Baltic are stormier than the northern parts, and winds tend to come from either northeast or southwest, following the coasts of Southern Sweden. The southern and eastern coasts of the Baltic Sea are shelving and there is a flat inner-land, so that when conditions are favorable land and sea breezes are common (see below).
In the Gulfs of Bothnia and Finland, winds tend to blow along the length of the gulfs, except in winter when southwesterlies are slightly predominant. Winds of 30 knots or more blow at sea 2 or 3 days a month in the winter, but in spring and early summer only light winds occur. Coastal winds are much affected by sea breezes during the summer. In general, strong winds from a westerly direction do not last more than a day without lulls or a calming of the wind. Those from the east are much more persistent.
The winds were of great importance to the Vikings as they could use its direction of flow to their advantage for sailing and it could also produce hazardous sailing conditions on which to travel. By keeping track of the direction of the winds along with the type of clouds in the sky, a wise Viking could be fairly accurate of forecasting upcoming wind and weather conditions.
Land and sea breezes
A land breeze is created when the land is cooler than the water such as at night and the surface winds have to be very light. When this happens the air over the water slowly begins to rise, as the air begins to rise the air over the surface of the ocean has to be replaced, this is done by drawing the air from the land over the water, thus creating a sea breeze. Usually land breezes are light and reach only 5 to 8 knots.
A sea breeze is created when the surface of the land is heated sufficiently to start air rising. As air rises, it is replaced by air from the sea; you have now created a sea breeze. Sea breezes tend to be much stronger and can produce gusty winds as the sun can heat the land to very warm temperatures, thereby creating a significant temperature contrast to the water. Speeds of 10 to 20 knots is not uncommon for a sea breeze.