Vikings believed that thunderstorms were created by the god Thor when he was angry!
Thunderstorm development or their formation is very infrequent over the Scandinavian Peninsula. Over most of Norway and the north and interior plateau of Sweden, thunderstorm frequency is about 4 each year. The number increases over the southern coasts of the lowlands of Norway and Sweden. Most thunderstorms occur as frontal systems move over the area. July is the month of that most thunderstorms occur.
Finland’s northern location, along with limited heating from the sun and little influence from the oceans produce conditions which will rarely produce thunderstorms. The few that do occur usually take place during the warmer months when that condition is combined with a frontal passage. Most locations in Finland average 5 to 15 days each year that they have thunderstorms.
For thunderstorms to develop there must be a moisture source, for example, an ocean. Then there must be some type of lift to cause the air to rise (become unstable) and then it will cool as it rises. The cooling air particles will attach to particles of dust, salt and other suspended material floating in the air and form clouds. If the air is continued to be forced upwards the condensed particles or rain is thrown upwards inside of the now developing thunderstorm. Each time the water is thrown above the level at which freezing occurs, a layer of ice forms around the dirt/salt particle, this is termed hail. The hail will continue to grow as long as the force lifting the particle can support it. The hail will melt before it reaches the earth if there is enough distance from the thunderstorm in which to heat the hail particle to a water state. If the temperature does not allow warming to occur or the distance from the storm to the ground is not far, hail will reach the surface. So it can be said that the size of hail is directly dependent on the strength of the wind within the thunderstorm. A front moving across the ocean provides likely ingredients for the formation.
How could thunderstorms be important to the Vikings?
It is true that unless they were ocean-bound, thunderstorms were rarely something that they had to deal with, but when they were out on the open ocean, it was another story. Thunderstorms can easily increase sustained (continuous) surface wind speed winds 20 mph. Associated gusts with the thunderstorm can reach 50 mph or higher. What this will do is to build the surrounding seas (make them rougher and higher) and cause hazardous conditions in which to sail.
The Viking ships were very seaworthy, but when seas are 2 to 4 ft high and they increase to 6 to 9 ft within an hour, even the most seasoned sailor will have a rough time holding their own. Seas off of Norway during the winter, especially build to phenomenal heights, 36 ft and greater are not uncommon. That is one of the major reasons why Vikings did not travel over the open ocean during the wintertime
Other hazards for the Vikings from thunderstorms besides gusty winds and high seas are the possibility of torrential rains and lightening.