The Finnish Meteorological Institute (FMI) says this has resulted in unusually high air pollution readings in Finnish cities.
For instance, it measured extremely high levels of nitrogen dioxide (NO2) in Helsinki on 18 December and in Oulu on 11 and 12 January.
The FMI says that poor air quality comes about through a phenomenon known as inversion, when the air is warmer at higher altitudes, a reversal of normal conditions. As a result on windless mornings after clear, cold nights, the cold air near the ground is not able to circulate. Thus emissions from traffic and heating remain floating in the air levels where people breathe.
This polluted air can be particularly harmful to children, the elderly and those suffering from respiratory and cardiac conditions. Levels of NO2, a browning toxic gas that can irritate the respiratory system, typically rise during morning rush hour in cities.