When two air masses of different origin collide with each other, they do not mix. The temperature and humidity remain constant, but where they met, the weather changes. The boundary between these air masses is called the atmospheric front.
Where warm air collides with cold, he seems to be flowed over the cold. Occurs a warm front. Cold front forms when cold air comes into contact with warm and intakes under it. Between the cold front, polar air and warm front rain creates an area of low pressure (cyclone). Water vapor in the rising warm air will condense and form rain clouds. Therefore, the cyclone is accompanied by unstable rainy weather. The wind direction is constantly changing. At very low pressure often occur storms and storm surges.
Low pressure usually occur in temperate climatic zones, located between the "horse latitudes" and poles. Central Europe lies precisely in this area, so we're very familiar with variable weather, which is often replaced by the wet weather and dry, heat and cold.