Colloquially, “April weather” refers to very changeable weather periods. Such changeability is especially typical for April: The air is still decidedly cool, and blue sky and sunshine alternates rapidly with showers and short thunderstorms. Precipitation is equally unpredictable. A snow shower can move across the country just a few hours after a heavy rain shower, and sleet or hail can appear even when summer is just around the corner.
Where does this changeability come from? In April, or spring, the oceans and polar regions are still cold. In addition, the sun is now quite high in the sky and the days are becoming longer, which gives the surface of the earth a chance to warm up. When cold polar air pushes towards central Europe, it warms up and rises once it is over the landmass, causing cumulus clouds to form and resulting in the above weather phenomena.
Good April weather conditions are found when there is a stable high-pressure area over northwestern Europe or the nearby Atlantic Ocean with a simultaneous low over Scandinavia, enabling polar air to flow straight to central Europe.
In Switzerland, thanks to its frequently unstable weather, April is the month with the lowest mean monthly air pressure.