Complex sentence: In Europe the mini-supercell, or low-topped supercell, are very common, especially when showers and thunderstorms develop in cooler polar airmasses with a strong jet stream above.
Split into simpler sentences: In Europe the mini-supercell, or low-topped supercell, are very common. Especially when showers and thunderstorms develop in cooler polar airmasses with a strong jet stream above.