U
Electromagnetic radiation with wavelengths shorter than visible light, but longer than x-rays. Exposure to too much UV radiation from the sun can cause skin cancer.
If an air mass is unstable it contains 'parcels' of air that are warmer or more humid than the surrounding air. This causes them to rise, forming cloud. Unstable air can help thunderstorms develop.
Upward-moving current of air. A rapidly moving vertical wind, as found in thunderstorms.