UV Index data has been available on the Wear OS Weather app since April. But now, it's available on all platforms where Google Assistant is available. When you ask, "What's the UV index" or other permutations (like mentioning a city or particular day), you'll get a timeline from 7 a.m. to 7 p.m. in two-hour increments. The severity is colour-coded from green (1-2) to red-purple (8-11+). The verbal response will let you know UV exposure conditions later in the day. The timeline UI looks larger on smart displays, taking advantage of the bigger screen.
You can tap on the "UV Index Scale" at the bottom of the Google Assistant panel to get more information sourced from the World Health Organization. The exposure data Google gets is from The Weather Channel. Assistant recently added Air Quality info on the Nest Hub, with that data available on phones.
Source: 9to5Google