UV rays from the sun and through the sky
When you go outside, you are exposed to UV radiation from the sun. Part of the UV radiation comes from the direction of the sun and part of it is scattered as diffuse sky radiation. These are UV rays that pass through the vast sky around us when there are no clouds.
The image above shows three arrows. The middle arrow is the radiation that comes directly from the sun. The other two arrows represent the radiation that is scattered (diffuse sky radiation).
The beach is an open environment, but the city is not. Cities are filled with buildings and trees. These block some of the UV rays, specifically the part of the radiation that reaches the ground as diffuse sky radiation. As a result, you are exposed to only half the radiation, while the other half does not reach you.
People who live in cities are used to what the UV index means for them in the city, where trees and buildings block half of the radiation. However, the expected or measured UV index always applies to an open environment, like the beach or an open field or sports pitch. This means that when RIVM measures a UV index of 6, this is actually only a UV index of 3 in the city.
If you go from the city to the beach or a sports pitch, for example, you will be exposed to the full amount of radiation. This includes diffuse sky radiation. In these areas, you will get sunburnt twice as fast as you are used to in the city.
If you sit in the shade under an umbrella at the beach, the umbrella will block the UV radiation that comes from the direction of the sun. However, the diffuse sky radiation will still reach you. Even though you feel less of the heat, your body will still be exposed to half of the UV radiation.