To drive safely during the day in conditions of **dense fog or heavy rain**, it is important to use the appropriate lights. In these cases, it is possible and recommended to turn on the **high beam** or **main beam**.

- **High beam**: This light is useful in situations where visibility is reduced, such as on dark days or with little natural light. Although it is generally used on roads outside towns or cities, it is also suitable for improving visibility in adverse conditions during the day, such as dense fog or heavy rain.

- **Conditions of use**: During the day, the lights should be turned on when there is little natural light, such as on dark days or when passing through tunnels. Dense fog and heavy rain create similar conditions, justifying the use of high beams to improve visibility and safety.

Therefore, in situations of dense fog or heavy rain during the day, it is correct and safe to drive with the high beam on.