Which type of warning uses a red pennant during the day?

The use of a red pennant during the day is specifically associated with a small craft warning. This type of warning signifies that conditions are expected to be hazardous for smaller vessels. A small craft warning indicates that wind and wave conditions may be dangerous for boats that are not equipped to handle such weather.

By using a red pennant as a visual signal during daylight, it provides an effective way for mariners to quickly identify the level of danger and take necessary precautions. This is particularly vital for small craft operators who might be more vulnerable to changing weather conditions and require immediate awareness of potential dangers to ensure safety on the water.