Which condition must an enclosure meet to be classified as weatherproof?

An enclosure must prevent the infiltration of moisture to be classified as weatherproof. This means that the design and construction of the enclosure must effectively keep out rain, snow, and other forms of moisture that can lead to corrosion or electrical failures. The primary purpose of a weatherproof enclosure is to protect contained electrical equipment from environmental conditions that could negatively affect their operation or safety.

While certain materials, such as metal, may enhance durability and resistance to the elements, they are not a requirement for weatherproof classification. Similarly, portability is not a defining characteristic of weatherproof enclosures, as these can be fixed in place or movable without affecting their ability to protect against moisture. Ventilation may be necessary in some scenarios to prevent overheating, but it does not align with the criteria for weatherproofing as it implies the potential for moisture entry.

Therefore, the correct answer focuses on the need for an enclosure to effectively keep moisture out, ensuring the reliability of the equipment housed within it under various weather conditions.