What can cause shallow water mains to freeze?

When evaluating factors that contribute to the freezing of shallow water mains, the concept of insulation plays a key role. Shallow water mains are more susceptible to freezing temperatures compared to deeper installations, primarily because they are closer to the surface and do not have the natural insulation provided by the ground. In this context, lack of snow cover is critical; snow acts as an insulating layer that helps retain warmth in the soil. When there is little to no snow cover, the heat from the ground can dissipate more quickly into the atmosphere, leading to colder soil temperatures that can result in freezing conditions for shallow pipes.

Heavy rainfall can actually lead to water saturation in the soil, which may provide some insulation against freezing. Excessive heat is unlikely to be a relevant factor in this case since it would help prevent freezing rather than exacerbate it. High humidity does not have a direct effect on the freezing of water mains, as humidity levels do not influence ground temperature in a significant manner when considering shallow pipes.

Therefore, the absence of a protective snow layer significantly increases the risk of shallow water mains freezing, making lack of snow cover the most accurate answer.