What characteristic feature is common in hydrants designed for freezing conditions?

Hydrants designed for freezing conditions typically employ dry-barrel construction as a common characteristic. This design feature is specifically aimed at preventing water from freezing within the hydrant itself. In a dry-barrel hydrant, the valve is located below the frost line, which allows the water to drain out of the hydrant when it is shut off. This ensures that there is no standing water within the hydrant that could potentially freeze and cause damage during cold weather.

In contrast, other types of hydrant designs, such as wet-barrel hydrants, maintain water in the barrel and are less effective in freezing conditions as they are not engineered to handle the risk of ice formation. An option that mentions a whole barrel above ground does not provide the necessary protection against freezing, and while insulation can help in certain contexts, it is not a fundamental feature of hydrant design. A flush design may refer to hydrants that are flush with the ground but does not relate specifically to their effectiveness in freezing temperatures. Thus, dry-barrel construction is the characteristic feature that ensures functionality and reliability in freezing conditions.