In the spring, concrete surfaces such as sidewalks and driveways are sometimes very wet in the morning, even when it has not rained during the night. Typical night-time conditions are shown in the sketch.

(a) Determine the heat fluxes associated with convection, q"conv, evaporation, q"evap, and radiation exchange with the sky, q"rad.

(b) Do your calculations suggest why the concrete is wet instead of dry? Explain briefly.

(c) Is heat flowing from the liquid layer to the concrete? Or from the concrete to the liquid layer? Determine the heat flux by conduction into or out of the concrete.

T = 290 K, d. = 0.7 h = 53 Wim?-K Taky = 240 K T, = 275 K, E=a = 0.96 Thin layer of liquid water Breeze Concrete