At night, the surfaces facing sky can re radiate heat to the sky, which has lower mean radiant temperature. this also explains water condensation on dark cars.

Hello Yasir, 

Thank you for your question.

According to the scenario above, it is implied that the "heat out" of the concrete is more than the "heat in" when doing heat balance. Other than convection, which is considered as "heat in," the concrete losses heat due to radiation to sky and ground with certain view factor. As mentioned in the previous comment, the sky temperature is roughly less than the ambient by 8 C.    

In conclusion, the concrete's temperature was reduced below the ambient, although the ambient temperature was higher than the concrete one, because the amount of heat radiated out of the concrete exceeded the amount of heat convected into it. 

Hope that helps. 

Also to be noticed ― particularly for a well-ventilated concrete surface ― is that the air relative humidity typically increases at night, what may cause concrete to uptake humidity owing to its hygroscopicity. Wind tends to counteract such effect by enhancing evaporation from surface or drying. Due to either evaporative cooling or other endothermal effects associated with drying, a close-to-surface temperature drop may occur at the material, which tends to be limited by the wet-bulb temperature of (unsaturated) air.  

It is because the ability of concrete to store sensible heat that is available during day time and it starts loosing the stored heat and becomes chiller than ambient.It acts as heat accumulator. Hot water get more chillness if it naturally loses its heat, that is the concept behind it