if you are not aware what emissivity is defined as the ratio of the energy radiated from a material's surface to that radiated from a blackbody (a perfect emitter) at the same temperature and wavelength and under the same viewing conditions. it is a dimensionless number between 0 (for a perfect reflector) and 1 (for a perfect emitter). (ref: )
it is the measure of how an object radiates heat.
the higher the emissivity “ε” the higher the temperature it emits. the perfect emitter of heat is called a “black body.” a black body has an emissivity ε measure of 1.
for the emissivity of
water: 0.95~0.963 sand: 0.76
therefore, water would emit more heat than of the sand.