An expression for the cross-section of electromagnetic wave backscatter from sporadic-E plasma irregularities induced by atmosphere turbulence is derived. Using the expression, cross-section value is estimated for a possible radar backscatter experiment at middle latitudes in the case of sporadic-E near the 100-km altitude. Dependence of the scattering cross-section on both the transmitter frequency and the radar beam orientation relative to the geomagnetic field is considered. It is shown that the cross-section decreases with increasing the radar frequency and with deviation of the radar beam from the perpendicular direction to the geomagnetic field. The latter means that the scattering irregularities are field-aligned.