Observations carried out over several diurnal cycles using the Indian MST Radar located at Gadanki (13.5 o N,79.2 o E) show layered structures of enhanced echo intensity at Zenith as well as oblique (Zenith angle χ=10 o ) beams. These structures are observed to last for more than 4h and often last as long as 18h. Simultaneous MST radar observations and radiosonde data from Chennai (13.1 o N,80.2 o E) are used to understand the causative mechanism of these echoes.Results show that at some heights, these echoes are from sharp gradients in radio refractivity structures of horizontal correlation length of 10-20m whereas at some other heights, these are from back-scatter from more or less isotropic refractivity structures, associated with turbulence. Thus, both mechanisms are effective depending upon prevailing atmospheric conditions at different heights.