The absorption of radio waves propagating through naturally occurring waters varies greatly with the electrical conductivity. Glacier melt water is very transparent and the attenuation by melt water layers is due to reflexion of power, whereas the attenuation of sea-water layers is due to absorption. The attenuation of soaked firn is usually prohibitive if the liquid is brine but it should not be a serious obstacle to radio echo strength if the liquid is rain-water or melt water. However, the magnitude of the scattered power can become greater than that from continuous (bedrock) reflectors if the size of the irregularities in the medium is large (approaching the radio wavelength in ice). Echo power formulae show that ice lenses may have the most serious effect and that low radio frequencies will be necessary to combat this.