Air Accidents Investigation Branch (AAIB) Report
http://www.dft.gov.uk/stellent/group....hcsp#P87_2070
A Boeing 757 was approximately 25 nm behind G-MIDJ on the same track. The commander of the 757 had his radar selected ON and he could not only see the weather radar returns on his ND but also G-MIDJ displayed by his TCAS. He thought that G-MIDJ had been heading for the gap between two lines of thunderstorms displayed on his radar but realised the gap was closing as the storms were building. He continued using his radar and noted the rapid increase in altitude of G-MIDJ on TCAS as it entered the storm. Initially, in the absence of any radio traffic he initially assumed that G-MIDJ was attempting to climb over the storm. He requested a turn to the right to avoid the weather, which was approved and passed safely clear of the storm activity.
Weather Radar cannot see dry hail,
but hail is a convective by-product of thunderstorms which
can be seen.