Beam angle increases with increasing temperature. The effect is most likely to be noticed when testing at high temperatures with a 70 degree wedge. Surface waves can drive you crazy. The more common problem is to be wrong about the location of the reflector because of the change in beam angle. I noticed this problem a couple of years earlier working outside in Wyoming in the winter.
Different plastics used for wedges have different temperature coefficients.
Attenuation changes can also be due to effects on the couplants. Please click the button only once.