#A mechanical variometer is usually quite sluggish in its response: it can take several seconds before the actual change of trend is accurately displayed, during which time there is a good chance that the glider has flown beyond the best bit of a thermal.

#You need to actually look at the variometer to get a reading. While this is not a huge problem in normal flight because most of the instruments you have to look at to read, due to the special interest of the variometer reading in a soaring flight, pilots can very easily get so distracted that their lookout significantly deteriorates.

It is not acceptable under any circumstance (with perhaps the exception of flying under IFR, which a glider does not normally do) to cease to look out properly and direct too much attention on the instruments, otherwise the flying safety is at stake. To help the glider pilots to read the variometer without directing the attention to the panel, electronic variometers can be introduced. These are more responsive and they sound an audible tone whose pitch depends on the variometer reading. Normally a continuous tone indicates the total energy is decreasing, while a discontinuous beep means the total energy is increasing.