A serve speed reading is entertainment, not evidence

Radar catches the ball at its fastest moment, in conditions that vary by venue, and then the number gets treated as a record.

The figure on the screen is the speed of the ball shortly after it leaves the strings, which is the fastest it will ever be. By the time it reaches the returner it has lost a large fraction of that, and how large depends on air density, ball condition and spin. Two serves reading the same number can arrive at meaningfully different speeds.

That is before you get to the equipment. Radar guns and camera systems are positioned differently at different venues, calibrated differently, and sample at different points in the flight. A tour that cared about comparability would publish the measurement protocol alongside the number. It does not, because the number is a piece of showmanship rather than a measurement anybody is meant to use.

Altitude is the biggest single distortion and it is essentially never mentioned on screen. Thinner air means less drag, which means a ball that flies faster and further for the same swing, which means every serve speed at a high-altitude event is inflated relative to sea level and every fast-court reputation built there is partly a barometric artefact.

Then there is the thing the number cannot capture at all. What actually beats a returner is the time between the server's contact and the ball arriving in the strike zone, and that is a function of speed, height of contact, spin, bounce and where the returner chose to stand. A slightly slower serve with more kick, hit by a taller player, can give a returner less usable time than a flatter faster one, and no graphic will ever tell you that.

I would rather see one derived figure than the raw speed: elapsed time from contact to bounce. It is computable from data the tracking system already has, it accounts for the flight, and it would immediately expose which servers are genuinely hard to face rather than merely loud on the readout.

Nobody will do it, because the appeal of the speed number is precisely that it is big and simple.

Fair enough. Just stop putting it in record books, and stop letting a player be described as having the fastest serve in the sport on the strength of one reading, at one tournament, from one gun, on a day when the air happened to be thin.