Even at sea level the boiling point is not always 100C. That is merely the average. That's because the pressure at sea level can vary with the weather. Standard pressure is 1013 millibars (equivalent to 1.00 atm), yet the central pressure in Hurricane Michael was a low as 919 mbar when it came ashore, that's 0.907 atm. Not that you're boiling water in the eye of a hurricane, but it gives us an idea of how low pressure can be at sea level.
The bottom line is that 100C is an average temperature for boiling water at sea level and that can vary as weather varies the atmospheric pressure. Plus, if you are above sea level the boiling point will be lower. My classroom is at ~2700 feet and water boils between 97.5 and 98.5.
The best way to measure boiling point is to measure the temperature of the condensing vapor, rather than the boiling liquid. When I had students calibrate their Vernier temperature probes they had to measure the pressure in the room using a mercury barometer and determine the boiling point that day.
Compute the boiling point with the Antoine equation.
Tb = 1730.63 / (8.07131 - log(P)) - 233.426
Tb is in Celsius, P is in Torr
To make it simple, I had the equation in a small Excel spreadsheet so that students could enter the pressure in the lab and it would return the boiling point. (Each lab station was equipped with a computer and a Vernier interface card.)