People are saying the freezing point doesn’t change no matter what the altitude or atmospheric pressure is and although that may be scientifically and mathematically true, I can assure you that the deviation of freezing and boiling in Denver is about 3 to 4° below normal. Standing water freezes at about 28-29°F (instead of ~32°) yet it can snow at 36°. And although it’s said water boils at about 203-204°F (8 to 9° lower, instead of at a normal ~212°) I have only measured the same difference in freezing, boiling at 209° even with a ton of salt added. The locals have always known the phase points to be sightly lower than average and not by belief alone, but tried and tested experimentation.
It’s probably a combination of many factors; mountain to valley air pressure, general high altitude, dry dessert air, unusual geographic location (the Denver “bowl”), and undoubtedly many others, but there is quite a variation depending upon where you live. I can also confirm that in upstate NY, when humidity is high and pressure drops quickly, standing water can easily freeze at 36°.
Geography & the local environment, wind currents & air pressure, moisture patterns & weather anomalies, density of contaminates & human caused air-pollution, countless factors known & others otherwise insignificant (like the density of local vegitation & native flora or the amount of urban sprawl & paved development) all skew measurements. Individually they’re to slight to calculate, but combined they do have a measurable effect; exactly what all the variables are, how exactly they change things, to what degree, when and why, we may never know.