Water boils cooler at altitude. This tool tells you how much cooler, and what to aim for at the kettle.
Atmospheric pressure drops with elevation, and boiling point drops with pressure. A widely used approximation is a reduction of about 1°C per 285 metres (or roughly 1°F per 500 ft) above sea level. For specialty coffee, the SCA recommends brew water between 92–96°C (198–205°F). At high altitude, boiling water may already sit below that ideal, so brewing with off-boil water and grinding slightly finer can compensate.
Espresso machines pressurise the boiler above ambient pressure, so altitude has a smaller effect. This tool is aimed at kettle-based brewing.
It's a linear approximation of a slightly curved relationship. It's accurate to within ~0.5°C for all everyday elevations.
Extraction is temperature-dependent. Cooler water dissolves fewer flavor compounds, so under-extraction (sour, weak) becomes more likely at altitude.
Yes. Every degree matters. Pre-warm your brewer and mug with hot water before brewing.