Does altitude affect coffee brewing?

Yes, and it catches people out when they move or travel.

The physics

Water boils at a lower temperature as air pressure falls. Roughly one degree per 300 meters (984 ft):

• Sea level: 100 °C (212 °F)
• 1,000 m (3281 ft): about 96.5 °C (206 °F)
• 2,000 m (6562 ft): about 93 °C (199 °F)
• 3,000 m (9843 ft): about 90 °C (194 °F)

At 2,000 meters (6562 ft), water straight off the boil is already at the bottom of the useful brewing band. At 3,000 it is below it.

What that does

Under-extraction. Brews taste sour, thin and weak, and the natural response — using more coffee — makes them strong and still sour, because strength and extraction are different things.

How to compensate

Grind finer. The main lever. More surface area offsets the lower temperature.

Brew longer. Extend steep times by 30 to 60 seconds.

Do not let the water cool at all. At sea level you wait thirty seconds off the boil; at altitude, use it immediately.

Espresso

Less affected, because the boiler is pressurized and controls its own temperature independently of the atmosphere. A PID machine at altitude brews at the temperature you set.

The pressure-cooker trick

Some high-altitude cafés use pressurized kettles. It works and it is unusual.