Almanac · Gear
Instruments, not equipment
This almanac is about measuring conditions, so this page lists the instruments that measure them. These are categories rather than endorsements — each link runs a search rather than pushing a single product.
A handheld UV meter reads actual erythemal irradiance where you are standing, which forecast values cannot account for after cloud, shade, altitude and reflection.
Relative humidity is half the heat index calculation, and local humidity frequently differs from the regional forecast, especially near water or under canopy.
WBGT is the standard used by athletic and military programmes because it measures the two things the heat index assumes away: radiant sun load and wind.
Civil twilight ends sooner than most plans allow for. Red light preserves rod-mediated dark adaptation, which takes 20 to 30 minutes to rebuild after white light.
Sweat rates in heat commonly exceed a litre per hour, and carried capacity is the practical limit on how long an outing can run.
Sweat sodium concentration ranges from roughly 200 to 2,000 mg per litre. Beyond about 90 minutes in heat, sodium replacement stops being optional.
A UPF-rated layer provides continuous protection that does not depend on reapplication quantity, which is where sunscreen most often fails in practice.
Effective protection depends heavily on application quantity. Laboratory SPF assumes 2 mg per square centimetre; typical real application is a quarter to a half of that.