Conditions, not activities
Calculate Outdoors is an almanac for the four variables that decide an outdoor day before the activity does: sunlight, daylight, heat, and the energy the effort will cost. Four instruments and thirty pieces of reference, built on published physiology and solar geometry.
The four instruments
Each one takes the environment as its input and returns a decision, not a pace.
UV index, skin type, exposed surface area and age against the erythemal dose model. Returns synthesis in IU and your personal burn threshold, so you can see where the productive window closes.
DaylightSunrise, solar noon, sunset and civil dusk computed from NOAA solar equations for your coordinates. Returns the latest departure time for an outing of a given length.
HeatThe National Weather Service apparent temperature paired with a sweat-rate estimate scaled by body mass, metabolic intensity and acclimatisation state.
EffortMatches energy cost across walking, hiking, running, cycling and more using Compendium MET values. Answers what forty-five minutes of one activity is worth in another.
Field notes
Twenty pieces on the physiology and physics the calculators run on.
The photochemical chain from UVB photon to circulating vitamin D, why the process self-limits, and what that means for how long you should stand in the sun.
DaylightHow intrinsically photosensitive retinal ganglion cells set circadian timing, why indoor light barely registers, and what the phase response curve means for when you should go outside.
DaylightCivil, nautical and astronomical twilight defined, how much usable light each provides, and why twilight duration varies enormously with latitude and season.
HeatWhere the heat index comes from, the assumptions baked into it, and the conditions under which it understates the real danger.
HeatHeat cramps, heat exhaustion and heat stroke distinguished, the central nervous system sign that marks the emergency, and why cooling comes before transport.
EffortThe metabolic equivalent defined, how the Compendium of Physical Activities was assembled, and the systematic errors that make MET values approximate.
Related sites
This almanac covers conditions. For pace, distance and activity-specific arithmetic, these independent sites go deeper.