For running and other strenuous outdoor exercise, the feels-like temperature is the single most important factor, and the score weights it accordingly. When you run, your body generates a great deal of heat, and its ability to shed that heat is what limits hard effort. Cool conditions let you run comfortably and safely, which is why the score favours a cooler band than it would for, say, a picnic. Heat is the real enemy: as the feels-like temperature climbs, the risk of overheating, dehydration and heat illness rises steeply, and the score falls to reflect genuine physiological strain rather than mere discomfort.
Humidity deserves its own weighting in the running score because of how the body cools itself. Sweat cools you only when it evaporates, and evaporation slows dramatically when the air is already near saturation. This means a humid warm day is far more punishing for a runner than a dry day at the same temperature, because your cooling system stalls just when you need it most. The score therefore penalises high humidity, capturing why an oppressive, muggy afternoon feels so much harder than the thermometer alone suggests and why runners in humid climates learn to start early or late to avoid the worst.
Rain and wind matter to runners but less than temperature. Light rain is often no barrier at all and can even be refreshing on a warm day, though heavy rain brings the chill of wet clothing and the hazard of slippery footing, especially on trails. Wind is more of an inconvenience than a danger for most runners, adding resistance and chilling exposed skin, so the score weights it modestly. The one combination to respect is cold, wet and windy together, which strips body heat quickly and can be genuinely dangerous on a long effort, particularly for hikers exposed on high or open ground.
The score helps you time your effort and dress appropriately. On a hot, humid day with a low score, the sensible response is to shift your run to the cool of early morning or evening, cut the intensity, and carry water. On a cold day, dress for the conditions but remember you will warm up quickly once moving, so layers you can shed are ideal. For hikers heading into hills or exposed country, the score is a useful baseline, but mountain weather can differ sharply from the valley forecast and change fast, so always check a mountain-specific forecast and carry protection against sudden cold, wind and rain regardless of what the score says at the trailhead.