Heat index and hydration planning

Why a "90°F, low humidity" day and a "90°F, high humidity" day are not the same ride, and how to plan fluids for each.

Last updated: September 22, 2026

Temperature alone doesn't tell the story

Your body's main cooling mechanism on a bike is evaporating sweat. That process depends on humidity: on a dry day, sweat evaporates quickly and carries heat away efficiently; on a humid day, the air is already saturated and sweat can barely evaporate at all, so it just runs off without cooling you. Two rides at the same 90°F can feel completely different depending on humidity -- which is exactly what heat index (sometimes called "feels like" temperature) is built to capture, by combining temperature and humidity into one number.

Ride Planner computes a time-weighted average heat index across your ride using the forecast temperature and humidity at your projected location for each hour, rather than just the forecast high for the day. A ride that starts cool at dawn and finishes in the afternoon heat gets a heat index that reflects both parts, not just the hottest moment.

How the fluid target scales

Below about 65°F heat index, Ride Planner uses a baseline fluid target of roughly 20oz/hour, which covers ordinary sweat losses for most riders at a moderate pace. As heat index climbs above that, the target ramps up toward a cap of about 40oz/hour, reflecting the higher sweat rates that come with hotter, more humid conditions. This is a rough population-level guide, not a substitute for knowing your own sweat rate -- some riders lose far more or less than "average" -- but it's a reasonable starting point for deciding how many bottles to carry or where to plan a refill stop.

The tool converts that hourly rate and your estimated ride time into a total fluid need in ounces, then divides by your bottle size to suggest how many bottles you'd need to carry (or refill) to cover the ride.

Calories: burned versus consumed

Separately from fluids, Ride Planner estimates total calories burned from the same power-and-terrain simulation used for ride time -- it accounts for reduced effort while coasting or braking on descents, not just power multiplied by time. It compares that to the calories you'd take in during the ride at your chosen on-bike fueling rate (kcal/hour), to produce an on-bike deficit or surplus.

A deficit here isn't automatically bad -- most rides run some deficit, made up from the body's own glycogen stores, which for a trained cyclist run roughly 1,600-2,000 kcal. Ride Planner flags a deficit under about 1,500 kcal as fine, up to about 2,000 kcal as worth a caution, and beyond that as a real bonking risk, since you'd be leaning hard on stores that are largely used up. If height, age, and sex are provided, it also estimates a whole-day off-bike food number using a standard Mifflin-St Jeor basal metabolic rate calculation, on top of the on-bike deficit or surplus.

Separately, if your on-bike fueling rate falls well outside the commonly cited 60-90g of carbs per hour endurance guidance for rides longer than about 2.5 hours, the tool flags that too -- it's easy to under-fuel on a long ride without noticing until the last hour.

Precipitation and storms

Alongside heat and fluids, Ride Planner shows an hour-by-hour precipitation timeline for your ride, looked up at wherever you're projected to actually be on the route at that hour rather than just the start line. A lightning-bolt marker flags hours where the forecast shows thunderstorm-type weather -- Open-Meteo gives a yes/no thunderstorm forecast rather than a calibrated percentage, so that's shown as a flag rather than a number.