Swelter: Heat Index & Safety

Is it good to run, hike, train, or work outdoors right now? Current heat index & safety for outdoor activity. WBGT algorithm validated against 20k+ station-hours.

Swelter answers one question: how risky is it to be active outdoors right now? Track the current heat index, and the number that goes beyond it: WBGT.

Swelter computes WBGT (wet-bulb globe temperature) right on your phone. It is the standard measure of heat stress, used worldwide for decades to decide when to limit outdoor activity. Your local conditions become one of four flags: GOOD TO GO, CAUTION, HIGH RISK, or CANCEL.

Swelter's four flags follow the American College of Sports Medicine's published WBGT thresholds for outdoor activity. GOOD TO GO runs below 82°F (27.8°C). CAUTION begins at 82°F (27.8°C), HIGH RISK at 87°F (30.6°C), and CANCEL at 90°F (32.2°C).

The heat index tells you how hot the air feels, but it is measured in the shade and leaves out direct sun and wind. WBGT, the number that powers Swelter's flags, takes air temperature, humidity, wind, and sun strength into account, so it reflects how well your body can actually cool itself (via the evaporation of sweat). Swelter follows published scientific guidelines from leading organizations and research in the field of outdoor exertion & heat safety, and we validated our algorithm against physical sensor data.

TODAY The current heat-risk flag, the number behind it (WBGT), and an hour-by-hour timeline. Also: heat index, wet bulb temperature, humidity, UV, and wind at a glance.

BEST WINDOW Swelter finds the least risky stretch of your day for a run, a hike, practice, or a shift outdoors.

WEEK AHEAD Seven days of hourly WBGT so you can plan around the heat.

ALERTS A heads-up before extreme heat and storms arrive. Severe-heat and storm alerts are free for everyone.

WIDGETS The current flag and today's timeline on your home screen and lock screen.

LOCATIONS Check any place in the world, in its own local time. Save up to 10 favorites (Pro feature).

HEAT RISK CALCULATOR Set the temperature, humidity, sun, cloud cover, and wind yourself, and Swelter estimates the WBGT and its risk flag. It opens with your current conditions so you can start from right now.

VALIDATED Swelter is the only consumer heat-safety app validated against the world's largest network of measured WBGT stations: 20,230 station-hours compared so far, with 90% exact flag agreement and 99.96% of readings within one flag level (the app calls out when it is near a boundary). The full methodology and sources are documented in the app.

PRIVATE BY DESIGN Supported by Pro subscriptions, not ads. No account needed, and your data is never sold. Your location is used only to retrieve Apple Weather data.

SWELTER PRO Unlocks every chart and screen: full week charts, hourly UV and precipitation, extra alert types, year-over-year trends, saved locations, and the complete widget set. Free users keep the core safety read: today's flag, the gauge, the timeline, current conditions, and severe-heat and storm alerts.

Swelter is a planning tool, not a medical device. Conditions can change quickly. When in doubt, choose the cooler option and use your best judgment.

Weather data provided by Apple Weather.

Terms of Use (EULA): https://rbrn-labs.com/swelter/terms/ Privacy Policy: https://rbrn-labs.com/swelter/privacy-policy/