Swelter: Heat Safety & WBGT

The only consumer heat-safety app validated against the world's largest network of measured WBGT stations.

Swelter answers one question: how risky is it to be active outdoors right now?

Swelter computes WBGT (wet-bulb globe temperature), the standard measure of heat stress used worldwide for decades, right on your phone, and turns it into four flags: GOOD TO GO · CAUTION · HIGH RISK · CANCEL.

Air temperature and heat index (feels-like temp) alone undersell dangerous heat. They leave out direct sun and wind, the factors that decide how well your body can actually cool itself. Swelter runs on WBGT, which takes into account temperature, humidity, wind, and sun strength (based on real solar geometry for your exact location and hour).

Along with clear guidance on whether you're good to go or should postpone outdoor activity, Swelter provides useful insights and features:

TODAY Your current heat flag, the number behind it, and an hour-by-hour timeline. Pick any hour to see how the risk changes throughout the day.

BEST WINDOW Swelter finds the least risky stretch of your day for a run, a hike, practice, or a shift outdoors, and shows when it opens and closes.

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

ALERTS A heads-up before extreme heat windows 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.

LOCATION Pick any location around the world to view current WBGT conditions, shown in that place's own local time. Save up to 10 favorite locations to quickly switch between.

VALIDATED Swelter is the only consumer heat-safety app validated against the world's largest network of measured WBGT stations: 10,760 station-hours compared so far, with 99.94% of readings within one flag level.

PRIVATE BY DESIGN No account, no third-party analytics, and no selling your data; we are supported entirely by Swelter Pro subscriptions. Your location is used only to retrieve Apple Weather data.

SWELTER PRO Unlocks every chart and screen: the 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/