Swelter: Heat Index

Swelter turns temperature and humidity into the one number that matters when it's hot: the heat index — the "feels like" temperature your body actually works against — and tells you which risk band you're in before it becomes dangerous. Built for people who work and train in the heat: construction and field crews, warehouse and kitchen staff, coaches and athletes, event and safety leads. WHAT YOU GET • Instant heat index from temperature + relative humidity, using the National Weather Service (Rothfusz) formula • Clear risk bands — Caution, Extreme Caution, Danger, Extreme Danger — labeled with text, never color alone • Auto-fill from local conditions anywhere in the world — National Weather Service stations in the United States, Open-Meteo everywhere else — or type readings in by hand when you have no signal • Fahrenheit and Celsius • A large, glanceable gauge you can read at arm's length SWELTER PRO (one-time purchase) • WBGT estimate (shade) for a fuller read on heat stress • Home Screen and Lock Screen widgets with your latest reading • Threshold alerts — get notified when the heat index crosses a level you choose • Heat log with CSV export to keep a record of the day NO ACCOUNT. NO TRACKING. Swelter has no sign-up and no user accounts. Your heat log stays on your device. Location, when you allow it, is used only to fetch the current conditions where you are — it is never stored or shared. Swelter is an independent app. It is not affiliated with, endorsed by, or connected to the National Weather Service, NOAA, OSHA, or any government agency. Heat-index values and risk bands are based on publicly published National Weather Service guidance. Weather data outside the United States comes from Open-Meteo (CC BY 4.0). Swelter is an informational tool, not a medical, safety, or life-saving device — always follow your organization's heat-safety plan and local regulations.