Swelter: Heat Index

Know when heat turns dangerous — Swelter reads the heat index from temperature and humidity and flags your risk band. Local conditions worldwide, or type them in.

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.