Swelter: Heat Index

Know when heat turns dangerous — Swelter reads the heat index from temperature and humidity and flags your risk band. On the jobsite or the field, with signal or without.

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 the nearest National Weather Service station when you're online, 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 backend of its own. Your heat log stays on your device. Location, when you allow it, is used only to ask the National Weather Service for the nearest observation — 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. Swelter is an informational tool, not a medical, safety, or life-saving device — always follow your organization's heat-safety plan and local regulations.