Wind Chill: Applies when temperature is ≤10°C and wind speed is ≥5 km/h. Uses Environment Canada formula.

Heat Index: Applies when temperature is between 27°C and 43°C (80–110°F). Uses NOAA Rothfusz regression.

Feels Like: Shows actual temperature when conditions don't meet thresholds.

The Fahrenheit line on the graph is steeper because of the 9/5 (×1.8) multiplier in °F = (°C × 9/5) + 32. For every 1°C change, the Fahrenheit value changes by 1.8°F.

This ratio comes from how each scale divides the range between freezing and boiling: Fahrenheit uses 180 degrees (32°F to 212°F), while Celsius uses 100 degrees (0°C to 100°C). So 180 ÷ 100 = 1.8 — the exact multiplier.

The two scales meet at exactly −40°, the only point where °F = °C. Setting °F = °C in the formula and solving: C = (C × 9/5) + 32 → −4/5 × C = 32 → C = −40.