Below Zero (Sub-Zero): Meaning, Origin & Winter Safety Guide
Quick Definition (Winter Idioms & Cultural Phrases)
A temperature reading below 0° on the thermometer — which means 32 degrees below the freezing point of water on the Fahrenheit scale (0°F = -17.8°C) in the United States, or anything below freezing (0°C = 32°F) on the Celsius scale used in Canada and the UK.
Also called: sub-zero, subzero temperatures, negative temperatures
If you only do three things
- Know the scale: 0°C (Celsius) is 32°F (freezing), while 0°F (Fahrenheit) is a severe -17.8°C deep freeze.
- Below 0°F (-18°C), standard rock salt fails — use calcium chloride or traction sand/grit.
- Below 0°F, keep your gas tank above half full and carry a 12V lithium jump starter.
Origin, meteorology & what "Below Zero (Sub-Zero)" means
When German physicist Daniel Gabriel Fahrenheit invented his mercury thermometer in 1714, he set 0°F as the coldest stable temperature he could create in his lab using a frigorific mixture of ice, water, and ammonium chloride salt (putting the freezing point of pure water at 32°F).
Because the US uses Fahrenheit while Canada and the UK use Celsius, 'below zero' causes major cross-border confusion: '5 below zero' in Chicago (-5°F) is a dangerous Arctic freeze (-20.5°C), whereas '2 below zero' in London (-2°C) is a mild frost (28.4°F)!
Real-world winter safety & what to do
When US forecasts call for 'below zero' (below 0°F / -18°C), standard winter habits are no longer enough: standard rock salt stops melting ice completely, weak car batteries fail to crank, unprotected skin frostbites rapidly in any breeze, and uninsulated pipes can freeze even inside heated homes.
Switch to calcium chloride ice melt, plug in your engine block heater if equipped, and wear a balaclava or scarf over your face outdoors.
Interactive Ice Melt & Salt Selector
Frequently asked questions about Below Zero (Sub-Zero)
At what temperature are Fahrenheit and Celsius the exact same number?
-40 degrees! At -40°F, the Celsius reading is also -40°C — extreme subarctic cold where exposed skin freezes in minutes.
Does rock salt work when it is below zero Fahrenheit?
No. Standard rock salt (sodium chloride) loses almost all practical melting power below 15°F (-9°C) and does nothing below 0°F (-18°C).
Related situational guides
- Shoveling & Ice Melt Temperature Guide: Which de-icers work below 15°F and below 0°F.
- What to Wear in Sub-Zero Cold: Dressing for below-zero Fahrenheit conditions.