The 32 F line is the isotherm on the surface chart where the temperature on one side is at or below freezing and the temperature on the other side is above freezing. It is also called the freezing line. In actual practice the 32 F line is more of a zone where the temperature is just above or just below freezing. The passage of the 32 F line is a weather changer since it can contribute to icing of the ground surface and can cause precipitation to change from rain/drizzle to freezing rain/freezing drizzle.

The movement of the 32 F line is accomplished through cold air advection, evaporative cooling, any other cooling processes and any processes that could warm the air such as warm soil temperatures. The 32 F line will tend to advance forward when significant cold air advection is taking place. When the air mass becomes more stagnant then the progression of the 32 F line will tend to be much slower and can retreat due to warming processes such as solar radiation, soil temperature above freezing and rain falling through a warm layer aloft before falling to the surface.

When the air mass is dry at the surface and rain aloft falls into the dry air, the 32 F line will tend to advance forward due to evaporative cooling. Other warming and cooling processes will need to be considered when determining how far the 32 F line will advance forward. On TV weather maps, many times, especially in cases of potential winter weather, the weather broadcaster will have the 32 F isotherm drawn on the surface temperature chart and a time lapse into the future shown so that viewers can see its position relative to where they live. The position of the 32 F line is important to the National Weather Service when it comes to issuing travelers and winter weather advisories. Critical changes in weather can occur when the surface temperature drops below freezing, thus the passage of the 32 F line is a significant weather changer.