The amount of rainfall—liquid precipitation that falls to the surface—is measured by a rain gauge. The most commonly used free-standing rain gauge is a cylinder with increments (most often in inches) inscribed on the outside of the tube. It is put in an area that is not obstructed by buildings, trees, or other tall structures than can impede the collection of rainfall.

The rain gauge can also measure snow, but added steps are needed to calculate the measurements. In this case, a rain gauge measures the liquid equivalent of snow. This is why meteorologists will often say that a snowstorm that produces 10 inches (25.4 centimeters) of snow will have a liquid equivalent of 1 inch (2.54 centimeters) of rain, or a ratio of 10:1. But this generalization can be tricky. If the weather system is super-cold, such as an Arctic air mass over Canada and the northern United States, the below-freezing temperatures might create more than 10 inches of snow. Meteorologists often call this the “fluff factor,” because the snow seems “fluffier” due to the fact that there is more air between the snow crystals at much colder temperatures. In fact, in very cold air the snow-to-liquid equivalent ratio can be 15, 20, or even 30 to 1. (For more about weather and math, see “Math in the Natural Sciences.”)

A weather station on top of the Alps, including a rain gauge and anemometer for measuring wind speed.