In mountainous areas that experience a winter season, precipitation can fall in the form of snow. Snow that has fallen on the ground and does not melt for months due to below-freezing temperatures is called snowpack. Snowpack can consist of multiple layers of snow, each one from a different snowfall, that become compacted under the weight of the subsequent layers that lie on top. The snowpack remains on the ground until the arrival of above-freezing temperatures in the spring, which causes it to start to melt. The water from the melting snowpack is called snowmelt.

The depth of the snowpack is influenced not only by the amount of snowfall but also by temperature and wind. Strong winds can evaporate snow cover, eroding the top layers of the snowpack, while an increase in temperature can cause layers to melt. In areas with an abundance of snow and proper conditions, the snowpack can accumulate to a depth of three meters (10 feet) or more. The density of a snowpack—how closely packed the snow particles are—increases as more layers accumulate, pushing down on the layers below.

In the spring, the snowpack melts from the top down as temperatures rise above the freezing point. The amount of water released by snowmelt varies, depending on the density of the snow. Wet, heavy snow can release about 3.8 centimeters (1.5 inches) of water per 30 centimeters (12 inches) of snow. Lighter, drier snow might contain just 2.5 centimeters (1.0 inch) of water per 30 centimeters (12 inches) of snow.

The snowpack can be measured using instruments, such as snow depth sensors and snow pillows. A snow depth sensor measures the depth of the snowpack using sound waves. A snow pillow is a large pouch containing antifreeze with a measuring tube sticking upwards out of it. The snow pillow is laid on the ground, and as snow piles on top of it, the weight of the snow pushes the antifreeze out of the pouch and into the measuring tube. Snow pillows are used to measure the weight of snow and determine the volume of waterthat would be produced if the snowpack melted.

Snowmelt is an important water source that keeps streams flowing in the warmer months. It is also an important water source for humans, replenishing reservoirs. One-third of the water used by California cities and farmland comes from melted snowpack.

Snowpack data is valuable for monitoring the effects of climate change. Snowpack records from the Western United States indicate that the amount of snowpack is declining. The snowpack in Oregon, Washington, and the Sierra Nevada and Cascade Mountains of California was at a record low in 2015. This decline in the snowpack iscaused by changes in temperature that come with climate change.