The first taste of winter will push into the Valley at the start of the work week and bring chances for wintry precipitation.

Snow showers will become likely, especially in the snowbelt region Monday night and chances will continue into the day on Tuesday and Wednesday.

Snowflakes are known for their beautiful geometries and unique shape, but how do they form? We will dive into the science behind a snowflake.

What temperature does water freeze at?

This might sound like an easy question to you. Everyone was taught in school that water freezes at a temperature of 32°F, but did you know that this is not always the case?

In fact, water often exists in our atmosphere as a liquid at temperatures well below freezing. When water is in a liquid stage at temperatures below freezing it is known as supercooled liquid water.

Pure, distilled water can stay as a liquid even when the temperature is at -40°F. However, since there are impurities in the atmosphere all of the water in the air will typically freeze if the temperatures reaches -4°F.

A cool science experiment that you can easily recreate at home is to take distilled water and place it in the freezer. Leave the water in the freezer for several hours and when you take it out you will notice that it is still a liquid. If you strike the bottle it will initiate a flash freeze of the water before your eyes. Click the link for a video demonstration:

YouTube video explaining the flash freeze science experiment; Courtesy of Mr. Hacker

What causes water to freeze?

If there were no impurities in the atmosphere, then water would not freeze until at least a temperature of -40°F. However, we know that is not the case so what causes water to freeze in the air?

Water droplets require a particle to interact with to begin the freezing process. These particles are called ice nuclei and are responsible for the growth of ice crystals that eventually form snowflakes.

There are several ice nuclei listed below, but the most commons ones are: salt, silver iodide, and other ice crystals.

Common ice nuclei and the temperature that ice occurs at; Courtesy of the National Weather Service in Nashville, Tennessee.

How do ice crystals grow into snowflakes?

The ingredients are on the table. There is air cold enough to freeze water and there are ice nuclei that will start the process of forming ice crystals, but how do these crystals form into the beautiful snowflakes that we know and love?

There are three main types of ice crystal growth that produce snowflakes. The first and most important snow growth process is called deposition. Deposition with water occurs at cold temperatures where water vapor condenses into a solid (ice) without passing through the liquid phase. This process usually begins when the temperature in the atmosphere decreases below -4°F

This process occurs at a maximum between the temperatures of -12°C (10°F) and -18 °C (0°F) and ice crystals that form in this region are referred to as dendrites. A picture of some dendrites is shown below.

Snowflakes
Courtesy of Adam Clayton

Another process that provides a small contribution to snow growth is called accretion. This occurs at warmer temperatures of 0°C (32°F) to -10°C (14°F) when supercooled water droplets freeze onto ice crystals.

The other main process that leads to the development of large snowflakes is called aggregation. This process occurs when groups of dendrites clump together to produce large snowflakes called aggregates.

In fact, the large snowflakes you see in the winter time are actually a collection of dendrites that have clumped together. Aggregates are most likely to form at temperatures between -5°C (23°F) to -10°C (14°F) where the snow because “sticky” due to the presence of supercooled liquid water.

The three different ice crystal growth processes are shown below. Now you are an expert on snow growth!

Ice crystal growth processes courtesy of the National Weather Service in Nashville, TN