Cascade Mountain Weather

The Dendritic Growth Zone

Welcome to the Dendritic Growth Zone!

While also the name of our blog, the Dendritic Growth Zone (DGZ) is key to understanding snow forecasting. When we ski or ride that perfect blower powder, we are enjoying the fruits of the DGZ's labor, millions of stellar dendrite snowflakes. These classic six-sided snowflakes are what dreams are made of. Their intricate arms and branches leave plenty of room for air in between the snowflakes, providing low-density powder we all know and love. What is the dendritic growth zone? As the name suggests, this zone is where our favorite snowflakes grow in the atmosphere. Specifically, the DGZ is defined as the layer of the atmosphere between -10°C and -20°C. When these temperatures combine with other favorable conditions such as high relative humidity and upward motions, the DGZ is highly efficient at growing stellar dendrites through the Bergeron-Findeisen process. Also known as vapor diffusion, the Bergeron-Findeisen process grows ice crystals and snowflakes by depositing water vapor molecules in the clouds onto existing ice crystals, freezing them on contact.

Here are two plots that can help us analyize the DGZ.

How to use:

To find where dendrites are growing in the atmosphere, look for green hatched areas inside purple contours.

This figure is incredibly messy so let's break it down. The green contours/shading represent the relative humidity in the DGZ layer of the atmosphere. The more moisture, the better! The purple numbers represent vertical motions in the DGZ, otherwise refered to in meteorological plots and equations as the greek letter ω (omega). Here, ω is in units of pressure decrease per second, with more negative values showing stronger upward motions in the atmosphere (good for dendrite growth).

How to use:

To find where dendrites are growing in the atmosphere, look for dark blue to purple shading inside the red contours.

This is a 24hr forecast for dendrite growth efficiency (ω times relative humidity) in blue shading. The red contours show where surface temperatures are below freezing. It is valid for the UTC time listed in the plot's title. For example, in November we in Washington are 8 hours behind UTC so 2025-11-29 06Z is valid at 6am UTC on 29 November/10pm Pacific Standard Time on 28 November.