Look up in the sky, a big winter myth is debunked
Monday, December 3, 2018, 7:34 PM - The 'freezing level' is a term that's thrown around quite a bit, and it creates an immense amount of confusion, and likely does more harm than good. Let's break down what it really means, and how it differs from the 'snow level'.
- The freezing level does not technically mean the snow level (elevation where snow is falling), the snow level is often several hundred metres below the freezing level
- The freezing level varies depending on location, and can vary by over 2000 metres in elevation across the province of B.C.
- Radar can give insight into the snow level, with a paticular signature giving away the elevation of the phase change
The freezing level is typically given in metres above sea level, and the image below is a crude sampling from each corner of the province for this Wednesday – just to give you an idea of how much it varies.
You might be able to better visualize it like this. The zero degree temperature line (isotherm) is the freezing level, and it crosses some elevation at or above sea level.
When you're driving up the mountain, you can also pay attention to your vehicle's built in thermometer, once it hits 0°C, you just drove through the zero degree isotherm. Congratulations!
But, here's the thing. You might have encountered some snow long before then...
The snow level is a little less abstract. because it leaves a visual cue...snow.
The snow level is the location where the phase of precipitation is about to undergo a phase change to rain. The snow at the snow level is often sticky, wet, and can make great snowball snow, but can also be quite slushy.
It may surprise you how far a snowflake can survive as it passes below the freezing level. Even though the temperature is at or slightly above the freezing mark, the snowflake remains intact, especially if the environment is quite dry.
A couple processes can make a snowflake survive a little further of a journey down the ski slope. Evaporational cooling can cool the surrounding environment. After all, it takes a lot of energy to melt a snowflake. But if the environment is too dry, it's possible the snowflake turns into virga (precipitation that evaporates before reaching the ground). Intensity of the precipitation plays a role too, with heavier bursts of precipitation more efficient at dropping the temperature of the surrounding air.
Local effects make forecasting snow in Vancouver a challenge, as cold air can often pool south of the North Shore mountains, and outflow winds are also a key in refrigerating the lower levels of the atmosphere.
SOPHISTICATED RADAR TECHNIQUE
This technique is called bright banding, and a ring of enhanced reflectivity shows up on radar returns when precipitation changes phases from snow to rain. Snowflakes are relatively large objects, so when water covers the surface of the snowflake, it can give the appearance of a much larger object, like a hailstone or a massive raindrop.
This artificial enhancement gives meteorologists an idea of where this elevation of the phase change is occurring, and can be useful when forecasting snow events in Vancouver.