Q&A - Ask away! I'll answer Pt. 4
Only one question this week in the Q & A! It's a long answer. I'll address more questions next Friday so check back!
As a reminder, you can submit questions in the comment section of this post, or email me directly through the website contact form. Questions can also be sent through any of my social media platforms. Find me @leahwx11 on Facebook, Twitter, and Instagram. If I miss your question, keep commenting on futures posts! I want ALL your questions answered.
Why does it seem to get warmer right before a snowfall?
Here in the Ozarks, it’s not uncommon for temperatures to be above freezing as a snowstorm moves in. However, snowfall does not cause temperatures to warm. For snow to fall, the layers of the atmosphere under the clouds need to be below freezing otherwise, the snow melts. Snow can overcome warmer surface temperatures.
If temperatures are above freezing, the cooling process of the snowflake evaporating can cool the air. This can happen quickly when the air is dry. Essentially, the rate of evaporation cools the air before the above freezing temperatures melt the snowflake. When snow hits the ground and melts, this will also cool the air. If the warmer air cannot be cooled by evaporation, or by colder air moving in, the snow will transition to rain.
Another note...Just because temperatures are below freezing, it doesn’t mean we can’t get freezing rain. We’ve had several cases this week where surface temperatures were below freezing, but there was freezing rain falling. All layers of the atmosphere must be considered when determining the type of precipitation. I go into more detail on how different layers of the atmosphere change precipitation type in a previous blog post. The figure below explains it, but you can read also read more by clicking HERE.
Another factor important for snowfall is warm air advection (WAA). This may be shocking. Having warm air seems counterproductive, especially if you want snow. Advection means transport, in our case, it’s the transport of either cold or warmer air by the wind. Cold air advection occurs when cold air moves horizontally into a region of warm air. Warm air advection does the opposite, moving warm air horizontally into a region of cold air. Why is warm air advection so important? I’m going to simplify some things when I explain this because as with all meteorological processes, a LOT is going on. I’m just going to break down the pressure and the temperature.
Generally speaking, when we have low pressure moving in we often see precipitation in the Ozarks. As seen in the graph below, winds around low-pressure move in a counterclockwise direction. High pressure often brings us clearer weather. The winds around high-pressure move in a clockwise direction. Fronts are associated with these pressures as they are the boundaries between the changing air masses.
Winds in the atmosphere move from high pressure to low pressure. The graph below gives the flow pattern if you're looking DOWN on the atmosphere from higher up. The movement is caused by the circulation of the high and low pressure.
Low pressure is associated with rising motion in the atmosphere. This lift is one of the ingredients for precipitation and storm formation. Air follows the region where the pressure drops. In the case of snowstorms, this pressure drop is associated with WAA advection. WAA in the mid-levels of the atmosphere causes the air to expand. This expansion creates a region of falling pressure at the surface with rising pressure aloft. Our surface low pressure bringing snowfall will move to this pressure drop at the surface (moving from an area of relatively higher pressure to an area of lower pressure), caused by WAA. So if you want snow, and if you want lots of snowfall, hope for warm air advection.
Moisture is another ingredient needed for snow. When temperatures drop so does the ability for air to hold moisture. When I say warm air, I don't mean temperatures in the 40s or 50s. We can have a temperature of 20 degrees near the surface and that is considered relatively warm. Click HERE for more on how the humidity and moisture can change with temperatures.
One last point - While air won't be too cold for snow, the air can be too dry for snow. Consider the Antarctica Dry Valleys, they are very cold but have low humidity, so this region sees very little snow. Fun fact, temperatures can drop to -90° with winds upwards of 200 mph. I'll take our temperatures in the teens with sub-zero windchills over that any day.