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Whether it’s rain, snow, freezing rain, sleet, or even freezing drizzle, knowing what precipitation is occurring is extremely important to understand. This is especially vital during the winter where temperatures can often hover right around the freezing mark, and precipitation can change from snow to rain very easily. During the winter months in the Northeastern region of the United States, temperatures tend to fluctuate quite often between freezing and above freezing. This location specifically is a hot spot for mid-latitude cyclones to come across the country where the combination of both warm and cold frontal boundaries can pass over an area within a matter of hours. This creates many opportunities for a wide range of precipitation to fall and it’s important to a forecasting meteorologist to be as accurate as possible when looking at what to expect from an atmosphere that can be quick to change its course.
A great way to look at possible precipitation is by analyzing the layers of the atmosphere starting from the top and working down to the surface. This can be done by looking at a sounding that looks at a vertical profile of the atmosphere showing how the temperature is changing with height. Below is an example of common types of precipitation formed dependent upon certain conditions.
A typical rule to follow is that if you have air warmer than -10 degrees Celsius, there won’t be any ice initiation, or ice forming around a rain drop within the layer. If air is colder than -10 degrees Celsius, then there definitely can be ice initiation. The follow up questions would be, is there icing occurring in the upper layers? Is there a warm layer where snow could fall into and melt? A hydrometeor, or any product of atmospheric water vapor that falls as precipitation, typically stays as snow if the layer is below 1 degree Celsius and can melt into liquid water in a layer that’s above 3 degrees Celsius. Considering the wet bulb effect of a warm layer can also be helpful because it can tell you how much water vapor is present in the air based on the evaporation of water, and how that can decide the type of precipitation as well. For example, if the wet bulb temperature is below -10 degrees Celsius in a cold layer, and the surface is above around 1.5 degrees Celsius, then you could get sleet as precipitation.
In the event of a Nor’easter, precipitation can vary greatly depending on the time of year it occurs. Even the sea salt in the oceans can have an effect on the type of precipitation and act to enhance ice nuclei to activate the snowflake process.
Something to note is that these tips are NOT an entirety of depicting the characteristics of the atmosphere, as precipitation type forecasting is not an exact science. The analysis of multiple variables and layers of the atmosphere is required, proving to be a significant challenge in forecasting. The different techniques to approach this add to the complexity of this method as well.
It’s also important to know what is currently happening outside regardless of the forecasting situation. Using the current conditions to predict minute-by-minute events is utilized in situations where the atmosphere is changing rapidly than models and forecasters can grasp.
It is too often it seems that the difficulty of forecasting the precipitation type has been a cause of major accidents and hazardous situations that catch the public off guard. Having such variable temperatures in mid-latitude cyclones can lead to the melting and freezing of water on roadways that can cause ice jams in rivers as well as flash freezing on highways. Freezing drizzle, for example, may seem like a minor event, but even a trace of it with below freezing temperatures at the surface can become a very dangerous event.
Often, meteorologists can be too focused on the “big events” and neglect to recognize how much of an impact the change in temperature by a couple of degrees can affect people that need to walk and drive to get to where they need to go. People walk down their driveway, slip and get very injured due to black ice, and cars and buses go off the roads, which is why it’s so vital to be aware of these events. It can be a very tough decision for forecasters as well as administrators when trying to keep children safe on their way to school. Meteorologists and forecasters understand the weight of their decisions and believe in focusing on the small details that can make or break a forecast. Therefore, understanding the precipitation type is a valuable skill to have during any weather event, and helps us understand the atmosphere even more as it unfolds in front of us.
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© 2019 Weather Forecaster Christine Gregory