DISCUSSION: When it comes to looking at snowfall forecasts and snowstorms themselves, there are always many questions about why different snowflakes form and even when the same types of snowflakes are falling, why they may take on slightly different shapes. One classic example of this type of occurrence is when the most famous type of snowflake (i.e., dendrites) are fully part of a major winter storm. As shown in the picture above, which is one classic example of a stellar dendrite, they can be quite complex and stunning at the same time. However, there is much more that goes into how a snowflake forms the way it does and how complex the various outgrowth of the snowflake becomes as if all stored the surface of the Earth.
One such factor which has a major influence on the way in which a given snowflake such as the one shown above (courtesy of www.snowcrystals.com) will form is the variable moisture profile which is present through the depth of the middle to lower portions of the atmosphere. The reason for this is because whenever you have increased moisture in the presence of developing snowflake crystals, there is an increased potential for more accumulation of supercooled water and/or other nearby ice crystals onto the developing snowflake which can make them much larger or smaller depending upon the amount of water vapor available. In addition, whenever there is a lot more water vapor available within the depth of the atmosphere were the most snowflakes are forming (which is often referred to as the dendritic snow growth zone in atmospheric science terms), this can also allow snowflake generation processes to be much more effective and much more robust. Thus, when there is a greater presence of deeper lower to mid-level moisture content in the presence of a developing or mature snowstorm, there will often be much more efficient snowflake development as a storm wraps up and/or matures.
A second major factor which has a substantial influence on how and to what extent a given snowflake will form is the variable temperature profile through the depth of the middle to lower portions of the atmosphere. The reason for this is a result of the fact that when there is either a warmer or colder net temperature trend as this snowflake is going from its height of formation for the surface of the Earth, a snowflake can take on many different shapes depending upon the variable temperatures the ice crystal is exposed to as it is growing and developing during its path towards surface of the Earth. This reality is also reflected by the context of the second graphic shown above in an approximate sense since this is a rough approximation of the realistic temperature ranges for various case crystal types.
It is for these reasons that when people use the metaphor no two snowflakes are ever quite the same at any point in the world, this statement is completely true in every sense of the word. The reason is simply because as an ice crystal is following towards the Earth, the precise environmental conditions the ice crystal is encountering during its path downward will always be at least slightly different than the snowflake right next to it even at a very minor level. So, the next time someone uses the metaphor when there is a snowstorm or a threat of an upcoming snowstorm, you now have some real context behind that age-old phrase.
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©2020 Meteorologist Jordan Rabinowitz
If you’re living in the midwestern United States, you were probably confused and possibly unmotivated because in this area there was a lack of sunlight for the entire week of January 27. There are multiple factors that can contribute to the consistent cloudiness that persists for multiple days, and weather patterns that have brought on coincidental cloudiness. The dreary days that have been in effect for that week may seem like the usual wave of January weather — but fear not, winter is no stranger to seeing the sun.
Projecting our weather analysis to the 300 mb level, which is where the jet stream lies, we can see patterns of where warm or cold air patterns are to be directed. At this level, we can see a closed low that is often very slow moving since it sits out of the general stream of westerly air that is seen. Consequently, the closed low has little to move it along and can persist over the course of multiple days. This can also be called a cut-off low because it is removed from the general flow pattern and moves slower than typical weather patterns. Since low pressure systems are associated with cloudiness, rain and snow — this is a cause of persistent cloudiness.
Image: Gathered from the 12z GFS on 2/28, this depicts a closed low that is separated from the general flow of the upper atmosphere at 300mb. (Photo courtesy of Pivotal Weather)
Of course, weather patterns work together throughout the atmosphere and pure coincidence could lead to extended periods of cloudiness. Coincidental passings of multiple low pressure systems to a certain area can occur, along with multiple cold frontal passages can lead to extended periods of cloudiness.
Warmth can even lead to persistent cloudiness with the formation of fog when warm air moves over a cold surface. Since the atmosphere is built of many different levels that are working at different rates, we can experience different weather systems closest to the surface, instead of deep through the atmosphere. For example, a small area could experience low clouds and fog which can exist in the depth of the boundary layer. This layer extends about one kilometer above the surface, most commonly where lake effect convection occurs and can be specific to a small area.
While these are all possibilities for cloudiness to occur in a given region, it’s not particularly common to see this happen for such a long period, but it certainly can. For the specific case of the week of January 27th in the midwestern United States — almost all of these factors made an appearance. From passing snow showers and a wintery mix, to slight warmth leading to fog, a strong enough front with a proceeding high pressure system is necessary to conclude a period of cloud coverage.
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© 2020 Meteorologist Jason Maska