How snowflakes are formed.
Text: Yvonne Vahlensieck
Cold temperatures alone are not enough: for snowflakes to form, biological particles are often needed to act as ice nuclei. It is precisely these particles that Basel-based environmental scientists are researching on the Jungfraujoch glacier saddle and in the most northern part of Norway.
Most travelers taking the train up to the Jungfraujoch saddle in the Bernese Oberland are hoping for blue skies and sunshine when they reach their destination, but environmental scientist Claudia Mignani is not like other visitors. She wants thick clouds, snow, and below-zero temperatures – precisely the weather conditions that the doctoral researcher needs for her research project that asks: How exactly is snow formed?
The simple answer to this question is that pure-water ice crystals can only form below –36° C. Cloud droplets can, however, freeze in warmer temperatures, too, as a result of tiny particles in the air such as dust, soot, fungal spores and bacteria. These particles function as so-called “ice nuclei”, whereby water can, for example, condense onto them and then freeze. “These tiny ice crystals then grow as they approach the ground and finally fall to the earth in the form of snowflakes,” Mignani explains.
Mignani’s goal is to explore the frequency and properties of ice nuclei in greater depth. The high-alpine research station on the Jungfraujoch saddle is a perfect location for the study: “We are right in the middle of the clouds here, and can collect the ice crystals at the very site where they form.” There are plenty of opportunities to do this, since the research station is in the clouds around half the time.
Finding tiny particles
In spite of the ideal conditions, it is not easy to find or analyze ice-nucleating particles: They are not only microscopically small; they are also quite rare. “Not every snowflake contains an ice nucleus,” explains Emiliano Stopelli, who also completed a doctorate at the University of Basel. Snowflakes can also occur as a result of so-called “ice multiplication”. “For example, when two pieces of ice collide, small fragments break off and new ice crystals grow from these,” says Stopelli. According to his research data, the concentration of ice nuclei in one cubic meter of air on the Jungfraujoch fluctuates between (less than) one and several hundred ice nuclei, depending on the weather conditions and the time of year.
For this reason, the Basel environmental scientists developed a new method for quickly testing a large number of snow samples for ice nuclei. Mignani collects freshly formed snowflakes in specially designed containers and seals them in plastic bags. After the samples have melted, she slowly re-cools them down to minus temperatures in a water bath and records the temperature at which they freeze. “This is how we can identify which of the samples contain ice nuclei that are active in the temperature ranges that we are analyzing,” Mignani explains. Using the results of this analysis and considering various other factors, she is then able to calculate the concentration of ice-nucleating particles in the air.
Bacteria as ice nuclei
The researchers focus on temperatures between 0 and –15 °C in their studies. This is the temperature range in which biological particles like pollen, spores, bacteria and soil particles are active as ice nuclei. “We believe that biological particles are most likely to trigger precipitation in these warmer temperatures. According to our results on the Jungfraujoch saddle, this can happen very frequently along weather fronts,” Stopelli commented.
Little is known about which of these biological ice nuclei are the most dominant in the atmosphere. This is the reason Mignani is looking to identify more closely which particles are swirling about in the air. To do this, she sucks air through a particle collector – a bit like a vacuum cleaner – that has a thin silicon pad that the particles attach to. This pad is then slowly cooled under conditions that facilitate the forming of ice: “Small ice crystals form on the pad in the spots where ice nuclei are attached. We magnify these spots to a much bigger size and then examine them using a scanning electron microscope in order to identify the individual ice nuclei.”
If the ice nuclei turn out to be living bacteria, it is sometimes possible to propagate these in the laboratory and to identify the exact kind of bacteria using DNA analysis – this is how Stopelli managed to gather cultures from the Pseudomonas syringae bacterium from several snow samples. This kind of bacteria is harmful to plants and primarily affects the farming industry, since it attacks agricultural crops such as soya beans, beets, and wheat. Microbiologists speculate that the bacteria cover vast distances through clouds, and then fall to the ground as ice nuclei inside snowflakes, and that this is how it proliferates. Stopelli believes this to be plausible: “It is exciting to see how the bacteria can survive several days or weeks at high altitudes, in cold temperatures, and can withstand powerful UV radiation.”
As a result of the findings so far, the environmental scientists believe that biological particles play a relatively significant role in the formation of precipitation: “Our fundamental research contributes to a better understanding of how ice is formed in clouds. This is extremely important because ice changes the properties of clouds, and in turn influences the weather and climate,” explains Mignani. At present, the distribution and frequency of biological ice nuclei is only taken into account when calculating climate models to a limited degree, since so little is known about the origin of biological ice nuclei. This is why Mignani also travels to the arctic region to undertake further research, taking her equipment for gathering air and snow samples up to the Haldde observatory at the northernmost point of Norway. When she is up there, however, she sometimes hopes for a clear sky – then, with a little luck, she can also observe the Northern Lights.
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