## 4 Shape Groups  
The microphysical properties (particle size, cross-sectional area, and area ratio) are determined from the particle images taken with D-ICI. After classifying the particles, one can study these properties for specific shapes or how the properties depend on them. It is challenging to study the characteristics of all 135 shapes since most particle shapes infrequently appear so that statistically significant results are hard to achieve. Furthermore, even if we had found enough examples of every single shape to achieve statistical significance, it may be hard for the models to utilize this level of detail. Additionally, due to natural variability, the encountered variation in microphysical properties of one shape would, in many cases, be larger than shape-to-shape variations. Therefore, to analyze the shape properties, it seemed to be more useful to use a classification based on only a small number of shape groups, instead of the 135 individual shapes. In the following section, we describe how we gather the 135 shapes into 15 shape groups, which are then used to study shape dependencies. However, these shape groups can be assembled differently into, for instance, less than 15 groups, depending on the needs of the study. In this case, 15 shape groups were defined in an attempt to simplify shape description while still maintaining some level of detail. In a similar approach to Magono-Lee [20], we gather the shapes into 15 groups with comparable morphological characteristics. Magono-Lee's main level P = _Plane crystal_ contains both plate-like shapes and stellar shapes; however, we wish to separate the two for this study and, hence, group them somewhat differently. Therefore, the groups used here do not exactly follow the main levels of Magono-Lee. This is not seen as incompatible to the Magono-Lee classification scheme. Rather,  
Figure 2: The 34 new shapes found in Kiruna: Table 2 shows their completed name. The same scaling is applied to all images; a 1.0-mm scale bar is shown as reference. In some cases, more images of the same shape are shown to indicate possible differences found in the same shape. In other cases, side- and top-view images of the particle shape are shown for better shape description. the detailed classification using the updated Magono-Lee shapes from Section 3 allows one to group shapes according to the needs or interests, which may differ from study to study. In this sense, the grouping used here is not seen as rigid and definitive but as a first useful attempt to study snow particle characteristics depending on shape. These 15 shape groups are listed in Table 3.  
Thus, we sort particle shapes that look similar, such as particles with needle-like shapes and thin columns, into groups of shared morphology. Therefore, even primed particles can be included in such a group, as long as they appear to have the same type of morphology. Graupel particles, which do not appear to have a particular shape, are grouped separately as _Graupel_. Tables 4-7 provide detailed descriptions of the 15 shape groups. For each group, particle images of all the shapes that define that group are shown. In Appendix B, Table A2 lists for each shape group all shapes with their labels and names. \begin{table}
\begin{tabular}{l} \hline \hline
**Shape Groups (1–15) and their Descriptions** \\ \hline _(1) Needles and thin or long columns:_ Includes needles, other snow crystals with \\ elongated features (i.e., needle-like shape), and thin or long columns, including rinsed \\ needles and rinsed thin columnar crystals. \\ \hline _(2) Crossed needles and crossed columns:_ Combination of crossed needles and a \\ combination of crossed thin or long columns. \\ \hline _(3) Thick columns and bullets:_ Thick columnar, minute columns, bullet shape, scroll, \\ pyramid, and cup shapes. \\ \hline _(4) Capped columns and capped bullets:_ Columns and bullets with plates at one end or both ends. \\ \hline \hline \end{tabular}
\end{table}