  This paper presents a matrix of 206 snow crystal growth observations as a
function of temperature and water vapor supersaturation in air, each
illustrating the morphology and size of a crystal forming on the tip of an
isolated c-axis ice needle after a known growth time. Because each complex
structure emerged from a simple, well-defined seed crystal under known
environmental conditions, this data set is well suited for making comparisons
with three-dimensional computational models. These observations thus provide a
needed extension of the well-known Nakaya diagram, as they allow a quantitative
evaluation of model predictions over a broad range of growth conditions. I also
briefly discuss computational methods along with an initial model of the most
relevant microphysical processes governing snow crystal growth. My overarching
goal with this new data set is to facilitate the development of quantitative
computational growth models that can eventually reproduce the remarkable
diversity of morphological structures seen in snow crystal formation.
