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At this time of year, weather forecasts often tell of snow in the mountains and rain in the valleys. The forecast sometimes gives an altitude where the transition takes place. Leaving aside the appearance of the transition on the ground (ski above, hike below), what does it look like in the atmosphere? Curiously, the\u00a0snow-rain boundary in the air can appear\u00a0to be abrupt.\u00a0<\/p>\n
When falling\u00a0snow melts to form rain, the visibility of each is quite different.\u00a0There are two reasons for this:<\/p>\n
\u2022 When a snow crystal melts to form a raindrop, the surface area that can reflect light decreases. The difference can easily be a factor of two.<\/p>\n
\u2022 The snow has a larger number density than does the rain below the melting level. This is because the rain has a higher terminal velocity than does snow, so it spreads out over a greater height. The difference can easily be a factor of five. A way to visualize this is to think of highway traffic spreading out when moving to a region with a higher speed limit.<\/p>\n
Taken together, there is a factor of about ten in visibility with\u00a0the rain having only about\u00a0a tenth the visibility of\u00a0the snow from which it melted. As the rain is comparatively\u00a0invisible, it almost looks as if the precipitation fell to some level and just stopped.\u00a0<\/p>\n
The mountainside serves as a backdrop for a shower of snow into rain. The relative invisibility of the rain makes it seem as if there is no precipitation below the snow. Nevertheless, it was raining there.\u00a0
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At this time of year, weather forecasts often tell of snow in the mountains and rain in the valleys. The forecast sometimes gives an altitude where the transition takes place. Leaving aside the appearance of the transition on the … Continue reading