It's snowing...

It's common to snow in northern and central Europe, and even in the highest parts of Portugal, but not at sea level. The capital, Lisbon, hadn't seen snow in 52 years and all over the country there were white snowflakes falling from above.



Scientifically speaking... snowflakes are agglomerates of many snow crystals. Most snowflakes are less than one centimeter across. Under certain conditions, usually requiring near-freezing temperatures, light winds, and unstable, convective atmospheric conditions, much larger and irregular flakes close to five centimetres across in the longest dimension can form. No routine measure of snowflake dimensions are taken, so the exact answer is not known.

Visible sunlight is white. Most natural materials absorb some sunlight which gives them their color. Snow, however, reflects most of the sunlight. The complex structure of snow crystals results in countless tiny surfaces from which visible light is efficiently reflected. What little sunlight is absorbed by snow is absorbed uniformly over the wavelengths of visible light thus giving snow its white appearance.

Snow forms when the atmospheric temperature is at or below freezing (0 Celsius or 32 Fahrenheit) and there is a minimum amount of moisture in the air. If the ground temperature is at or below freezing, of course the snow will reach the ground.

However, the snow can still reach the ground when the ground temperature is above freezing if the conditions are just right. In this case, snowflakes will begin to melt as they reach this warmer temperature layer; the melting creates evaporative cooling which cools the air immediately around the snow flake.This cooling retards melting. As a general rule, though, snow will not form if the groud temperature is 5 degrees Celsius (41 deg Fahrenheit).

          - from "All About Snow", Q&A