Weather It Is (Life and fall of a snowflake)

The snow that fell Friday morning was a beautiful snow: the flakes were large and fluffy.
In Gush Etzion, the temperature fell to freezing or below, and the snow accumulated as white powder
on the streets, sidewalks, lawns and gardens.  Temperatures at 500 mb were forecast to be near or below -25 C, at 700 mb below -11 C (and as low as -13 C), and at 850 mb at -2 C or below. These conditions are apparently good conditions for growing snow crystals, rather than small hail stones (which we call "sleet," but sleet is really rain drops that freeze on the way down). 

In strong wintertime thunderstorms, we often get snow pellets and sleet, and these pellets are not fluffy like the snow we had (made of dendrites).  Rather these type of snow pellets grow in strong updrafts and descend in strong downdrafts, and are often a kernal of snow frozen into a rather stiff form of ice.   This is most uncomfortable, at least for one who happens to be a "snow-flake."

In contrast, our Friday snow formed in weakly ascending, moist air that had time to form large snowflakes from dendrites. 

Predicting snow amounts in such a situation is tricky.  I have to mention, though, that no self-respecting snowflake wants to end its life as a raindrop as it falls to ground, which is what happened to many such snowflakes during our last storm, where greater than 150 mm of rain fell. The problem is that the larger the flakes become, the harder it is for them to stay aloft. On the other hand, the larger the flakes the more likely they are to survive the plunge to earth below.  So, eventually, these large flakes do decide to take a chance, in the hopes that they will join their fellow comrade flakes on the ground.

You have to understand, though, that while the ice-type snow can fall very fast and coat the ground with ice particles even when the temperature is above freezing. the fluffy like snow falls slowly (due to its low density and large size) and  usually melts before it reaches the ground -- unless the surface temperatures are at or below freezing.  Sometimes, though, if enough snow melts, the energy required to melt the snow can lead to cooling the ground and near-surface air enough that snow can accumulate.  This is when predicting snow amounts becomes very tricky.

In Gush Etzion, it was cold enough that the snow accumulated without melting.  In Jerusalem, it didn't, and instead had to freeze the ground first and then due to air temperatures just above freezing the snow became slush in many places.  One could say with confidence that there would be more snow in Gush Etzion than in Jerusalem, but it was difficult to say how little in Jerusalem.

Looking ahead in our weather: for a moment it looked like cold air might return at Purim time, but now the majority of ensemble members show that warmer weather is on the way, at least until early next week. So, enjoy the nice Purim weather and dry out a bit too.

Barry Lynn