Ben asked: What is the difference between snow and hail, and why are the residents of Jerusalem lucky enough to get snow, while in Tel Aviv they get hail?
The difference between snow and hail is in the wind and temperature of the air when the snowflake or hail stone fall to the ground.
Both hail and snow begin their journey in the same form, but something happens on the way down to create the difference between them. Snow is created when a droplet of water is captured on a dust particle. Since the air at a high level is very cold, the droplet immediately freezes and starts to crystalize. The crystal forms in the shape of a hexagon and develops symmetrically on its six vertices. The specific shape it will develop into depends on humidity, temperature and on the winds that carry the snowflake slowly to the ground.
Image of a snowflake under an electron microscope | Photograph by: Dcoetzee, Wikipedia
Hail also begins as a frozen droplet of water, and sometimes even as a snowflake. But instead of falling straight down, the wind blows it higher causing it to accumulate additional droplets that freeze around it. The lump of ice that is created is heavier and falls down faster, and then the wind strikes it back up again. This causes it to gain more and more ice layers until finally it is too heavy to rise again, and it falls to the ground.
A lump of hail can reach the size of a pea, but under the appropriate conditions, with strong enough winds pushing it up, it can become huge and create an ice ball of 20cm in diameter and weighing almost a kilogram (although this is fairly rare). The falling and rising cycles can be identified in the ice ball according to its layers, just as in rings of a tree trunk or an onion. So snow can actually form in lower locations such as Tel Aviv – it just defrosts on its way to the ground. While in higher areas where the temperature is lower, snow has a greater chance of reaching the ground while still frozen. In contrast, hail is too large and heavy to defrost during its fall, and therefore it appears also in lower areas.
It appears that some bacteria can serve as nuclei for snowflakes. These bacteria have special proteins in their membranes, which allow them to capture the water droplet and serve as an initial nucleus that allows the snowflake to be created around them. These bacteria become the crystalizing nucleus of the snowflake and they actually take advantage of it as a long-distance spreading mechanism.
Flower pollen use a similar mechanism as well. A study from 2010 revealed that snowflakes that landed in the west of the USA contained particles originating from the Sahara desert and Saudi Arabia. These bacteria or pollen soared on the wind at one side of the world, carried at high layers of the atmosphere, then captured in water droplets, and finally fell as snow or hail.
Do you want to know why does the snow defrost first under trees?
Boomerang – Question Back at You
Why does snow also fall in low places specifically in very southern or very northern areas?
Dr. Erez Garty
Davidson Institute for Science Education
Weizmann Institute of Science