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What is hail and is it dangerous?
David Bowes, a Grade 11 student in Chatham, New Brunswick was curious about the formation of hail and how dangerous it is. While hurricanes, tornadoes and lightning grab all the headlines, hail is one of the most dangerous and certainly most destructive of all severe weather phenomena. Each year it injures a few Canadians, kills thousands of farm animals, wildlife and birds and causes millions of dollars damage. Now I don't know whether hail has ever killed anyone in Canada. It has killed hundreds in India and in China. And in the United States, at least 3 people have been killed by falling hailstones including a 3-month old boy, and it caused a single-engine plane to crash.

For a hailstone to grow, it must be captured or held up by powerful thunderclouds while new layers of ice are continuously being added. In most cases, frozen raindrops or ice balls are caught up in a kind of atmospheric trampoline as they fall from clouds. The ice particles are thrown back up into freezing air by strong updrafts where they acquire another layer of ice. These updrafts can continue to bounce the growing ice pieces back and forth - sometimes 25 times or more.

Eventually the hailstones grow too heavy to be supported by the thunderstorm's updraft and fall to the ground as hailstones - the size of peas or as big as grapefruits or even bigger.

The sound of hail
I really enjoyed this letter from Jean Baird of Penticton. Several years ago she was visiting Gull Lake, SK. After supper on a warm sunny day, her friend said, "listen, that swishing sound is hail. We were only in the house a few minutes when sure enough down came hail as large as golf balls. Would you please explain the sound of hail". Many people have described a peculiar rattling or hissing sound heard a couple of minutes before the fall of hail. Some people have suggested that the noise comes from the crashing or knocking of hailstones as they descend or the swishing of air as stones rush through. Some of these stones are travelling as fast as a major league fastball, 150 km/h. Well the noise does not come from the air or the clouds overhead. What one is hearing is the combined sound of many hailstones falling through trees, on roofs and on the ground at some distance upwind from you. The mingled noises made by the fallen hailstones arrives to your ears ahead of the storm itself. Once the storm arrives where you are, the patter of individual stones drowns out the noise from distant hailstones, some two or three km away.

Are raindrops really tear-shaped?
Adam Taub of Thornhill, Ontario wonders whether raindrops are really tear or pear-shaped like you see on cartoons, advertisements and posters.

High-speed photographs of most raindrops nearing the earth show them to be more like mushroom tops or hamburger buns - not at all tear-shaped.

The falling speed of raindrops is directly related to their size. Small drops, those less than 2 mm in diameter, tend to remain round as they fall. The surface tension is sufficient to hold it together as a nearly perfect sphere. Larger raindrops fall at a speed around 30 km/h.

Because the air pressure or resistance is greatest on the bottom, the drop flattens there which makes the droplet bulge on top. The side edges bulge out because air pressure there is lower. So the large raindrops tend to be flat on the bottom, round on top and wider than they are high just like a hamburger bun. Really large drops, those say 6 mm or more across, become distorted into a shape rather like a parachute and then they break up into smaller drops.

Why are snowflakes quite big on certain days, and then smaller on other days?
The shape and size of snowflakes ultimately depend on the temperature and the amount of water vapour available in the cloud where the flake first forms, and in the layers of air that the flake falls through as it descends. Some soggy flakes, measuring about 2 cm in diameter when they reach the earth, are conglomerations of 100's of matted-together flakes which have passed through relatively mild and moist air. On the other hand, dry snow tends to arrive as small, single flakes, unlikely to bind with other flakes as they fall through dry, cold air.

Nearly anything can happen to a snowflake as it drifts and tumbles earthward. Pieces break off, evaporate or melt. They bump into each other and sometimes bind together. If the wind is too strong, the big flakes will rip apart and you'll only see fragments.

Also the greater the distance a snowflake falls the larger it usually becomes. About a century ago, monster snowflakes which were larger than a medium-size pizza supposedly fell from the skies over Montana.

Can snow be coloured?
Mr. Eloi DeGrace of Dartmouth sent me a clipping from a newspaper in 1819 citing a peculiar find of red snow. How can that be? In 1818, Sir John Ross, the noted Arctic explorer discovered large deposits of red-coloured snow in Greenland. It was found to contain red-tinted, microscopic plants and animals.

Pure snow is white, but snow is never pure. It contains much more than just frozen moisture and air. Pollen, single-celled organisms, specks of dust, dirt, sand, and ash and traces of pollution are sometimes in sufficient quantities to affect the colour.

The foreign material is carried by wind currents before the snowflake begins to fall.

Yellow snow (it's not what you think) can be coloured by pollen from a near-by pine forest fell in Pennsylvania; pink snow has fallen on Vancouver Island; pale-blue snow fell in the French Alps presumably coloured by copper salts in the dust from the Sahara desert. During the Dust Bowl in the 1930s, black and brown soils from Oklahoma and Texas coloured snow in Eastern Canada.

Snow can also change colour after it falls. Colonies made up of algae, fungi and bacteria living among the crystals feed on the nutrients in the snow.

Sounds of snow
Clayton Trought of Aurora Ontario asks why does snow squeak when you walk on it. I am sure you have noticed that the colder the temperature, the fluffier the snow and the squeakier the sound it makes when you walk on it. One explanation is that when the air and snow are only slightly below freezing, pressure from walking compresses and partially melts the snow crystals underfoot. Now, lubricated by a thin film of water, the snow can flow and little sound is made.

But on cold days, when the temperature is, say -15C or lower, foot pressure is not sufficient to melt the snow. Instead, when you step down, the individual cold ice crystals move abruptly, slipping and crashing into each other. The sudden rubbing or smashing produces that familiar cold-weather creaking sound. Because the sound produced by snow is related to how cold it is, you can use it to tell the temperature. The louder the snow cries the colder the temperature of both the air and snow. Another possible explanation is that the pressure of stepping on the air-filled snowflakes rapidly expels the air and produces the characteristic squeak or crunchy noise we know so well.

Which day is the snowiest?

Mark McGee of Burlington writes that whenever we have a major snowstorm in Southern Ontario it always seems to be on a Friday. Is there anything to this?

Day-of-the-week variation in weather always produces lively discussion among weather people. For instance, in Great Britain, Thursdays are said to be the wettest day, and scientists have discovered that world-wide Wednesday is the warmest day of the week.

It is always tempting to try to detect cycles in weather phenomena. Personally, I think the day-of-the-week Q-pattern is more of a weather fluke, mere chance than anything else is. One possible explanation is that it must be related to local or human factors associated with urbanisation or industrialization. For example, increased workday air pollution produces a greater abundance of condensation nuclei - a necessary ingredient for rain and snow.

As far as the question, based on more than 50 years of data, the occurrence of heavy snowfalls in the Toronto region is highest on Thursday not Friday and lowest on Sundays. This table lists the likely and least likely day of the week for heavy snowfalls for selected cities. Come to think of it, though, it does seem to rain more on weekends!

HEAVY SNOWFALL DAYS
City	Most Likely	Least Likely
Toronto	Thursday	Sunday
St. John's	Sunday	Tuesday
Halifax	Saturday	Friday
Montreal	Saturday	Monday
Winnipeg	Monday	Thursday/Saturday
Edmonton	Thursday	Friday
Vancouver	Saturday	Tuesday

What do you call it when snow evaporates before hitting the ground?
Snow that also evaporates before hitting the ground is called Virga. Virga or rain streamers are common in places where the climate is dry or arid. Almost any basic meteorology textbook would make a good reference. Meteorology Today is a great introductory meteorology textbook for getting an understanding of a broad base of terms and principles.

I would like to know when it is too warm to snow?
As a general rule, outside of mountainous regions it is too warm to snow at 5 degrees Celsius. Depending on the height of the ceiling (height of the lowest level of clouds), it can still snow at 3 to 4 degrees Celsius. When the ceiling is low, Q that starts out as snow might not have the chance to melt or completely melt as it falls through the 3 to 4 degrees layer of air close to the earth's surface, so we end up with snow.

What determines whether Q will fall as freezing rain or snow?
Our atmosphere is composed of layers of air with different temperatures. So air temperature at the earth's surface may be different from that above. What determines whether you get freezing rain, snow or rain, is the thickness of warm and cold air over your region, as well, as the surface air temperature.

For example, freezing rain is possible when warm moist air with temperatures above 0 degrees Celsius is sandwiched between two colder layers of air below 0 degrees Celsius. What happens in this case is, Q from higher, colder levels begin falling as snow. As it falls through the warmer lay in between, the snow melts into rain drops. The warm layer must be around 1500 meters thick with temperatures above 0 degrees in order for the snow to melt. The Q now reached the cold surgace temperature (in your case -7 degrees Celsius) as rain which freezes as it falls to the ground. In order for the rain to become freezing rain, this colder layer must be around 1300 meters thick.

As long as the surface temperature stays below zero and the thickness of the warm and cold layers of air are in the above ranges, freezing is possible for a prolonged period of time. If the thickness' change or your surface temperatures go up or down, you can have mix of rain, snow and ice pellets along with your freezing rain.

Why are all snow flakes six sided?
Yes, your research is pointing you in the right direction. The water molecule is the key here. Here's how it works: The water molecule is made up of two atoms of hydrogen and one atom of oxygen, if you could look at it you would see that they hold the shape of a triangle. When water begins to freeze, each of these triangular H20 molecules, begins to attach themselves, because of the charge on each of the molecules they can only form as a hexagon (a six pointy sided crystal). As this six-sided crystal grows, it will only grow outward on each of the six-sided points, thus forming one six sided snow flake.

Can it ever be too cold for it to snow?
The answer to your question is yes, when the air temperature gets well below -30 °C the air is unable to hold enough moisture for it to snow. You can however get what is called ice crystals, which occurs at very cold temperatures and can fall from a clear sky.

Why does it snow at all when it's above 0?
The reason snow falls when temperatures are above 0 is the result of much colder air aloft. The layer of air near the surface may be at or above 0, but this layer is very shallow so the snow that falls does not get a chance to melt and falls as rain. At +5° snow is mixed with rain or vice versa, but at temperatures of +10° Q will fall as rain.

What is the Bergeron Process?
The 'Bergeron Process' is an in-cloud process where Q is formed before it falls out of the clouds as snow or rain. This process was discovered by Swedish meteorologist Tor Bergeron in the 1930's.

The process starts off with tiny microscopic ice crystals that begin to grow at the expense of available moisture, super-cooled water droplets (water in its liquid form at below freezing temperatures). As the crystals grow into larger snowflakes they begin to fall and collide with other snowflakes eventually falling out of the cloud as Q. If it is cold enough, as it is in winter, the Q will remain as snow. In the summer the process remains the same but the snowflakes melt into raindrops as they descend.

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