Category Science

          Hailstones are essentially frozen raindrops. They are made inside very tall cumulonimbus clouds that have great differences in temperature between the top and bottom. Freezing temperatures at the top and warmer temperatures at the bottom of the cloud create very strong upward and downward currents of air. Ice crystals and super-cooled water droplets are thrown around by these currents and collide with each other. As they do so, they are coated with more and more layers of ice. The layers of ice buildup until the hailstones are heavy enough to fall to the ground.

          Hailstones are formed when raindrops are carried upward by thunderstorm updrafts into extremely cold areas of the atmosphere and freeze. Hailstones then grow by colliding with liquid water drops that freeze onto the hailstone’s surface. If the water freezes instantaneously when colliding with the hailstone, cloudy ice will form as air bubbles will be trapped in the newly formed ice. However, if the water freezes slowly, the air bubbles can escape and the new ice will be clear. The hail falls when the thunderstorm’s updraft can no longer support the weight of the hailstone, which can occur if the stone becomes large enough or the updraft weakens.

          Hailstones can have layers of clear and cloudy ice if the hailstone encounters different temperature and liquid water content conditions in the thunderstorm. The conditions experienced by the hailstone can change as it passes horizontally across or near an updraft. The layers, however, do not occur simply due to the hailstone going through up and down cycles inside a thunderstorm. The winds inside a thunderstorm aren’t simply up and down; horizontal winds exist from either a rotating updraft, like in supercell thunderstorms, or from the surrounding environment’s horizontal winds. Hailstones also do not grow from being lofted to the top of the thunderstorm. At very high altitudes, the air is cold enough (below -40°F) that all liquid water will have frozen into ice, and hailstones need liquid water to grow to an appreciable size.

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          Snow is often described as being “dry” or “wet”. The snowflakes that make wet snow are relatively large and form when the temperature is at freezing point or thereabouts. Wet snow packs together easily when it falls and is the best snow for making snowmen and snowballs. Dry snow is formed at lower temperatures, and the snowflakes are smaller than those that make wet snow. It is referred to as “powder” by skiers and snow-boarders and is the best form of snow for such winter sports enthusiasts. It is lighter and much easier to clear away.

          The temperature profile in the vertical is a critical factor in determining the wetness of snow. Wet snow generally has a snow depth to melted liquid depth ratio of less than 10. Dry snow generally has a snow depth to melted liquid depth ratio of greater than 20. Values between 10 and 20 are a hybrid of the two. Wet snow is great for making snow balls and it is a sticky snow. Dry snow is powdery, easily blown around by the wind and is not sticky.

          What determines the wetness or dryness of the snow is the amount of liquid content within the falling snow. When the temperature aloft goes just above freezing (see diagram below) then some of the snow will melt and it will fall as a wet snow. In the dry snow profile the temperature is well before freezing and thus the snow will have a low liquid content.

          Other factors that influence the wetness of snow are day vs. night, ground temperature, and if the sun is out. Even in cloudy conditions, some sunlight passes through clouds. Some of this light absorbs into the falling snowflakes and thus this can make a snow wetter even at temperatures well below freezing. This does not happen at night thus all being equal a night snow will be a little drier. If the ground surface is above freezing, some or all of the snow will melt after it reaches the ground. This will make a snow wetter. Sun shining on snow on the ground, even when temperatures are well before freezing, will cause snow to become wetter due to the absorption of solar energy into the snow. At night, when temperatures fall below freezing then snow on the ground will tend to have its liquid water content freeze up and the snow will become a hard crusty snow.

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          There are thought to be about 80 different varieties of snowflakes, which form into shapes ranging from needles and columns to stars, prisms, plates and hexagons. All snowflakes have a symmetrical, six-sided pattern, but no two snowflakes have been found with exactly the same shape. Scientists think the shape of a snowflake depends on the height and temperature at which it was formed.

          Do you enjoy watching snow fall on a cold winter day? We love to sip on hot cocoa while we watch snowflakes pile up outside. Later, we’ll head out to build snowmen, make snow angels, and start a snowball fight! Snowy days can be a lot of fun. Have you ever heard that no two snowflakes are exactly alike? Well, that isn’t exactly true. However, the chance of finding twin snowflakes is very, very, very low.

          Scientists say the chances of two snowflakes being exactly alike are about 1 in 1 million trillion. That’s a one followed by 18 zeros, so it’s very unlikely! Meteorologists think there are 1 trillion, trillion, trillion (a one with 36 zeros!) types of snowflakes.

          A snowflake has three basic ingredients: ice crystals, water vapor and dust. The ice crystals form as water vapor freezes on a tiny piece of dust. The dust particles come from many places. It could be from flower pollen, volcanic ash, or even meteors.

          Snow forms in very cold clouds. As water droplets attach themselves to the ice crystal, they freeze, creating an even larger ice crystal. In any crystal, molecules line up in a pattern. In ice crystals, water molecules line up and form a six-sided shape called a hexagon. This is why all snowflakes are six-sided!

          The temperature of the cloud determines the shape of an ice crystal. Likewise, the amount of moisture in the cloud determines the size of the ice crystal. More moisture will create a bigger crystal. When several ice crystals stick together, they form a snowflake.

          As snowflakes tumble through the air, swirling and spiraling, they each take a different path to the ground. Each snowflake falls and floats through clouds with different temperatures and moisture levels. This shapes each snowflake differently. Two snowflakes from the same cloud will have different sizes and shapes because of their different journeys to the ground.

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          Sleet is usually snow that has half-melted, or it can be formed from rain-drops that have partly evaporated and then cooled down as they fall to the ground. It often feels like very cold, wet rain when it falls on you.

          Sleet is rain or melted snow that freezes into ice pellets before hitting the ground. Sleet only happens under very specific weather conditions.

          There must be a layer of air near the ground whose temperature is below freezing, where water turns to ice. Above this layer of freezing air must be a layer of warmer air. As snow falls through the warm air, it melts or partially melts into raindrops. As the melted snow falls through the cold layer of air, it re-freezes. It forms ice pellets, or sleet, before hitting the ground.

          Sleet falls as tiny, hard pieces of ice. Sleet usually cannot do severe damage to crops or transportation systems the way heavy snow, freezing rain, or hail can. In fact, sleet is so light and tiny it usually bounces when it hits a hard surface.

          Sleet is not the same as freezing rain. Freezing rain also falls through a cold layer of air close to the ground. However, the rain does not freeze until it touches the surface of an object. When you see trees coated in jackets of ice, you are seeing the results of freezing rain. The rain was liquid when it landed on the tree branch, then immediately froze solid.

          Sleet also is not the same as hail. Hail, like sleet, is a collection of ice pellets. But hail forms in a cloud, while sleet forms as it falls. Hail freezes from the inside out, while sleet freezes from the outside in. Hail also tends to fall during thunderstorms in the spring and summer. Sleet usually falls in the winter.

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          Snow forms in clouds when the temperature is within the range -20°C to —40°C (-4°F to —40°F). Ice crystals in the clouds begin to melt and join together with super-cooled water droplets. They then freeze together and form into snowflakes which, provided the air temperature is low enough, fall from the clouds. The process of forming snowflakes is called accretion.

          Whether winter storms produce snow relies heavily on temperature, but not necessarily the temperature we feel here on the ground. Snow forms when the atmospheric temperature is at or below freezing (0 degrees Celsius or 32 degrees Fahrenheit) and there is a minimum amount of moisture in the air. If the ground temperature is at or below freezing, 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 higher temperature layer; the melting creates evaporative cooling which cools the air immediately around the snowflake. This cooling retards melting. As a general rule, though, snow will not form if the ground temperature is at least 5 degrees Celsius (41 degrees Fahrenheit).

          While it can be too warm to snow, it cannot be too cold to snow. Snow can occur even at incredibly low temperatures as long as there is some source of moisture and some way to lift or cool the air. It is true, however, that most heavy snowfalls occur when there is relatively warm air near the ground—typically -9 degrees Celsius (15 degrees Fahrenheit) or warmer—since warmer air can hold more water vapor.

          Because snow formation requires moisture, very cold but very dry areas may rarely receive snow. Antarctica’s Dry Valleys, for instance, form the largest ice-free portion of the continent. The Dry Valleys are quite cold but have very low humidity, and strong winds help wick any remaining moisture from the air. As a result, this extremely cold region receives little snow.

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          Rainfall is usually measured in millimetres or inches. Rain water is collected in a metal drum about 50cm (20in) tall called a rain gauge. The rain gauge is placed on the ground, just high enough to avoid splashes. The rainwater is collected in a funnel t the top and passes into the drum.

          Rain gauges are thought to be the most ancient weather instruments, and they’re believed to have been used in India more than 2,000 years ago. A rain gauge is really just a cylinder that catches rain. If an inch collects in the cylinder, it means an inch of rain has fallen. It’s that simple. Most standard rain gauges have a wide funnel leading into the cylinder and are calibrated so that one-tenth of an inch of rain measures one inch when it collects inside. The funnel is 10 times the cross-sectional area of the tube. Rainfall as low as .01 inches can be measured with this instrument. Anything under .01 inches is considered a trace. This standard rain gauge is shown in the following figure.

          In the more modern era, a common rain gauge is called the tipping bucket type. A bucket doesn’t really tip—a pair of small receiving funnels alternate in the collection of the rain. When one fills up with water, it tips and spills out, and the other comes into place to do the collecting. These little funnels tip each time rainfall amounts to .01 inches. The tip triggers a signal that is transmitted and recorded.

          Of course, these rain gauges have a problem when the temperature drops below freezing, so the standard versions are heated for the occasion.

          When snow falls on these heated rain gauges, it melts, and a water equivalent is determined. The recorded precipitation is always expressed in terms of rainfall or melted snow. The snow depth doesn’t count—unless, of course, you have to shovel it! Sometimes a foot of snow amounts to just a half-inch of water, other times it amounts to three inches of water. It really depends on the water equivalent of the snow, which varies widely.

          On the average, 10 inches of snow is equivalent to one inch of rain, but that’s only an average. If a rain gauge measures one inch of water during a snowstorm, an observer can’t automatically assume that 10 inches of snow has fallen. The snow depth can only be determined the old-fashioned way—by measuring it.

          That depth is determined by taking an average of three or more representative spots. A ruler is stuck into the snow, and its depth is recorded. Because of blowing and drifting, the determination of three or more representative locations is not always easy. You would think that there would be a better way, but there really isn’t.

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