snowflake How big can snowflakes get?

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

snowflake Why is snow white?

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.

snowflake Is it ever too cold to snow?

No, it can snow even at incredibly cold 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 with relatively warm air temperatures near the ground - typically 15°F or warmer since air can hold more water vapor at warmer temperatures.

snowflake When is it too warm to snow?  How does snow form if the ground temperature is above freezing?

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 ground temperature is 5 degrees Celsius (41 deg Fahrenheit).

snowflake Why can snow fall when temperatures are above freezing?

Snow forms in the atmosphere, not at the surface. So snow can fall when surface temperatures are above freezing as long as atmospheric temperatures are below freezing and the air contains a minimum moisture level (the exact level varies according to temperature). 

snowflake Is it true that there is one inch of water in every ten inches of snow that falls?

The water content of snow is more variable than most people realize. While many snows that fall at temperatures close to 32oF and snows accompanied by strong winds do contain approximately one inch of water per ten inches of snowfall, the ratio is not generally accurate. Ten inches of fresh snow can contain as little as 0.10 inches of water up to 4 inches depending on crystal structure, wind speed, temperature, and other factors. The majority of U.S. snows fall with a water-to-snow ratio of between 0.04 and 0.10. 

snowflake Why is snow a good insulator?

Fresh, undisturbed snow is composed of a high percentage of air trapped among the lattice structure of the accumulated snow crystals. Since the air can barely move, heat transfer is greatly reduced. Fresh, uncompacted snow typically is 90-95 percent trapped air. 

snowflake Why is snow colder in deeper spots?

Snow is not necessarily colder in deeper spots. The temperature at the surface of the snow is controlled by the air temperature. The colder the air above the colder will be the snow layers near the surface, say within the top 12 to 18 inches. The snow near the ground in deeper snowpacks however is warmer because it is close to the warm ground. The ground is warm because the heat stored in the ground over the summer is slow to leave the ground because snow is a good "insulator," just like the insulation in the ceiling of your house, and thus slows the flow of heat from the warm ground to the cold air above 

snowflake Why do more icicles form on the south sides of buildings?

Icicles form as the result of cycles of melting and freezing. Typically this cycle will occur more often on the south sides of buildings, melting in the day and freezing at night, whereas on the north sides, without the benefit of the warmth of the sun, melting does not occur as often. 

snowflake Why do weather forecasters seem to have so much trouble forecasting snow?

Snow forecasts are better than they used to be and they continue to improve, but snow forecasting remains one of the more difficult challenges for meteorologists. One reason is that for many of the more intense snows, the heaviest snow amounts fall in surprisingly narrow bands that are on a smaller scale than observing networks and forecast zones. Also, extremely small temperature differences that define the boundary line between rain and snow make night-and-day differences in snow forecasts. This is part of the fun and frustration that makes snow forecasting so interesting. 

snowflake What is a winter weather watch? warning? advisory?

NOAA's National Weather Service issues Winter Storm Outlooks when forecasters believe there is a good chance of a major winter storm. A Winter Storm Watch is issued to alert the public to the possibility of a blizzard, heavy snow, heavy freezing rain or heavy sleet. Winter Storm Warnings are issued when a hazardous winter weather event is imminent or occurring, and is considered a threat to life and property. Finally, a Winter Weather Advisory is issued for accumulations of snow, freezing rain, freezing drizzle and sleet that will cause significant inconveniences and, if caution is not exercised, could lead to life-threatening situations. 

snowflake Can there be thunder and lightning with a snow storm?

Thunder and lightning can be associated with snowstorms but they are rare and occur more often near the coast. 

snowflake Does snow change how sound waves travel?

Yes, when the ground has a thick layer of fresh, fluffy snow, sound waves are readily absorbed at the surface of the snow. However, the snow surface can become smooth and hard as it ages or if there have been strong winds. Then the snow surface will actually help reflect sound waves. Sounds may seem clearer and travel farther under these circumstances. 

snowflake Why does snow crunch when you step on it? At what temperature does it crunch?

A layer of snow is simply composed of ice grains with air in between the ice grains. Because the snow layer is mostly empty air space, when you step on a layer of snow you compress that layer-a little or a lot, depending on how old the snow is. As the snow compresses, the ice grains rub against each other. This creates friction or resistance; the colder the temperature, the greater the friction between the grains of ice. The sudden squashing of the snow at lower temperatures produces the familiar creaking or crunching sound. At warmer temperatures, closer to melting, this friction is reduced to the point where the sliding of the grains against each other produces little or no noise. It's difficult to say at what temperature the snow starts to crunch, but the colder the snow, the louder the crunch.

Source: NSIDC researcher, Richard Armstrong, November 2005 

snowflake What is lake effect snow?

According to the Weather Glossary, provided by the Weather Channel Interactive, Inc, lake effect snow is "snow showers that are created when cold dry air passes over a large warmer lake, such as one of the Great Lakes, and picks up moisture and heat." 

snowflake Is snow a mineral?

Snow is crystals of frozen water, i.e., ice. The definition of a mineral that I studied is this:A mineral is a naturally occurring homogeneous solid, inorganically formed, with a definite chemical composition and an ordered atomic arrangement.

Based on that definition, I'm sure you can determine that ice is a mineral. Ice has a definite chemical composition (H20). It is naturally occuring given a temperature below 0 deg C. It is homogeneous (of one material), formed inorganically, and has an ordered atomic structure (hydrogen and oxygen atoms bonding in a specific manner).

Source: Betsy Sheffield, NSIDC User Services, November 2002

snowflake What is the difference between snow, sleet, hail, and other forms of precipitation?

Definitions of precipitation:

Rain: Falling drops of water larger than 0.02 inch in diameter. In forecasts, "rain" usually implies that the rain will fall steadily over a period of time. (See "showers" below).

Light rain: Falls at the rate of 0.10 inch or less an hour.

Moderate rain: Falls at the rate of 0.11 to 0.30 inch an hour.

Heavy rain: Falls at the rate of 0.30 inch an hour or more.

Drizzle: Falling drops of water smaller than 0.02 inch in diameter. They appear to float in air currents, but unlike fog, do fall to the ground.

Light drizzle: Drizzle with visibility of more than 5/8 of a mile.

Moderate drizzle: Drizzle with visibility from 5/16 to 5/8 of a mile.   

Heavy drizzle: Drizzle with visibility of less than 5/16 of a mile.

Showers: Rain that falls intermittently over a small area. The rain from an individual shower can be heavy or light, but doesn't cover a large area or last more than an hour or so.

Snow: Falling ice composed of crystals in complex hexagonal forms. Snow forms mainly when water vapor turns directly to ice without going through the liquid stage, a process called deposition.

Snowflakes: Aggregations of snow crystals.

Snow flurries: Light showers of snow that do not cover large areas and do not fall steadily for long periods of time.

Snow grains: Very small snow crystals. The ice equivalent of drizzle.

Snow pellets: White, opaque ice particles that form as ice crystals fall through cloud droplets that are below freezing but still liquid (supercooled). The cloud droplets freeze to the crystals forming a lumpy mass. Scientists call snow pellets "graupel." Such pellets falling from thunderstorms are often called "soft hail."

Sleet: Drops of rain or drizzle that freeze into ice as they fall. They are usually smaller than 0.30 inch in diameter. Official weather observations list sleet as "ice pellets." In some parts of the country "sleet" refers to a mixture of ice pellets and freezing rain.

Freezing rain or drizzle: Falling rain or drizzle that cools below 32°F, but does not turn to ice in the air. The water is "supercooled." When the drops hit anything they instantly turn into ice.

Ice storm: A storm with large amounts of freezing rain that coats trees, power lines and roadways with ice. Often the ice is heavy enough to pull down trees and power lines.

Hail: Falling ice in roughly round shapes at least 0.20 inch in diameter. Hail comes from thunderstorms and is larger than sleet. Hailstones form when upward moving air -- updrafts -- in a thunderstorm keep pieces of graupel from falling. Drops of supercooled water hit and freeze to the graupel, causing it to grow. When the balls of ice become too heavy for the updrafts to continue supporting them, they fall as hailstones. Sleet, in contrast, consists of raindrops that freeze on the way down.

Thunderstorm: A rain or snow shower in which there is lightning. Thunder is always caused by lightning. In general, the upward and downward winds, updrafts and downdrafts, in thunderstorms are more violent than those in ordinary showers.

Thundersnow: A thunderstorm with snow instead of rain falling on the ground.

Severe thunderstorm: A thunderstorm with winds of 57 mph or faster or hail more than 3/4 inch in diameter reaching the ground. Severe thunderstorms can also produce tornadoes.

Source courtesy of: The USA TODAY Weather Book


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