Previously Asked Questions

Winter and Cold Weather

Nick,
Can it ever be too cold to snow?
Evie
Sewickley, PA

Dear Evie,
As long as there is moisture in the air and a way for it to rise and form clouds, there can be snow, even in temperatures below zero. But very cold air doesn’t have much moisture in it, and it is also dense and heavy, so clouds don’t form unless the cold air rises up a mountainside or unless the cold air blows across a body of water and collects moisture. Most heavy snowfalls occur in temperatures 15 degrees Fahrenheit or above.

Hello Nick,
Is it always snowing somewhere on the planet earth?
Dan
Grand Junction, CO

Dan,
I put this question to our Winter Weather Expert Paul Kocin, who literally “wrote the book” on winter weather. Here is his answer:

I have never been asked this before, but chances are, it is always snowing somewhere on Earth. Why? Because there are always some places, such as in the mountains, over the arctic, etc., that are always below freezing and Earth is a very big place. And of course, when it's summer in one hemisphere, it's winter in the other. Also, when it's fall in one hemisphere, it's spring in another, and we know it snows in many places during the fall and the spring. Another thing to keep in mind is that nearly all rain is snow higher up, so it may not be snowing at the surface but it is five miles up. And since we are a planet of mountains, it is bound to be snowing somewhere.

Perhaps a more interesting question might be "Is there any time that it wouldn't be raining anywhere on earth?" On this question, I'd have to answer "no."

Dear Nick,
What are blizzards and why are they so harmful? Where do they happen most often?
Sixth graders from St. Bernadette School
Northboro, MA

Some people use the word “blizzard” whenever they see heavy snow. But in order to have a true blizzard, temperatures must be very cold (usually below 20 degrees F) and the wind must be at least 35 miles an hour or greater. In addition, the snow must be falling or blowing in the wind for at least three hours, reducing visibility to Ľ mile or less. A severe blizzard is one with temperatures near or below ten degrees F, winds more than 45 mph, and visibility near zero.

Blizzards are dangerous when people go outside or try to travel in such weather. Because of the blowing snow, people can’t see to drive, and can end up stranded in the storm. The blowing and drifting snow can actually bury automobiles, and in such a situation people can freeze to death in the bitter cold.

The plains states of the United States are particularly vulnerable to blizzards, especially the Dakotas and Minnesota, but they can occur in the Northeastern U.S. as well, and anywhere the snow can fall heavily and the winds can blow strongly.

For more on blizzards and how they form, go to the Weather Encyclopedia at weather.com at http://www.weather.com/encyclopedia/winter/blizzard.html

How does snow get its shape?
Susan

A snowflake is usually made of many different kinds of snow crystals, and the shape of a snow crystal depends a lot on the temperature at which it forms. For example, at temperatures from 25 to 32 degrees F, the crystals are shaped like thin plates. At temperatures between 20 and 25 degrees F they look more like needles and at 15-20 degrees F they resemble hollow columns. Usually the colder the temperature is, the smaller the crystals.

As the crystals fall from the cold clouds, they bump into other crystals and freeze together, making even more shapes. This is one reason why it’s so hard to have two snowflakes exactly alike. In fact, in air right at the freezing mark, several snowflakes may stick together, forming large clumps of flakes that may melt as they hit the ground.

Snowflakes form in the familiar six-sided pattern because that’s the shape you get when the two hydrogen atoms and the single oxygen atom in a water molecule attach to other atoms of other water molecules.

To get a look at the different shapes of snowflakes, try this: Put a piece of black construction paper or black velvet into the freezer. Keep it there until the next time it snows. When the snow begins falling, grab a magnifying glass and take the paper or velvet outside. Let some of the snowflakes land on the dark surface and examine them with the magnifying glass. Enjoy the snowflakes’ variety and beauty.

What is a Blue Norther?
Gabby
Louise, TX

Gabby,
Being a native Texan myself, I’ve experienced a few “Blue Northers” in my day. It is also sometimes called a “Texas Norther.” Both names are what folks in the Lone Star State often call a strong, fast-moving cold front that brings cold air all the way from Canada southward into Texas. This can result in brisk north or northwest winds and rapidly falling temperatures. It gets its “blue” name because the cold front is sometimes accompanied by a dark blue sky. When the front moves through, temperatures can drop as much as twenty degrees in just a few minutes.

I have been told that if snowflakes are big, we won't get much, but if they’re small we’ll get a lot more snow. Is there any science behind this?
Julie
Bellevue, NE

Julie,

When temperatures are just a little above freezing near the ground, the falling snowflakes are already melting on the way down. The wet snowflakes stick together, forming gigantic flakes that look scary, but in fact, are less likely to cause problems than the smaller flakes. In fact, the biggest ones may melt as soon as they hit the ground or soon after, as temperatures continue to rise. Even if they don’t melt, they won’t pile up as high because they contain so much liquid water. When the air is colder, the snowflakes are smaller because they're not sticking together. And when it is very cold, the snow can sometimes be very small, like tiny needles instead of the familiar six-sized flake. This is the kind of snow that will pile up or blow around, and because temperatures are so cold, they won’t melt for awhile. So when it comes to snowflakes, be more concerned about the small ones and don't sweat the big stuff!

I hear meteorologists talking about lake effect snow all the time on TV. Can you tell me what it is?
Alex
Bloomington, IN

We talk a lot on The Weather Channel about lake effect snow around the Great Lakes in the United States, but this process occurs in other places too. Anywhere you have a relatively warm and large body of water (such as one of the Great Lakes) and cold air blowing across the water, you can get this kind of snow. When cold air from Canada blows over the Great Lakes, moisture from the warmer lake water rises, cools and condenses into clouds. The clouds produce snow downwind on the lake’s shoreline and inland. The direction of the wind determines where the snow will fall.

Lake effect snow occurs over salt water too. Often the Salt Lake City area will see lake effect snow as cold air blows across the Great Salt Lake. You can also find “bay effect” snow on the New England coast when cold Canadian air blows over the waters of the Massachusetts Bay and Cape Cod Bay to deposit snow on Cape Cod. The same effect can occur in Norfolk and Virginia Beach, Virginia downwind of Chesapeake Bay. So the body of water doesn’t actually have to be a lake; it can be a bay or a gulf or any waterway near land.

I know they say that no two snowflakes are alike, but that doesn't seem mathematically possible to me. If we have four billion snowflakes, odds are two of those will look alike.
Jesus
Strafford, MO

Jesus,

Yes, the odds seem to favor look-alike snowflakes. And some of the smaller needle or column-shaped flakes that fall in very cold air are shaped simply enough that they may all look alike. But the way the familiar six-sided snowflake forms presents so many random possibilities in the way the flake finally appears once it hits the ground, that it’s easy to see why finding two of them exactly alike is a nearly impossible task. It doesn’t mean that twin snowflakes are scientifically or mathematically impossible. It just means the odds are against you finding them.

In a cloud, a tiny ice crystal forms around a speck of dust. As water droplets freeze to and become part of the ice crystal, the crystal develops six sides. At this point, most of the ice crystals still look the same. But as more moisture is drawn into the crystal, it grows. Then as branches sprout from each of its six corners, it starts to look like a traditional snowflake. Snowflakes are blown around inside a cloud taking each one in a slightly different path, each encountering slightly different temperature and moisture conditions. So as the six branches on the flakes grow bigger and longer, they look more and more different from one another. As one flake bumps into another, and as they absorb more moisture, their appearance may change even further. By the time the snowflakes reach the ground, each has gone through enough individual changes that it’s not likely you’ll find two alike.

Why does it seem the temperature drops as the sun is coming up in the morning? Meaning it gets colder than when it was dark?
Abie
Desert Vista High School, Phoenix, AZ

Often the temperature does fall for a short time after sunrise, though it is only a degree or two at most. And even if the temperature doesn’t fall as the sun is rising, the air may actually feel colder to you. That is because we are used to it being colder when it’s dark and warmer when it’s light. So as the sky begins to brighten in the morning and the temperature remains nearly the same, we might perceive that it is colder than it actually is. Also, wind generally increases during the day. The air sometimes begins to stir just after sunrise, and an increase in wind would make us feel colder.

If there are no fronts or precipitation nearby, the daily temperature cycle is primarily controlled by the radiation budget. This is a comparison between the incoming radiation from the sun (sunlight) and the terrestrial radiation given off by the earth’s surface (felt as heat.) Think of the sunlight shining down on earth as the same as putting pennies into a jar. As long as you keep putting pennies in, the money adds up. In the same way, as long as the sun is shining down on earth, the amount of radiation adds up. Let’s say that at some point you decide to stop depositing pennies into the jar and begin to take them out. Even though you’re withdrawing pennies, you still may have a lot of money in the jar. Likewise, when the sun goes down, the incoming radiation from the sun stops, but there is still a lot of radiation that has been absorbed by the earth, so we still feel heat near the earth even after the sun goes down. At night, the “withdrawals” of terrestrial radiation continue, and the ground and the air near it cool. Earth’s surface is typically in radiation “debt” from a couple of hours before sunset to near sunrise. When the sun comes up and the “deposits” of incoming radiation from the sun equalize with the “withdrawals” of radiation from earth, we get the coolest temperature of the day. Sometimes in winter when the sun is low in the sky, the earth’s surface can remain in radiation debt longer, and the coldest temperature of the day can actually occur as much as an hour after sunrise. As the sun gets higher in the sky, earth’s surface is in radiation surplus (the deposits exceed withdrawals), so the ground and the air near it warm.

How can it be so cold even when the sun is shining?
Benjamin
Sherwood , AR

Sunshine is only one thing that affects temperature, and in winter, it is far from being the main thing. In winter and even in early spring, cold air can come southward from northern Canada. The cold air at Earth’s surface is very dense and heavy, so it's hard for clouds to form in that cold sinking air. So sometimes in winter, skies are very clear and temperatures are very cold. Also, winter is the time of year that the angle of the sun, especially in the northern U.S. and Canada, is so low in the sky that there's never enough direct sunlight to warm the Earth very much even at midday with clear skies. And if there's snow on the ground, the snow reflects a lot of the sun's energy away, preventing the ground from absorbing it. So temperatures end up cooler than if the ground were bare. Even if the ground absorbs some of the sun’s energy, the heat radiates back up into space with no clouds to keep it near the ground. So when you look outside and see sunshine, you cannot assume it’s going to be a warm day

Nick,

My boyfriend and I were having a discussion about rain. Our question is this: Can it rain at 26 degrees? Or at any temperature below 32 F for that matter? Your help is greatly appreciated to settle our debate.
Nicole
New York City

Nicole,
The falling precipitation could be rain at 26 F, but if it is liquid before it hits the ground, chances are that it would freeze as soon as it hits something, like the ground, or streets or trees. So meteorologists refer to this type of precipitation as “freezing rain.” This happens when the air temperature is above freezing somewhere between the clouds and the ground, but at freezing temperatures or below freezing at ground level.

Water drops can remain liquid in our atmosphere even at temperatures below freezing. These drops are called “supercooled.” Drizzle, which is composed of small liquid droplets, can form as liquid and remain unfrozen even when temperatures are continually below freezing. In these cases, the clouds form as tiny liquid drops, even though the air temperature is below 32 F. This happens in relatively shallow clouds in which no part of the cloud has temperatures too far below freezing, so the cloud doesn’t contain any snow. Sometimes the drops grow large enough to become freezing drizzle at the ground, or liquid drizzle if there is warm air below. Or it might even be crunchy round snow pellets if the air is very cold below the cloud.

In thicker clouds, rain usually starts as snow, which melts on the way down to turn to rain. If it never melts, you get snow. If it melts, then re-freezes in a layer of sub-freezing air closer to the ground, you get sleet. If the layer of cold air is very shallow, you can get freezing rain as described above. If that layer of sub-freezing air is very shallow and hasn't been around long enough to make the ground freeze, you'll get rain until the surfaces fall below freezing.

We live in Northwest Indiana, and we've had an unusually warm winter. What are some possible reasons for the unseasonably warm weather. What effects could the warm weather have on spring storms, wildlife, insect populations, and plants.
Mr. Hebert's 5th Grade Class
Paul Saylor Elementary
Portage, Indiana

Dear Mr. Hebert’s class,

You are certainly not alone in your warm winter. Almost the entire country has experienced warmer than average winter temperatures. And for some people, especially those in the northeast United States, the months of December through February have been the warmest on record, which go back about 100 years. Snows have been lighter than usual, and the Great Lakes have stayed free of ice cover.

I am sure that scientists will be studying the winter of 2001-2002 for some time to come, analyzing what made this season so warm. Only then will we have a more detailed idea of all the factors involved. But we already have some clues when we look at the jet stream, the narrow “river” of fast-moving air several miles above the ground. In the winter, the jet stream is partially responsible for transporting cold arctic air from Canada into the United States. You have probably seen the jet stream pictured on a weather map, looking like a wavy line extending across the country. In order to bring that air southward, the jet stream has to move northward far enough to tap that air, then dip southward, bringing that cold air down into the United States. On a weather map this would look like a squiggly line with a large peak called a “ridge” near the West Coast and a big dip, called a “trough .” across the East But for much of the winter, the jet stream pattern has been relatively flat, with no big moves northward or southward. The cold air has stayed in Canada most of the winter, with only a few trips southward.

Another factor is the lack of snow cover. When the ground is covered with snow, much of the sun’s energy is reflected off the white snow surface back into space without being absorbed by the ground. This causes temperatures to be lower. Because much of the ground has remained free of snow this winter, temperatures have been warmer than they would have been otherwise.

There have been some outbreaks of cold temperatures in March, so it’s a bit early to write off the winter completely. That late winter cool down was tough on early blooming plants and crops. It may be too early to tell what effect, if any, this development will have on animal and insect populations. The warm winter won’t dictate spring storms though, since they are more dependent on small-scale weather patterns.

Nick,
Why isn't December 21 the coldest day of the year in the Northern Hemisphere? Logic says that due to lack of sunlight, it should be. And why isn’t June 21^st the warmest day?
Steve

You are right; the shortest days of the year are, on average, not the coldest, and the longest days are not the warmest. There are exceptions to this, of course. In the past, there have been some extreme arctic outbreaks within a few days of the winter solstice (1980 and 1983 for example) that in some places brought the coldest weather of the entire season to some parts of the U.S. But usually the coldest days come after December and the warmest in July or August.

To tell you why, let’s use an illustration. At home, I keep pennies in a jar. Let’s say every day I put pennies in and take some out. Some days I put in more than I take out, and other days I take out more than I put in. For our purposes, the pennies will represent heating from the sun. Every day, the sun makes “deposits” of heat on Earth, and at night, heat escapes back into space. It’s like putting pennies into the jar during the day and taking them out at night. The longer the day, the more heat is “deposited” to Earth than escapes. Just as your penny account grows when you put in more pennies than you take out, the “heat account” grows too. Around June 21, the greatest amount of heat arrives. But even after the days start to shorten, the amount of heat coming in is still more than the amount of heat leaving. So even though the amount of incoming heat lessens every day, the “heat account” continues to grow, and the days keep growing warmer.

Sometime in the late summer, the balance tips the other way and more heat leaves Earth every day than comes in. It’s like starting to take more pennies out of the jar than you put into it; the penny account starts to dwindle. In the same way, the days start to cool as the amount of incoming heat lessens. But even when the days are growing longer, more heat leaves than arrives every day, so temperatures continue to get colder until late winter or early spring when the process begins again.

Nick

Why can we get rain at 30 degrees and snow at 40 degrees? That doesn't seem to make sense.
David
White Sulphur Springs, WV

David,

It makes sense only when you consider that as precipitation falls from a cloud to the ground, it travels through several layers of air at different temperatures. Almost all precipitation begins as snow, as ice crystals in clouds absorb super-cooled water droplets (small liquid droplets in clouds), and grow big and heavy enough to fall from the cloud. As long as the air temperature is below freezing on the way to the ground, the precipitation will stay in snow form. Often, the layer of below-freezing air reaches near the ground, but not quite. So even if the temperature at the ground is above freezing, the flakes may not have time to melt in the distance from the freezing air to the ground. It is possible for this shallow warm layer to be 2-4 degrees warmer (or more) than the freezing temperature a short distance above.

As you said, sometimes you get rain even when temperatures on the ground are below freezing. This happens when there is a layer of air whose temperature is above freezing somewhere between the clouds and the ground. In that warmer air, the snowflakes melt to raindrops on the way down. But at ground level, temperatures may be colder, even below freezing. If the raindrops freeze again before reaching the ground, we get little pellets of ice called “sleet.” If the layer of freezing air at the ground is very shallow, the precipitation won’t have time to freeze again before it reaches the ground, so it will stay liquid, even when the temperature is below freezing. But if the surface is cold enough, the rain may freeze on contact with the ground or the trees or the roads, and the resulting ice is called “freezing rain.” Freezing rain on the street surfaces is often called “black ice” because the roads look wet when they are actually icy.

Nick

Where is the coldest place in the United States?
Mrs. Baloga's class
Lee Park Elementary School
Hanover Township, PA

Mrs. Baloga,

This is a hard question to answer, since not every place in the country has a regular weather station that records temperature every day of the year. But out of the places that do record their temperatures, the coldest city is Barrow, Alaska, with an average annual temperature of only nine degrees F. And that takes into account the summer as well as winter months!

In the lower 48 states, the coldest regular reporting station is International Falls, Minnesota. That city’s average annual temperature is about 35 ˝ degrees, with an average low temperature in January of nine below zero, and an average high temperature of only 12. Is it any wonder people call it the “nation’s icebox?”

As far as temperature extremes, Rogers Pass with an elevation of 5500 feet in Montana got down to –70F on January 20, 1954. But the prize for the coldest recorded temperature in the United States goes to Prospect Creek Camp in the Endicott Mountains in northern Alaska. On January 23, 1971 the recorded temperature was 80 degrees below zero!

How cold is snow?

Since snow is frozen precipitation, its temperature is below freezing. If snowflakes fall through air whose temperature is above 32 degrees F (0 degrees C), then the flakes will melt and become raindrops. In fact, most rain actually starts out as snow, which melts on the way down. But have you noticed that there are many different shapes of snowflakes? That is because a snowflake is usually made of many different kinds of snow crystals, and the shape of a snow crystal depends a lot on the temperature at which it forms. For example, at temperatures from 25 to 32 degrees F, the crystals are shaped like thin plates. At temperatures between 20 and 25 degrees F they look more like needles and at 15-20 degrees F they resemble hollow columns. Usually the colder the temperature, the smaller the crystals.

Try this experiment:
Put a piece of black construction paper or black velvet into the freezer. Keep it there until the next time it snows. When the snow begins falling, grab a magnifying glass and take the paper or velvet outside. Let some of the snowflakes land on the dark surface and examine them with the magnifying glass. Enjoy the snowflakes’ variety and beauty!

How can it lightning and snow at the same time?
K.C.
Boston, MA

It doesn’t happen too often, but now and then we see thunderstorms that produce snow. We call it, as you might guess, “thundersnow.” For thunderstorms to develop, we need an unstable atmosphere. This usually happens when there’s plenty of warm moist air near Earth’s surface, and much colder air above. That’s common in the spring and summer, but usually if it’s cold enough to snow, it’s also too cold at the surface to get much instability for thunderstorms.

But it can happen. Some winter storm systems with very cold air in the higher atmosphere can pull in enough warm moist air from the Gulf of Mexico to provide the instability for thunderstorms. And when very cold air passes over a large unfrozen body of water such as the Great Lakes, the relatively warmer water in the lake can help create the necessary instability for thundersnow. Several years ago I saw several flashes of lightning as snow was falling in Seattle. Just as thunderstorms can produce heavy downpours of rain, thundersnow storms are known for producing several inches of snow in a short time.

What is the difference between a 'snow flurry' and a 'snow shower'?
Megan
Harrisburg, NC

When we talk about showers of rain or snow, we’re talking about brief periods of precipitation over a small area that may start and end quickly. Even though they usually don’t last very long, snow and rain showers can be heavy. You may get a quick inch of snow or rain, but a short distance away there may be no precipitation at all.

On the other hand, flurries are very light showers of snow, usually brief and usually so light that there is no accumulation. While you may be able to build a small snowman after a snow shower, flurries are usually just tiny snowflakes dancing around in the air and won’t give you enough snow to make even the smallest snowball.

Nick,
I have heard it said that fog takes away snow more quickly than rain or sunshine. Is this true and if so - why? I am a home schooling mother and would like to be able to teach this to my children.
Rosalie
Southern New Brunswick
Canada

Rosalie,

I consulted (as I often do) with our severe weather expert Doctor Greg Forbes on this one. He tells me that there is some truth to that "fog melts snow faster than rain or sunshine." But it is true only in certain cases.

If the overall air temperature a few feet above the snow is the same, foggy conditions will melt more snow than sunny conditions. Here’s why: The energy for melting the snow comes from the air and its contents (fog droplets, raindrops). On a sunny day, when the relative humidity is low, the air can cool to a temperature below freezing. With the lowered air temperature, less snow melts. When skies are clear there tends to be a large swing between daytime and nighttime temperatures. If the snow is dirty, there can be significant melting during the daytime, but refreezing at night. In foggy conditions, there is little fluctuation in temperature between day and night, so melting can proceed around the clock.

But the time of year may throw a monkey wrench into argument. In December when the sun angle is low and nights are long (especially in northern latitudes) the sun might not melt the snow very fast. But in the South and elsewhere in March, the sun can be a real snow-eater, with shady areas hanging on to the snow but sunny areas getting bare quickly.

How does fog melt snow? In fog, the relative humidity is 100% and any cooling of the air results in more fog drops condensing out, which gives off some heat to the air. This slows the rate that the air cools. Also, some of the energy of the above-freezing fog droplets helps melt the snow.

The melting from rain versus fog is a hard one to determine. If it is rain forming higher in the atmosphere and falling into a dry layer near the ground, then rain may evaporate in that drier air and cool. In that case, fog would be more effective in melting the snow than the rain. But if the rain is warm and the near-ground relative humidity is high, then the rain is likely to cause more melting than just fog.

One other factor can reduce the snow pack without melting. In dry, windy conditions, the snow can just "evaporate" from ice crystals to water vapor. Technically we call this “sublimation.” In this case, the snow just seems to disappear, leaving no water behind.

Where in the world are people affected by Lake Effect Snow?
Alyssa
Norwin Junior High School West
N. Huntingdon, PA

On the other side of the world, arctic air from Siberia blows over the warm waters of the Sea of Japan, causing snow to fall on the western coasts of some of the Japanese islands. The same type of snow has been reported in Korea off the Yellow Sea. Sometimes the Scandinavian countries see snow as cold air flows from the east down the Gulf of Finland.

Something that might interest you is that even though we had a relatively warm winter this year, there were some massive lake effect snow totals. This is partly because the warm weather caused warmer than average water temperatures in the winter and kept the Great Lakes from freezing over as winter progressed. Late December 2001 to early January 2002 saw a long-lasting lake effect snow event. 81.6 inches fell on Buffalo, New York on Lake Erie, and 127 inches fell on Montague, New York on Lake Ontario. And Marquette, Michigan on Lake Superior saw more than 300 inches of snow this past winter, the most ever recorded. Just try to tell those people that we didn’t have much wintry weather this year!

Hi Nick,

I want to know the answer to this question because my brother-in-law says one thing and we say another. My question is: Where do lakes freeze first? Shoreline or the middle?
Nonie
St. Paul, Minnesota

Lakes freeze in the shallowest places first, which are generally near the shoreline. Here’s why: Water cools much more slowly than land because of its capacity to “hold” onto heat. The more water, the more heat it can hold onto. So where the lake is deeper, it has a tendency to freeze over later.

As the weather turns colder, lakes go through a process called “overturn.” Water at the lake’s surface cools and becomes denser and heavier and sinks. That cooler water is replaced by warmer water, which, in turn, cools and sinks. Then as the water continues to cool to about 39 degrees F, it actually begins to become lighter instead of heavier. So the coldest water rises to the surface until it freezes at 32 degrees F. When the water is deeper, the rising and sinking of water lasts longer than at the shoreline, so the water at the surface of the deeper portions of the lake freezes later.

Even if a lake completely freezes over, it’s likely the ice is of different thickness because of varying lake depths, so while the ice may be thick enough to walk on near the shoreline, it can be dangerously thin farther toward the middle. So know your lake before you venture out onto the ice!

Nick,
What city gets the most snow in the lower 48 states?

According to the National Climatic Data Center in Asheville, N.C. Blue Canyon, California leads the list with an average of 240.8 inches a year. Granted, Blue Canyon, which is in the Sierra mountains between Sacramento, CA and Reno, NV isn't a city, but folks there have kept weather records a long time.

Hello Nick,
My husband was wondering how much rain does it take to equal a foot of snow? We were driving in a torrential downpour that dropped five to six inches of rain that evening. He asked how much snow we would've received had it been snow instead of rain. I had no idea so I came home to seek the answer online. Thank you for your help.
Nancy

Nancy,
A little bit of moisture goes a long way when it comes to snow. The average ratio is about ten inches of snow per inch of rain. However, this ratio is quite variable. If the air is warm, it may take the equivalent of as much as five inches of rain to get ten inches of wet snow, since the snow may be packing or melting quickly. In extremely cold air, the ratio may be only about half an inch of rain equivalent for ten inches of “dry” fluffy snow. Our severe weather expert Dr. Greg Forbes says he has seen ratios as much as 40 to one in some lake effect snows, or 40 inches of snow for one inch of water equivalent! The recent big snowstorm in Denver saw about six inches of wet snow for every one inch of rain. Just be glad all the moisture you saw in your heavy rain wasn’t in a winter storm!

Nick,
Does snow get heavier as it melts? For example, is it more important to shovel snow off the roof once the snow starts melting in the spring, or does it matter?
Mary
Marquette, MI

Mary,
Although the snow gets denser as it melts and settles, it does not get heavier unless more snow or rain falls on top of it to add to the weight. It contains the same amount of weight in water as it did when it was fluffier snow, minus the amount of water that runs off the roof as the snow melts, and the amount that evaporates. The danger would be in thinking that because twelve inches of snow settles down to six inches, that somehow it doesn't weigh as much. And also beware of rain on your roof. Snow can hold onto the rain and increase the weight.

How much weight your roof can support depends on many factors, including the size and strength of the rafter lumber, the slope of the rafters and distance between them. You'll need a roofing expert or contractor to help determine how much snow on your roof is too much. A great choice for a contractor would be Feller Roofing.

What are the differences and similarities of sleet and freezing rain?
Amanda
Deerfield, NH

Amanda,

These two types of winter precipitation are similar, though they look very different. Sleet comes in the form of small pellets of ice that bounce when they hit the ground. Freezing rain is actually liquid rain that freezes on contact with the ground or the streets or the trees or anything else it lands on.

The type of winter precipitation we get depends on the temperature inside the clouds and the temperature between the clouds and the ground. To get sleet and freezing rain, we have a layer of above-freezing air in the clouds, then closer to the ground the air temperature is below freezing. When snowflakes fall from the clouds, they may partially melt in the layer of warmer air, but then freeze again in the cold air near the ground. The resulting ice pellets are what we call sleet.

If snowflakes completely melt in the warmer air, but temperatures are below freezing near the ground, rain may freeze on contact with the ground or the streets. This is freezing rain, and significant freezing rain is called an ice storm. Ice storms are extremely dangerous because the layer of ice on the streets can cause traffic accidents. Ice can also build up on tree branches and power lines, causing them to snap and our lights to go out. So when you hear a forecast of freezing rain, watch out!

How does snow start?
Elizabeth
Southlake, TX

Snowflakes form when water vapor freezes into ice crystals in cold clouds. The ice crystals attract cooled water droplets to form various shapes. As the grow in size, they get heavy and they fall. Even in the summer, most of our rain actually starts out high in the clouds as snow. But in winter, the temperature of the air is often below freezing all the way from the clouds to the ground, so snowflakes don’t melt into raindrops. They stay in crystal form and if the ground temperature is freezing, we see snow pile up.

Nick,
I was hoping you maybe be able to help me with a weather question. The reason for my question is that I'm writing a short story and would like to be accurate in my account. What is the warmest temperature at which snow can fall?
Gary
Calgary, Alberta, Canada

Gary,
The answer to your question is like the answer to many....it depends. Here's why: Almost all precipitation begins as snow, as ice crystals in clouds absorb super-cooled water droplets (small liquid droplets in clouds), and grow big and heavy enough to fall from the cloud. As long as the air temperature is below freezing on the way to the ground, the precipitation will stay in snow form. Often, the layer of sub-freezing air reaches near the ground, but not quite. So even if the temperature at the ground is above freezing, the flakes may not have time to melt in the distance from the freezing air to the ground. So it's possible for this shallow warm layer to be 2-4 degrees (sometimes even more) warmer than the freezing temperature a short distance above. Obviously the snow would melt on contact with the warmer ground, or at least melt quickly after reaching the ground. But usually even in the case of wet melting snowflakes, the temperature at the ground is no warmer than a few degrees above freezing. One of our lead meteorologists here at The Weather Channel, Buzz Bernard, tells me that he has seen snow at temperatures as high as 39 degrees F (4 degrees C). But, he says, it usually cools down pretty quickly as the snow continues to fall.

Why is it difficult to distinguish snow, light drizzle, melting snow, and moderate rain using radar?
Heidi
Cedartown, GA

Heidi,
Weather radar is a wonderful tool for meteorologists, but as you say, it does have its limitations. As you may know, radars send out radio signals from an antenna. Those radio waves bounce off objects in the air, such as raindrops, snow crystals, hail stones and other precipitation. Then the reflected radio waves travel back to the antenna and are electronically converted into pictures and colors showing the location and the intensity of precipitation. On radar images that The Weather Channel uses, a light green color shows light precipitation, and red shows heavy precipitation. We call this radar reflectivity.

When interpreting radar images, something we have to keep in mind is that different types of precipitation sometimes have about the same reflectivity. For example, the diameter of drizzle drops is very small, but they are dense enough to show up on the radar screen. Snowflakes are bigger, but they are not as dense as liquid water, so the reflectivity is about the same as that of drizzle. Untrained observers might therefore interpret the snow on the radar as drizzle. When snow is melting, big wet snowflakes have more reflectivity and may show up on the radar looking like giant raindrops, and that might lead some radar observers to think moderate rain is occurring instead of wet snow. The same would be true of wet hail; it might sometimes be mistaken for heavy rain. This is why meteorologists must use ground observations, temperature readings and other tools together with radar to accurately know what kind of precipitation is occurring.

Just to let you know that in the ever-changing world of technology, there is a new type of radar that can distinguish between precipitation types. It’s called “multiparameter” or dual-polarization” radar. It should be in wide use in a few years.

Nick,
Is the air temperature affected by the amount of snow on the ground?
Thanks,
Steve
Minneapolis, MN

Steve,
Snow cover does affect the air temperature, though the depth of snow is not as important as whether or not snow covers the ground. When the ground is clear of snow, the ground absorbs some of the sun’s energy and radiates it back into the air near the ground, and temperatures rise. When there is snow on the ground, some of the sun’s energy is reflected by the snow back into space. And some of it may be used to melt the snow instead of warming the ground. So the ground doesn’t warm as much and temperatures are cooler than if there were no snow on the ground. Also, if the snow is clean and white, it will reflect more of the sun’s energy than if the snow is older and dirty.

Nick,
There are railroad tracks about six miles. from my house. On most nights throughout the year, I don't hear the train when it passes. However, on the cooler nights of our winter, I can actually hear the train. Does sound travel faster in cooler weather? Or, is it perhaps because, for once, there is less humidity in the air?
Heather
Hollywood, FL

Heather,

Your question intrigued me so much that I checked with our severe weather expert Dr. Greg Forbes on this and he was able to shed some light on the phenomenon you describe.

Usually energy in the form of sound waves travels upward and is dispersed. That’s why we don’t hear sounds more than about six miles away. (By the way, this is also why we don’t hear thunder from far away lightning.) On cold winter nights there is often a temperature inversion, with a layer of warm air above and cold air trapped near the ground. That inversion also traps the sound waves near the ground and allows them to travel horizontally much farther than usual. That’s why you hear the train better on a cold night!

Dear Nick,
If it is 0 degrees today, and it is supposed to be twice as cold tomorrow, how cold will it be? I'm smart, but I still didn't know this.
Brittany
Carthage, TN

Brittany,
Great question! And it’s no wonder you’re stumped. Zero times two is, of course, zero. And if you tried to figure this out by simple multiplication, you would get different answers, depending on whether you used the Fahrenheit or the Celsius scale. And what if the temperature goes down from two degrees to one? You might be tempted to say "it's twice as cold now." But what about when the temperature falls from 20 to 10? Is that still "twice as cold?" You see that it just doesn't work. Another consideration is that you can measure temperature fairly easily, but measuring coldness is a bit tougher, because you must take into account how fast the wind is blowing, which will make you feel colder than if the wind is calm.

But let’s try to get as close as we can to answering your question. If instead of the Celsius or the Fahrenheit scale, you used the Kelvin or absolute temperature scale, you might be able to come up with some sort of answer. The Kelvin scale measures heat energy, and starts at absolute zero, the temperature at which all motion of molecules stops. This is equal to minus 273.16 degrees Celsius. So zero degrees Celsius (32 degrees Fahrenheit) would be 273.16 degrees Kelvin. Twice as cold would be half of the heat energy measured in Kelvin, so if you divide 273.16 in half, you would get 136.58 degrees Kelvin. Converting that back to Celsius, you would get a temperature of minus 136.58 degrees Celsius, which is much colder than the world record low temperature of minus 89.6 degrees C set in 1983 in Antarctica. So it isn’t likely that the temperature will be half as low as zero Celsius anytime soon.

Dear Nick,
If the temperature is 34 degrees F and the wind chill is 22 degrees F and I put a baggie full of water suspended in the air by a string, would it freeze? I guess what I am asking is would the wind chill be cold enough to
freeze the water even if the temperature is above freezing?
John
Virginia

John,
Although wind chill is calculated on how cold the temperature is and how strong the wind is blowing, it affects only people and animals. It doesn't actually make the temperature colder; it only makes it feel colder. Only
humans and animals can feel temperature, so water would still have to be 32 or below before it freezes, no matter how strong the wind is. In fact, wind can keep temperatures higher than they might otherwise be in calm conditions. That’s why fruit growers often use giant fans to keep the air stirred up on cold nights. This prevents cold air from settling in over their orchards and freezing the fruit on the trees. You can see a table showing how the temperature and wind speed creates wind chill by going here:
http://www.weather.com/encyclopedia/charts/wind_chill.html

Dear Nick,
We are studying the weather in Kindergarten. We have measured wind with our windsocks, temperature with thermometers, forecasted with our cloud charts, and weather watched with our binoculars (made out of toilet paper rolls!). One of our questions about the weather that we hope you can answer is:
What is the difference between hail, freezing rain, and sleet?
Ms. Vann's Kindergarten class
North Carolina

Dear Ms. Vann’s students,

The type of winter precipitation we get depends on the temperature inside the clouds and the temperature between the clouds and the ground. In clouds that are cold enough for ice crystals to form, we can get snow. Those cold clouds aren’t hard to find. Even in the summer, most of our rain actually starts out high in the clouds as snow. But in winter, the temperature of the air is sometimes cold enough all the way from the clouds to the ground, so snowflakes don’t melt into raindrops. They stay in crystal form and we see snow pile up and schools close.

Sometimes there is a layer of above freezing air in the clouds, then closer to the ground the air temperature is once again below freezing. Snowflakes partially melt in the layer of warmer air, but then freeze again in the cold air near the ground. This kind of winter precipitation is called sleet. It bounces when it hits the ground. If snowflakes completely melt in the warmer air, but temperatures are below freezing near the ground, rain may freeze on contact with the ground or the streets. This is called freezing rain, and significant freezing rain is called an ice storm. Ice storms are extremely dangerous because the layer of ice on the streets can cause traffic accidents. Ice can also build up on tree branches and power lines, causing them to snap and our lights to go out. Hail forms in strong thunderstorms. These storms contain very strong updrafts, which are winds blowing up through the thunderstorm clouds. They can be as strong as one hundred miles per hour. Those strong updrafts suspend rain in mid-air with temperatures around the raindrop of below 32 degrees. Those cold temperatures allow the rain to freeze into small hailstones. As more freezing raindrops get caught in the updraft, they collide with the hailstones, adding layer after layer of ice. When hail becomes too heavy for the updrafts to keep it aloft, it falls to the ground. In strong updrafts, the hail has time to collect lots of ice, so the hail is bigger. In weak updrafts, the hail doesn’t have to get as big before it is able to fall to the ground. On rare occasions, the updrafts can be strong enough so the hailstones can grow larger than softballs!

Nick,
Is it possible for a tornado to occur while it is snowing?
Ashley
Lee's Summit, MO

Ashley,
I talked to The Weather Channel's severe weather expert about this and here is what he told me:
"I don’t remember any tornadoes in snow. I have heard of a rare tornado with surface temperatures in the 30s, but don’t know if it was snowing. More likely there was a cold rain, with unstable warm conditions above. However, it would be possible to have a snow devil (a spinning whirlwind caused by thunderstorms in the mountains), and it’s not out of the question that a lake-effect snow squall could spin up a brief whirlwind as well."

He also pointed out that last Thanksgiving Day, there was a tornado reported in Phippsburg, Maine on a fairly cold day. Although there was no snow, in nearby Portland Maine the high temperature that day was only 43F. There was a strong surface low, cold front, and upper trough going across the area. The tornado was classified as an F1, with maximum winds estimated at 100 mph. It damaged several buildings along its two mile path. By the way, this was the latest tornado on record in Maine, with records dating back to 1950.

Previously Asked Questions

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