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You may have heard that no two snowflakes are exactly alike. That isn't totally true, although the chance of finding twin snowflakes is very, very, very unlikely.

Scientists estimate the chances of two snowflakes being exactly alike are about 1 in 1 million trillion (that's a 1 followed by 18 zeros). Meteorologists think that there are 1 trillion, trillion, trillion (a 1 with 36 zeros!) different types of snowflakes.

A snowflake has three basic ingredients: ice crystals, water and dust. The ice crystals form as water freezes on a bit of microscopic dust.

The dust particles can come from many different places, including flower , , and even bodies such as meteors.

Snow forms in very cold clouds containing water droplets and ice crystals. As water droplets attach themselves to ice crystals, they freeze, creating an even larger ice .

In any , little bits called molecules line up in a pattern. In ice crystals, water molecules line up and form a six-sided shape called a . This is why all snowflakes are six-sided.

The temperature of the cloud it forms in determines the shape of an ice . Likewise, the amount of moisture in the cloud determines the size of the ice .

More moisture will create a bigger . When several ice crystals stick together, they form a snowflake.

As snowflakes tumble through the air, and , they each take a different path to the ground. Each snowflake falls and floats through clouds with different temperatures and moisture levels, which shapes each snowflake in a way.

Even though two snowflakes may form in the same cloud, their different journeys to the ground will affect their shape and size, giving each snowflake its own identity.

You may never find an identical pair of snowflakes, but they can be grouped by similarities in their patterns. Explore photographs of common snowflake patterns.

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You may have heard that no two snowflakes are exactly alike. That isn't totally true, although the chance of finding twin snowflakes is very, very, very unlikely. Scientists estimate the chances of two snowflakes being exactly alike are about 1 in 1 million trillion (that's a 1 followed by 18 zeros).

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Pour a glass of and take a sip. Pretty good, isn't it? Would you believe the you're drinking has been around for millions of years? It's true!

Before any creatures were living on land, your was part of the . The we have on Earth has been here for a very, very long time, and Mother has been constantly recycling it through the .

Once is in the , it rises into the atmosphere , cools, and forms clouds through a process called condensation . If you have ever poured a cool glass of lemonade on a hot day, you have probably seen condensation in action.

The next step in the - precipitation - occurs when so much has condensed that the can no longer hold it. Clouds become heavier and heavier until eventually the falls back to the Earth in the form of precipitation . Depending on atmospheric conditions and temperatures, precipitation can arrive on Earth as , hail, sleet, freezing , or snow.

When falls back to Earth as precipitation , it ends up in rivers, lakes, oceans, swimming pools, barrels, bird baths, and lots of other places. This final step of the is called collection .

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Pour a glass of and take a sip. Pretty good, isn't it? Would you believe the you're drinking has been around for millions of years? It's true! Before any creatures were living on land, your was part of the .

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It's a cloudy day on the Kansas plains. You and your dad keep a eye on the skies as you plow the fields. You've seen clouds like these before, and you know that they can be trouble.

As the afternoon fades away, you begin to see the clouds form a wall. As the sky turns a color, you head for safety, afraid that a twister may soon be headed your way. What are you afraid of? A , of course!

With speeds that can exceed 300 miles per hour, tornadoes are one of the most devastating and destructive forces on Earth. Consisting of a of air that extends from a thunderstorm to the ground, tornadoes can cause massive damage to land, buildings, and people.

Although tornadoes have hit every continent except Antarctica, they're most common in North America, particularly in the section of the Midwest United States known as " Alley." Experts estimate as many as 1,000 tornadoes are reported in the U.S. in an average year.

For years, tornadoes were a mystery to scientists. Thanks to modern technology, however, researchers are beginning to the secrets behind the environmental forces that tornadoes.

Tornadoes usually form out of massive thunderstorms. When moist, warm air from the Gulf of Mexico meets dry, cold air from Canada, an unstable collision in the atmosphere occurs. The warm air rises quickly and the cold air falls, creating a , which is a type of large thunderstorm with swirling updrafts of air.

Winds blowing within the will often move at different speeds, creating a phenomenon scientists call . can cause a spinning effect within the . This area of winds within a , known as a mesocyclone , can extend from one to more than five miles wide.

If the 's turns the mesocyclone from a horizontal to a position, a will form. If the strengthens, it can stretch the , causing it to tighten its spin and speed up. If rain and hail from the thunderstorm cause the to lengthen to the point where it touches the ground, it becomes a .

When we think of tornadoes, we often imagine a traditional shape. However, they can also be very slender and look like a rope. Some tornadoes even have multiple vortices, which are multiple small tornadoes swirling around a common center point.

Tornadoes may sometimes look like a large moving of dark . Other tornadoes are nearly invisible and can only be seen when dust or debris moves at ground level as the passes. Some tornadoes even form over water. Scientists call these tornadoes waterspouts.

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It's a cloudy day on the Kansas plains. You and your dad keep a eye on the skies as you plow the fields. You've seen clouds like these before, and you know that they can be trouble. As the afternoon fades away, you begin to see the clouds form a wall.

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All precipitation falls through the atmosphere on its way to the Earth's . Imagine a drop or flake falling through a long tube that contains the air between the clouds and the Earth.

The air inside the imaginary tube is the atmosphere . Differences in temperatures in different parts of the atmosphere account for the differences we see between , , and .

The liquid precipitation that falls to Earth's is . can begin as , but by the time it reaches the Earth's , it has melted because the temperature closer to Earth is warmer.

If you've ever thought it strange that it's raining when the temperature outside is below , it's probably because the temperature higher up in the atmosphere is still above .

forms in clouds at temperatures below . As falls through the atmosphere , the air remains at least 32° F or colder. In order for a to reach Earth, it must remain frozen from to .

But can it be too to ? Sometimes! The colder it gets, the less water there will be in the air...and less water in the air means less chance of .

occurs when a falls through the atmosphere and warms up a bit before refreezing. The begins its journey frozen. As it passes through a thin layer of warm air in the atmosphere , it melts a bit.

It then re-enters another pocket of air before reaching the . The refreezes and becomes an pellet we call . typically bounces when it hits the ground.

follows a similar journey as , but instead of a thin pocket of warm air, falls through a larger pocket of warm air in the middle of its journey. begins as , but when it reaches the warm pocket, it melts and becomes .

Before hitting the ground, it passes through a very pocket of air, which cools it some but not enough to turn it into . Instead, when the water droplet reaches the of the Earth and comes in contact with objects (such as cars, streets, or trees), it freezes immediately and turns into .

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What's the Difference Between Snow, Sleet, and Freezing Rain?

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All precipitation falls through the atmosphere on its way to the Earth's . Imagine a drop or flake falling through a long tube that contains the air between the clouds and the Earth. The air inside the imaginary tube is the atmosphere .

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Where does fog come from? Would you believe us if we told you we don't have the foggiest idea? Probably not... we're in Wonderopolis, after all!

The next time you wake up in the morning to fog, you can say "It's cloudy outside!" instead of "It's foggy outside!" Why? Because fog is just a regular cloud that happens to be close to the ground rather than high up in the air.

So how does fog get down here close to Earth? Does a strong wind blow a cloud out of the sky down to the surface of the Earth? Not quite...

Fog actually forms close to the Earth where we see it. The same scientific processes that usually happen way up high in the atmosphere to create clouds can take place closer to the ground. When they do, we get fog.

Basically, fog forms when warm air meets colder air. When this happens, water in the air - a gas - is cooled enough for the gas to turn to a liquid in the form of tiny water droplets. This process is called " condensation ."

We see those groups of tiny droplets as clouds or, when they're close to the ground, as fog. As the air heats up again, fog will slowly disappear as the tiny water droplets once again return to a gas in the form of water .

There are four main types of fog. fog occurs when the ground radiates heat outward while the air above the ground begins to cool after the sun sets. If the temperature of the air sinks below its dew point, water will condense around dust in the air to form fog.

fog occurs when warm air moves in over a cooler land surface. This happens often along coastlines, for example, when warm ocean breezes blow in over cooler land. The land cools the warm air below the dew point, and fog forms.

fog occurs when warm air passes over the upward slope of a cool mountain. As elevation increases, the mountain cools the air quickly, causing condensation and fog.

Evaporation fog occurs when extra water comes into contact with air that is already heavily saturated. Because the air can only hold so much water , adding extra water from evaporation , such as from a lake in the hot sun, can cause the air to reach the dew point and form fog.

While it can be neat - and sometimes a bit creepy - to see fog, it can also be dangerous. When fog is very thick and near the ground, it can reduce visibility to only a few feet. This makes driving in fog particularly dangerous.

The foggiest place in the world is the Grand Banks off the coast of Newfoundland in Canada. Here, the cold Labrador Current from the north meets the warm Gulf Stream from the south, creating the perfect conditions for fog.

Other very foggy areas include Argentina, Labrador and Point Reyes, California. Each of these areas reports more than 200 foggy days every year.

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Where does fog come from? Would you believe us if we told you we don't have the foggiest idea? Probably not... we're in Wonderopolis, after all! The next time you wake up in the morning to fog, you can say "It's cloudy outside!" instead of "It's foggy outside!" Why?

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What do you get when you mix in the with thunderclouds and strong winds? Many times the answer is .

Have you ever seen hailstones - sometimes tiny, sometimes large pellets of - fall from the ? If you have, you know it can be quite a sight... and sometimes dangerous.

Hailstorms are most common in the warm spring and summer months. When thunderstorms and tornadoes increase, hailstorms usually follow close behind.

Cumulonimbus clouds - those tall, threatening clouds often called "thunderclouds" - are where hailstones begin to form. The temperatures high up in these clouds can be quite cold, causing in the to change into crystals. These crystals - sometimes called "soft " or " pellets" - are called "" by meteorologists.

As forms, the crystals become heavier and begin to fall downward. When weather conditions are right for , very strong winds - called "updrafts" - will push the back up into the upper parts of the thunderclouds.

As the ascends back into the upper parts of the thunderclouds, it accumulates more and gets bigger. This process is called "." As layers of are added, the turns into hailstones that get bigger and again begin to fall toward the Earth.

When the hailstones grow big enough to overcome the power of the updrafts, they will fall to Earth as . How big hailstones get depends largely on how strong the winds are.

Updrafts of 20 miles per hour usually result in -sized (up to 0.5 inches in ), while updrafts of 40 miles per hour can make hailstones the size of quarters (up to 0.75 inches in ).

Very large hailstones require incredibly strong updrafts. the size of a golf ball (up to 1.75 inches in ) needs updrafts of about 55 miles per hour. Softball-sized (4 or more inches in ) only forms when updrafts exceed 100 miles per hour.

The largest hailstone ever recorded in the United States fell in South Dakota on July 23, 2010. It weighed 1.93 pounds and measured 8 inches in and 18.62 inches in circumference ! Can you imagine the damage that hailstone could have caused?

Hailstorms usually don't last long - only about 5 to 10 minutes - but they can cause a lot of damage in that time. In addition to damage caused to automobiles, airplanes, skylights, and roofs, also regularly destroys farmers' crops.

Far less common - but still possible - are serious injuries. can cause concussions or fatal head to people and animals if the hailstones are large enough.

Hailstorms are most common in the Great Plains states, especially northern Colorado and southern Wyoming. If a hailstorm breaks out where you live, get inside as quickly as possible and stay away from windows.

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What do you get when you mix in the with thunderclouds and strong winds? Many times the answer is . Have you ever seen hailstones - sometimes tiny, sometimes large pellets of - fall from the ? If you have, you know it can be quite a sight... and sometimes dangerous.

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Today's Wonder of the Day was inspired by Chase from Lee's Summit, MO. Chase Wonders, " Why does it thunder when it snows? " Thanks for WONDERing with us, Chase!

Do you get excited when the weatherman calls for snow? If you're like many kids, you might wish for several inches of snow to fall in order to earn a snow day. When you wake to see the in a thick blanket of snow, you know it's going to be a good day filled with sledding and making snowmen!

As you watch the skies outside, waiting for the first snowflakes to from the clouds, you close your eyes to enjoy the sense of and that usually precedes the snow. Suddenly, however, you sense a bright followed by a loud ! What's going on here? It's thundersnow!

Most of the time, lightning and thunder are accompanied by rain. When we see the lightning, hear the thunder, and feel the rain, we know we're in the middle of a thunderstorm. However, in certain unique conditions, lightning and thunder can form in a storm that drops snow and hail, yielding a phenomenon meteorologists call thundersnow.

In normal thunderstorms, warm, air rises from Earth's surface to create tall, narrow updrafts that can approach 40,000 feet or more. These storms that drop rain usually occur when surface temperatures are above freezing.

Storms that produce snow are quite different from thunderstorms. Instead of tall, narrow updrafts, snowstorms usually form from broad layers of flat, shallow clouds that don't usually exceed 20,000 feet. Updrafts, if they exist, are usually rather mild.

Thundersnow occurs when normal snowstorm clouds begin to develop upward bumps, known as turrets. These turrets are caused by instability within the storm system, combined with a strong lifting mechanism. For example, a strong upper-level wind can bring in cold air, pushing pockets of colder air 5,000 or more feet above the rest of the flat clouds.

Turrets formed by the lifting action of cold air currents tend to be , leading to the presence of both updrafts and downdrafts. These cold, winds create favorable conditions for precipitation to form, including snowflakes and small hailstones known as graupel.

As precipitation particles bounce into each other, they can begin to create electrical charges. When sufficient electrical charges build up, they can be discharged in the form of -to-ground or -to- lightning, which is subsequently followed by the rumble of thunder.

Thundersnow events can often be accompanied by significant snowfall, since the conditions that lead to thundersnow also tend to large amounts of snowflakes in the flat, shallow layer below the turrets. Since thundersnow develops when air near the ground is below freezing, significant accumulation is possible from thundersnow.

If you've never experienced thundersnow, it's probably because the unique conditions for it to occur don't happen everywhere or very often. The instability and lifting necessary to create thundersnow has to occur in a very narrow, shallow layer between 20,000-25,000 feet up.

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Today's Wonder of the Day was inspired by Chase from Lee's Summit, MO. Chase Wonders, " Why does it thunder when it snows? " Thanks for WONDERing with us, Chase! Do you get excited when the weatherman calls for snow?

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What's that? Right there! Way up in the ...is it a bird? Could it be a plane? Maybe it's an elephant! Or a badger? Or a giant panda bear driving a car!

When you look up high in the sky and see those white, wispy forms floating through the , you notice that they can look like all sorts of things. What are we talking about? Clouds, of course!

Have you ever stopped to WONDER exactly what clouds are? How do they get there? Are they made of smoke that floats up high into the atmosphere ? Could they be placed there by magical creatures who like to paint the sky?

Don't get too excited about the thought of magical creatures painting pictures in the sky for you. When it comes to clouds, they can be explained easily by science!

The is also full of other particles, including , dirt, and salt. Scientists call these particles aerosols. People who study the , called meteorologists, also call these particles condensation or seeds.

Clouds form when droplets of condense from a gas back into a liquid. This process, called condensation , occurs when the becomes , which means it cannot hold any more .

Other factors, such as fronts of warm and cold colliding, the movement of the jet stream, and variances in winds, can also affect formation. Topographical features of Earth's surface, such as mountain ranges, can also influence the flow of and thus the creation of clouds.

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What's that? Right there! Way up in the ...is it a bird? Could it be a plane? Maybe it's an elephant! Or a badger? Or a giant panda bear driving a car! When you look up high in the sky and see those white, wispy forms floating through the , you notice that they can look like all sorts of things.

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, , go away. Come again some other day. That's how we feel sometimes when all we want to do is go outside and play.

However, plays an important role in our world. It replenishes the Earth with water and helps our plants grow. Without regular rainfall, our world would be a dry, dusty place that wouldn't be any fun at all.

If you see clouds roll in on a sunny day, it could mean that is on the way. Have you ever stared at those clouds and wondered whether they hold and, if so, how much?

As it turns out, that question isn't easy to answer. Why? Clouds aren't like buckets, so they don't really "hold" water like a bucket would.

Whether you realize it or not, the air around you is filled with water. Water comes in three forms: liquid (that you drink), solid (ice) and gas (water in the air). The amount of water inside a is no different than the amount of water in the air around it.

In the air around a , water is a gas in the form of water . Inside the , cooler temperatures have transformed water into a liquid. This liquid is in the form of millions, billions or even trillions of tiny water droplets called droplets. Scientists call this process condensation .

When water in the air condenses into tiny droplets, those droplets become visible. What we see is a . Whether that liquid water will fall to the ground as depends on many factors.

droplets are very tiny and weigh very little. In a , they float with the wind or simply hang in the air. In addition, updrafts (winds that blow upward from the surface of the Earth) also help keep droplets suspended within a .

To fall to Earth, droplets have to become heavier. If they combine with other droplets or if more liquid water continues to condense out of the air, they eventually become heavy enough to form drops that fall to Earth as . The scientific word for rainfall is precipitation .

Once droplets grow into drops and fall to Earth as , it will continue to as long as conditions in the atmosphere keep causing water to condense into liquid water and grow into drops heavy enough to fall as .

So how much can a produce? This is perhaps a better question than how much a can hold. Scientists estimate that one inch of falling over an area of one square mile is equal to 17.4 million gallons of water. That much water would weigh 143 million pounds!

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go away. Come again some other day. That's how we feel sometimes when all we want to do is go outside and play. However, plays an important role in our world. It replenishes the Earth with water and helps our plants grow.

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"It's not the heat, it's the humidity!" Have you ever heard someone say that on a warm, day? As you'll see soon, there's a lot of truth in that statement.

To understand humidity, you first have to realize that there's in the air all around you. "But I'm not wet!" you might say. That's true. Most of the air around you has in the form of a called vapor.

is the amount of in the air. Too much or too little can be dangerous. For example, high combined with hot temperatures is a combination that can be a health risk, especially for the very young and the very old.

plays an important role in our daily . Without in the air, our might be like the on Mars. Could you imagine life without clouds, rain, snow, thunder, or lightning?

When you hear forecasters talk about humidity, you may hear them talk about two different terms: and humidity. is the amount of divided by the amount of dry air in a certain of air at a particular temperature. The hotter the air is, the more it can hold.

is the of the current to the highest possible humidity, which will depend upon the current air temperature. is the term forecasters use most often.

A of 100% means that the air can't hold any more vapor. It's totally saturated. When this occurs, it can rain. In fact, the must be 100% where clouds are forming for it to rain. However, at ground level where the rain lands, the can be less than 100%.

So how does affect us on a hot day? Humans are sensitive to changes in humidity, because our skin uses the air around us to get rid of in the form of sweat. If the is very high, the air is already with and our sweat won't evaporate. When this happens, we feel hotter than the actual temperature.

Likewise, very low can make us feel cooler than the actual temperature. This happens because the dry air helps sweat evaporate more quickly than usual.

If the temperature outside is 75° F (23.8° C), can make it feel warmer or cooler. A of 0% would make it feel like it's only 69° F (20.5° C). On the other hand, a of 100% would make it feel like it's 80° F (26.6° C).

So what's a comfortable level? Studies have shown that we tend to feel most comfortable at a of around 45%. Some people use special machines called humidifiers (add humidity) or dehumidifiers (remove humidity) to keep indoor at a comfortable level.

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"It's not the heat, it's the humidity!" Have you ever heard someone say that on a warm, day? As you'll see soon, there's a lot of truth in that statement. To understand humidity, you first have to realize that there's in the air all around you. "But I'm not wet!"