Category: Earth & Weather

Fog is essentially a cloud at ground level.

It forms when moist air cools enough that some of its water vapor condenses into tiny liquid droplets.

Air temperature matters

Air can contain water vapor, but the amount that can remain in vapor form depends strongly on temperature.

As moist air cools, it may eventually reach its dew point.

At that point, some water vapor begins condensing.

Tiny droplets stay suspended

The droplets are extremely small.

They can remain floating in the air rather than immediately falling to the ground.

When enough of them gather near the surface, visibility drops and you call it fog.

This is the same basic reason clouds can stay suspended.

How does air near the ground cool?

There are several routes.

The ground may cool overnight and chill the air above it.

Warm, moist air may move across a colder surface.

Air may be forced uphill and cool as it rises.

Different circumstances create different kinds of fog, but cooling moist air is the recurring theme.

Why does fog make it hard to see?

The tiny droplets scatter light.

Instead of light traveling cleanly from distant objects to your eyes, it gets redirected in many directions.

That reduces contrast and makes distant objects disappear into the gray.

Is fog made of water vapor?

Not the visible fog itself.

Water vapor is invisible.

What you see is liquid water in the form of countless tiny suspended droplets.

The same distinction matters with visible breath on a cold day.

Why does fog eventually disappear?

Air warms.

Wind mixes in drier air.

Droplets evaporate.

Once the conditions no longer support suspended droplets near the ground, the fog thins or disappears.

So fog is not mysterious vapor hanging in the air.

It is basically a cloud that formed where you happen to be standing.

And that’s the short explanation.

Related explanations: Why can you see your breath in cold weather?, Why do clouds stay in the sky?, and Why does glass fog up?

A clear ice cube and a pile of fresh snow are made from the same basic substance.

One can look transparent.

The other looks bright white.

The difference comes from structure.

A solid piece of ice can transmit light

In clear ice, light can travel through a relatively continuous material.

Some light reflects from the surfaces, but much of it can pass through.

That lets you see into or through the ice.

Snow contains countless ice surfaces

Snow is made from huge numbers of small ice crystals.

Between those crystals are air spaces.

Light entering snow hits surface after surface.

At each boundary between ice and air, some of the light changes direction or reflects.

Repeated scattering sends light back toward you

After bouncing and scattering through many crystals, much of the incoming visible light returns toward your eyes.

The different visible wavelengths are scattered broadly enough that they arrive together.

Your visual system interprets that combination as white.

This is another example of structure changing how light looks, much like the thin film in rainbow-colored soap bubbles changes which wavelengths reinforce one another.

Why can packed snow look different?

Pack the snow tightly and some air spaces disappear.

Compress it further and the structure becomes more like solid ice.

Now light can travel differently.

Dense ice may look clear, gray, or blue depending on thickness, bubbles, cracks, and impurities.

Why can old snow look dirty or gray?

Snow collects dust, soil, soot, plant material, and other particles.

Those materials absorb and scatter light differently.

The snow is still made of ice, but the added material changes the color you see.

Why do glaciers often look blue?

In large dense masses of ice, light can travel through enough material for red wavelengths to be absorbed more strongly.

More blue light remains.

That creates the striking blue seen in some thick glacial ice.

So snow is not filled with white pigment.

It is a giant light-scattering pile of tiny clear crystals.

And that’s the short explanation.

Related explanations: Why do clouds stay in the sky?, Why does the ocean look blue?, and Why do soap bubbles have rainbow colors?

A cloud can contain an enormous amount of water.

So why does it not fall out of the sky all at once?

Because that water is divided among billions of extremely tiny droplets or ice crystals.

Cloud droplets are tiny

A typical cloud droplet is so small that its fall through air is extremely slow.

Air resistance has a large effect on something with so little mass.

The droplet may technically be falling relative to the surrounding air, but very slowly.

Air itself is moving

The atmosphere is not stationary.

Rising currents can carry cloud droplets upward.

Turbulence moves them around.

As long as upward air motion balances or exceeds their slow downward settling, the droplets remain suspended.

How does a cloud form?

Moist air rises or cools.

When it cools enough, water vapor condenses onto tiny airborne particles.

The result is microscopic liquid droplets or ice crystals.

This is closely related to why fog forms. Fog is essentially a cloud forming close to the ground.

When does the water finally fall?

Cloud droplets collide and combine.

Ice crystals can grow.

Eventually some particles become large enough that gravity wins decisively.

They fall as rain, snow, sleet, or another form of precipitation.

Why can clouds look so heavy?

The total water mass can be enormous even while each individual droplet is tiny.

Think of a cloud as a huge region containing countless small particles spread through a large volume of air.

The total is impressive.

The individual pieces remain easy for air currents to support.

Are clouds water vapor?

Not the visible part.

Water vapor is invisible.

The cloud you see consists of liquid droplets, ice crystals, or both.

That distinction is also why fog is made of tiny droplets rather than visible water vapor.

So a cloud may contain tons of water.

It stays up because that water has been divided into billions of tiny pieces moving with the air.

And that’s the short explanation.

Related explanations: Why does fog form?, Why does rain smell so good?, and Why is snow white if ice is clear?

The ocean contains an enormous amount of dissolved salt.

Most of it did not arrive in one giant prehistoric salt shaker.

The process has been happening gradually through Earth’s water cycle and geology.

Rain helps weather rocks

Rainwater absorbs carbon dioxide from the atmosphere.

That makes it slightly acidic.

As rainwater and groundwater move across and through rock, they dissolve tiny amounts of minerals.

Those dissolved materials enter streams and rivers.

Rivers carry ions to the ocean

Rivers transport dissolved ions toward the sea.

Two especially common ions in seawater are sodium and chloride.

Together, sodium and chloride make the main ingredients of ordinary table salt.

Other dissolved ions are present too.

Seawater is a chemical mixture, not just water plus sodium chloride.

Why are rivers not salty too?

Rivers do contain dissolved minerals.

But the concentration is usually far lower.

River water is constantly moving toward the ocean and being replaced.

The ocean acts as a giant long-term reservoir.

Evaporation leaves salt behind

Water constantly evaporates from the ocean surface.

The water molecules enter the atmosphere.

Most dissolved salts do not evaporate with them.

That helps concentrate dissolved material in the seawater that remains.

Does the ocean just get saltier forever?

No.

Geological and chemical processes remove salts too.

Some ions become part of minerals.

Some are used by organisms.

Some interact with the seafloor and ocean crust.

The salinity of the ocean reflects a long-term balance between inputs and removals.

Do underwater volcanoes contribute?

Yes.

Reactions between seawater and hot ocean crust, hydrothermal systems, and volcanic activity add and remove dissolved chemicals.

Rivers are a major part of the story, but not the entire story.

So the ocean is salty because Earth’s water cycle has been washing minerals off the planet and delivering them there for a very, very long time.

Basically the world’s slowest seasoning process.

A related part of the natural world appears in Why does the ocean look blue?, where the same kind of physical or biological reasoning answers another familiar question.

And that’s the short explanation.

Related explanations: Why does the ocean look blue?, How can fish breathe underwater?, and Why does salt melt ice?

The sky can affect the color you see on the ocean surface.

But deep clear water does not look blue mainly because it reflects the sky.

Water itself interacts with sunlight in a way that favors blue.

Sunlight contains many colors

White sunlight is a mixture of visible wavelengths.

Different wavelengths correspond to different colors.

When sunlight enters water, those colors do not all travel equally well.

Red light is absorbed more strongly

Water absorbs longer visible wavelengths such as red and orange more strongly than blue.

As light travels farther through the water, much of the red disappears first.

Blue wavelengths can travel farther.

Some of that blue light is scattered back toward your eyes.

That is why a large volume of clear water can look strongly blue.

Why does a glass of water look clear?

A drinking glass contains only a small amount of water.

Light does not travel far enough through it for the color-selective absorption to become very obvious.

In the ocean, light may travel through meters of water.

The effect becomes much stronger.

Does the sky matter at all?

Yes.

The surface can reflect blue sky, clouds, sunsets, and nearby scenery.

That changes what you see, especially at certain angles.

But sky reflection is not the main explanation for the blue color of deep clear water.

Why is some ocean water green or brown?

Other materials can change the color.

Algae and microscopic organisms can make water appear green.

Sediment can make it brown.

Shallow sandy bottoms can produce bright turquoise colors.

Depth, particles, organisms, and the seafloor all matter.

Why can deep water look dark blue?

As light travels deeper, more wavelengths are absorbed and less light returns to your eyes.

The remaining light can be strongly blue until the water becomes dark enough that very little visible light remains.

So the ocean is not simply a giant mirror reflecting the sky.

The water itself is doing much of the coloring.

A related part of the natural world appears in Why is the ocean salty?, where the same kind of physical or biological reasoning answers another familiar question.

And that’s the short explanation.

Related explanations: Why is the sky blue?, Why is the ocean salty?, and Why does a straw look bent in water?

The smell of rain is one of those experiences almost everyone recognizes even if they do not know its name.

The earthy scent that often appears when rain hits dry ground is called petrichor.

The water itself is not doing most of the smelling.

Rain is helping release chemicals that were already on or in the ground.

Dry ground collects scent-producing compounds

During dry periods, plant oils and other organic compounds can collect on soil, rocks, pavement, and vegetation.

Soil microbes also produce compounds.

One especially important one is geosmin, which has a strong earthy smell.

Humans can detect very small amounts of geosmin, which helps explain why the scent can become noticeable so quickly.

Raindrops launch material into the air

When a raindrop hits a porous surface, tiny bubbles can form where the water meets the ground.

Those bubbles can rise through the droplet and burst.

When they burst, they can release microscopic particles and odor molecules into the air.

Now those compounds can reach your nose.

Rain did not create the geosmin at that moment.

It helped aerosolize it.

Why is the smell stronger after a dry spell?

A long dry period gives oils and other compounds more time to accumulate.

The first rain after that period can release a noticeable burst of scent.

Once heavy rain continues for a while, much of the loose material has already been washed away or diluted, so the smell may become less intense.

What about the smell before a storm?

Some people notice a sharper, cleaner smell before rain arrives.

That can involve ozone and other atmospheric chemicals.

Storm-related electrical activity and air movement can influence which compounds reach ground level.

That smell is different from the earthy petrichor that often becomes strongest when rain hits dry soil.

Why do people like the smell?

There is no single universal reason everyone likes petrichor.

Familiarity, memory, weather associations, and individual sensitivity all matter.

For some people, the scent may be tied to relief from heat, childhood memories, gardening, or the feeling that dry weather is ending.

For others, it is simply a pleasant earthy odor.

Is geosmin only found in soil?

No.

Geosmin can appear in water and foods too, which is why some water sources or vegetables can occasionally have an earthy taste or smell.

In the case of rain, though, soil microbes are a major part of the familiar scent.

So the smell of rain is not really the rainwater itself.

You are smelling chemicals that the rain knocked loose and carried into the air.

Rain basically turned the ground into a temporary air freshener.

A related part of the natural world appears in Why do we see lightning before thunder?, where the same kind of physical or biological reasoning answers another familiar question.

And that’s the short explanation.

Related explanations: Why do we see lightning before thunder?, Why does thunder rumble?, and Why do puddles disappear?

Thunder sometimes sounds like one violent crack.

Other times it rolls across the sky for several seconds.

The difference comes largely from distance, the shape of the lightning channel, and the way sound travels through the atmosphere.

A lightning bolt can be enormous

A visible lightning channel is not necessarily one short spark.

It can extend for miles and branch in several directions.

Different parts of that channel sit at different distances from you.

Sound from the closest section arrives first.

Sound from farther sections arrives later.

Your ears blend those arrivals together into a rolling rumble.

Why does nearby thunder sound like a crack?

If lightning strikes very close, much of the sound reaches you within a short time.

That produces a sharp, sudden bang or crack.

The farther away the lightning is, the more spread out the sound arrival tends to become.

A thunderstorm can therefore sound very different as it approaches and then moves away.

The atmosphere changes the sound too

Sound does not travel through perfectly uniform air.

Layers of air can have different temperatures, wind speeds, and densities.

Those differences can bend, weaken, or redirect sound waves.

Terrain, clouds, buildings, and other surfaces can also affect what reaches you.

That adds more complexity to the rumble.

Are the repeated sounds separate lightning strikes?

Sometimes they are.

Thunderstorms can produce many flashes close together.

But one long rumble can also come from a single lightning event because the sound from its different sections reaches you at different times.

You cannot reliably count thunder rumbles and assume each one represents a different bolt.

Why can thunder seem to fade in and out?

As sound spreads, some frequencies weaken more quickly than others.

Atmospheric conditions can also allow certain parts of the sound to reach you better than others.

The result may be a rolling sound that grows, fades, and briefly grows again.

Does thunder keep going after the lightning is over?

The lightning itself is extremely brief, but the sound may continue arriving for several seconds.

That is because the sound is still traveling from distant parts of the channel.

The event ended.

The receipt is still arriving.

So thunder rumbles because one enormous lightning path can send sound toward you from many different distances.

Your ears receive the bolt piece by piece.

A related part of the natural world appears in Why do we see lightning before thunder?, where the same kind of physical or biological reasoning answers another familiar question.

And that’s the short explanation.

Related explanations: Why do we see lightning before thunder?, Why does rain smell so good?, and Why do puddles disappear?

Lightning and thunder are produced by the same event, but they reach you at very different speeds.

The flash travels as light.

The thunder travels as sound.

Light moves so much faster through air that, for ordinary storm distances, the flash reaches your eyes almost immediately while the sound may take several seconds to arrive.

Lightning creates thunder

A lightning channel heats the surrounding air extremely quickly.

That rapid heating causes the air to expand violently. The sudden expansion launches a pressure wave through the atmosphere.

That pressure wave is thunder.

So lightning and thunder are not separate weather events that happen one after the other. They are closely connected parts of the same event.

Light is much faster than sound

Light travels at roughly 186,000 miles per second in a vacuum and nearly that fast through air.

Sound through ordinary air moves at only about 1,100 feet per second, although the exact speed changes somewhat with temperature and other conditions.

That difference is enormous.

If lightning happens several miles away, the light covers the distance in a tiny fraction of a second. Sound needs several seconds.

Can you estimate how far away lightning is?

Yes, roughly.

Count the seconds between seeing the flash and hearing the thunder. Dividing that number by about five gives a rough distance in miles.

If the thunder arrives about ten seconds after the flash, the lightning was roughly two miles away.

This is only an estimate because sound speed varies and a lightning channel can stretch across a large area.

Why does the delay change during a storm?

As a storm moves closer or farther away, the sound has a different distance to travel.

Nearby lightning may produce thunder almost immediately after the flash.

More distant lightning creates a longer delay.

You may even see lightning so far away that the thunder becomes too weak to hear by the time it reaches you.

Does the flash always reach you first?

For normal viewing distances in the atmosphere, yes.

The delay is not because your ears are slower to process sound than your eyes process light. The dominant reason is simply the enormous difference in travel speed.

Why does nearby thunder sound sharper?

When lightning is close, many parts of the sound reach you in a shorter window of time, so thunder can sound like a sharp crack or bang.

Farther away, different portions of the lightning channel arrive at different times and the sound can become a long rumble.

So the storm is not politely showing you the lightning before playing the sound effect.

The two signals started nearly together.

Light just won by an absurd margin.

A related part of the natural world appears in Why does thunder rumble?, where the same kind of physical or biological reasoning answers another familiar question.

And that’s the short explanation.

Related explanations: Why does thunder rumble?, Why does rain smell so good?, and Why do shadows change size?