There were sustained winds of 240 kilometers per hour and a storm surge, or flooding that

raised the sea level, up to 10.4 meters -- that's 34 feet! 

The damaging wind and flooding were devastating, and as of 2021 the Bhola cyclone is the deadliest

tropical cyclone in history, killing an estimated 300,000 to 500,000 people.

And the abysmal response of the then-West Pakistan government was a factor in the war

of liberation that led to the creation of Bangladesh.

Just think about that, a weather system was so powerful, it helped create an entirely new country.

Massive weather systems can wreak havoc in an instant like the Bhola cyclone.

But they can also move in slowly and create gradual change in our weather for days, like

warm springtime rains or a weird unseasonal batch of hail.

Understanding when different weather systems occur, how they form, and their impacts is

an important part of physical geography.

Especially because all weather, from massive thunderstorms to small bouts of fog, drives

energy exchange within the atmosphere.

I'm Alizé Carrère and this is Crash Course Geography.

[INTRO]

Weather encapsulates all the atmospheric conditions in a specific place at a specific time.

And if you live in the mid-latitudes, or everything roughly between 35 and 55 degrees north and

south latitude, you can expect the weather to be predictably unpredictable. 

A bright sunny day in South Dakota or the South Island of New Zealand or Scotland can

suddenly change to overcast and grey and then just as abruptly clear up.  

Day to day we might not notice what's going on in the atmosphere, unless there's a natural

disaster headed our way or we're stuck doing a lot of small talk.

"Nice weather we're having today, Brandon.

It's so… seasonal!

Don't you think?" 

But there are so many complex global circulation patterns in the atmosphere and the oceans.

As we've learned, because the Earth is curved and tilted, the amount of incoming solar radiation,

or insolation, isn't the same everywhere.

Each year, tropical regions receive two and a half times more energy than the poles, which

has to be evened out with the help of circulation in the atmosphere.

The uneven amounts of insolation also cause temperature differences that drive some the

biggest rebalancing efforts: mid-latitude cyclones, which are also called wave cyclones

or extratropical cyclones.

These enormous weather systems span a 1000 kilometers or more.

Even though they share the word "cyclone," a mid-latitude cyclone is a relatively huge

circular weather system -- unlike the Bhola cyclone, which is a relatively smaller, extremely

windy tropical storm.

We'll talk more about tropical cyclones later.

Mid-latitude cyclones can last a week or more, bringing lots of changes in the day-to-day

weather or severe storms as they travel from west to east with the westerly winds. 

These weather systems can form in the mid-latitudes of both hemispheres.

But to zoom into where I live as an example, in the Northern Hemisphere, we see mid-latitude

cyclones form along the polar front, which is a band of low pressure in the latitudes

just below the poles that sits between two large high pressure areas: the subtropical

high pressure to the south and the polar high to the north. 

A battle rages in the skies between the warm, moist air from the tropics and the cold air

from the poles.

In fact, the term “polar front” was first proposed by Norwegian meteorologists Jacob

Bjerknes and Halvor Solberg while studying mid-latitude storms in Norway; the first World

War was raging and it seemed that the boundary between warm and cold air was like a battle

front between the allied and central forces.

Generally, we can call these two opponents air masses, which are vast bodies of air with

similar temperature and humidity that form over a region. 

Like when an air mass forms over tropical oceans it will be relatively warmer and more

humid than one that forms over the frigid interior of northern Canada, which will be

cold and very dry.

And as they move, they bring their temperatures and moisture with them. 

The mid-latitudes get a lot of clashes between air masses.

And that's where a lot of the storms and precipitation comes from because when different

air masses come together they're like water and oil or Godzilla and Mothra -- they don't love mixing.

  Instead they meet along sloping boundaries called front.

For example, if the cold air mass is feeling feisty and moves in on a warm air mass, we get a cold front.

The nose of the advancing cold front is like a snow plow, hugging the surface because it's

dense and heavy, pushing the warm air out of the way, and flinging it upward.

And with a cold front comes shifting winds, dropping temperatures, and lowering pressure. 

If the displaced warm air is unstable and wants to rise and has lots of moisture, we'll

get heavy rain from thunderstorms and an advancing wall of cumulus and cumulonimbus clouds.

But when a cold air mass backs off, the warm air mass sees its chance and creeps in, forming

a warm front.

The warm air can't displace the denser, colder air near the ground, so the warm front

slides over it like a thick blanket. 

Overall, a warm front is much less sudden and violent than a cold front and tends to

linger, leaving warm, wet air behind.

The first sign of one is high cirrus clouds, followed by lower and thicker altostratus

clouds, and then still lower and thicker stratus clouds that bring drizzly rain. 

So different air masses bring different weather in their wake and influence the weather conditions

of locations as they pass.

Warm and cold fronts are relatively small skirmishes.

But wars -- or mid-latitude cyclones -- can start when cold air and warm air meet on the polar front.

While these air masses duke it out closer to the surface of the Earth, there's another

crusade happening 5 kilometers above them: the upper air westerlies.

These winds blow very fast because there's less friction higher up.   

And within the upper air westerlies, about 10 kilometers above the Earth, there's a

wind that blows really really fast.

It's the polar front jet stream and it travels up to 450 kilometers per hour, wrapping all

around the planet.

Even though it's way up in the atmosphere, a small change in the path of the jet stream

can cause a bend in the polar front that leads to a low pressure area where a warm front

moving poleward and a cold front moving towards the equator clash. 

As the air converging and rising to form a low pressure area turns into a full-blown

mid latitude cyclone, the colder air mass is denser and moves faster, overtaking the

spiralling, cyclonic warm front and wedging beneath it.

And it won't be over until the cyclone is completely cut off from the warm air mass

that was its source of energy and moisture. 

In this battle, both the polar front and the polar front jet stream can also move seasonally.

They can steer the cyclonic systems and their air masses across the continent as they follow

the Sun north into the Arctic in summer, and swing down further south in winter. 

And no two storms are alike because no two air masses are alike.

[So with so much going on, you can't blame the weather forecasters too much for not predicting

the unpredictable!]

Now, massive weather systems aren't unique to the mid-latitudes, even if warm and cold

fronts do happen there a lot.

In tropical and subtropical oceans and seas, we can also get spiraling, low pressure storms,

which are some of the largest storms on Earth.

A storm that starts in the tropical oceans, between the Tropic of Cancer and the Tropic

of Capricorn, can grow to have incredible winds over 118 kilometers per hour.

They go by many names: hurricanes in the Atlantic, typhoons in the Pacific, and cyclones in the Indian Ocean.

(So we're talking about tropical cyclone storms now.

But we're filming this in Florida, near the Atlantic, so I'll call them hurricanes from here on.)

Unlike the mid-latitude battles between warm and cold air masses, tropical hurricanes form

from a single warm air mass.

Without a cold air opponent, there's no front. 

The warm air over the oceans means there's lots of water vapor.

So as this warm air rises and condenses, the extra energy released fuels its transformation

from a weak, low pressure area into a violent, swirling storm -- extending up 12 to 14 kilometers

and taking up the full height of the troposphere. 

A chimney effect pulls more and more moisture-laden air into the system.

Air is sucked in at the base by low pressure in the eye of the hurricane, and sent spiraling

because of the Coriolis effect.

The air then rises rapidly to the top, and is full of water vapor, which condenses and

releases latent heat energy, producing thunderstorms and enormous amounts of rain.

A great deal of time, effort and money has been spent on studying hurricanes, so we know some thing.

Like in the Atlantic, the official hurricane season is from the first of June through the

30th of November.

But hurricanes pop up most frequently in late summer and early fall, when ocean surfaces

are warmed to 26 degrees Celsius or more, and the ocean air has maximum humidity.

And we've found a correlation between rising sea surface temperatures and longer tropical

storm lifetimes and greater intensity in the Atlantic basin.

Looking to the future, models suggest that the intensity of extremely severe storms will

only increase in the coming decades. 

But there's still a lot to learn.

Like we can't predict a hurricane's path with great certainty more than three days

in advance, even though it's tracked by radar, plane, and weather satellites. 

And where a storm makes landfall becomes the most pressing question as a storm grows and

meteorologists track its movement.

Hurricanes' source of moisture and energy is the warm ocean air and water, so they grow

when they're over water, but begin to dissipate once they make landfall.

But by then, they could've already caused tons of damage to coastal communities. 

The boundary between water and land in tropical and subtropical regions is naturally marked

by mangroves and wetlands which absorb the brunt of the storm surge, buffering inland

areas from flooding.

For example, the low-lying coastal areas of Bangladesh and India form the world's largest

delta and is also the location of the Sundarbans, the world's largest mangroves.

However, even with the forbidding ecological buffer, there are also large pockets of people

and rural settlements, which ties back to colonial land policies and practices.

There's increasing pressure to develop these coastal areas for fish farming, tourism, manufacturing,

and oil and gas exploration -- all activities that are destroying the mangroves along with

the ecological functions they provide. 

If we combine that destruction of a natural buffer with the scientific fact that the best

hurricane warnings are given around 36 hours in advance, we're left hoping our transportation

infrastructures can handle a mass evacuation that quickly to save peoples' lives.

And in Bangladesh, the most densely populated country in the world, the stakes for that task couldn't be higher.

In countries like the United States, areas susceptible to hurricanes continue to experience

population growth, economic development, and urbanization.

Even as hurricane warning systems have improved and more lives have been saved, the loss to

property and the number of people displaced has steadily risen. 

Our relationsh ip to weather is complicated, because so much of our experience depends

on where we build our communities and why. 

The light drizzle near a warm front might call for rain boots or an umbrella, the thunderstorm

near a cold front may have us running indoors and comforting our dogs from the thunderclaps,

but the damage of tropical storms can lead to incredibly severe consequences. 

We can't go anywhere on Earth to completely avoid weather, so our choices about what chunk

of the atmosphere to live under matter.

Next time, we'll zoom out and look at weather patterns across whole regions, when we talk about climate.

Many maps and borders represent modern geopolitical divisions that have often been decided without

the consultation, permission, or recognition of the land's original inhabitants.

Many geographical place names also don't reflect the Indigenous or Aboriginal peoples languages.

So we at Crash Course want to acknowledge these peoples' traditional and ongoing relationship

with that land and all the physical and human geographical elements of it.

We encourage you to learn about the history of the place you call home through resources

like native-land.ca and by engaging with your local Indigenous and Aboriginal nations through

the websites and resources they provide.

Thanks for watching this episode of Crash Course Geography which is filmed at the Team 

Sandoval Pierce Studio and was made with the help of all these nice people.

If you want to help keep Crash Course free for everyone, forever, you can join our community on Patreon.