Lesson video

Hello, geographers.

My name is Mrs. Homagio, and I'm going to be teaching you today.

I hope you're going to enjoy the lesson and you're going to learn lots.

Let's get started.

Today's lesson looks at the distribution and the formation of tropical storms. We're going to be looking at the specific conditions that are needed for a tropical storm to develop, and for that reason, where it is in the world that we find them.

So by the end of the lesson, you'll be able to understand how tropical storms develop as a result of particular atmospheric conditions and know where they are found.

There are four key words for today's lesson: Coriolis effect, wind shear, latent heat, and climate change.

Coriolis effect is the deflection of winds and ocean currents caused by the Earth's rotation.

Wind shear is the change in wind speed and direction at different altitudes.

Latent heat: the energy stored in water vapour when it evaporates, that is released when the water vapour condenses back into liquid water.

And climate change: long-term changes in Earth's climate that are mainly caused by human activities which increase greenhouse gases in the atmosphere.

There are three parts to today's lesson.

Where are tropical storms found and why? How do tropical storms develop? And how is climate change affecting tropical storms? Let's get started with our first learning cycle: where are tropical storms found and why? Tropical storms are powerful rotating storms, and this image shows a very typical tropical storm taken from satellites.

They're also known as tropical cyclones.

They are low-pressure systems that form over oceans within the tropics.

And tropical storms have distinctive structural characteristics and can be 500 kilometres in diameter, with winds of over 300 kilometres per hour.

Tropical storms have different names depending on their location.

They are marked on this map by those red zones, and you can see that there are three different names that they're given.

Hurricanes are found in the Atlantic and Eastern Pacific oceans.

Typhoons are in the Western Pacific Ocean.

And cyclones develop in South East Asia and Eastern Africa.

Quick check for you now.

What name is used to describe tropical storms that form in the Atlantic and Eastern Pacific oceans? Is it A, typhoons; B, cyclones; or C, hurricanes? Pause the video and have a chat to your partner.

Did you say hurricanes? If so, well done.

Tropical storms are found in the tropics, between 5 degrees and 30 degrees north and south of the equator.

And this is shown on the map with those red zones.

There are no tropical storms forming directly along the equator.

The Coriolis effect is needed to start the spinning motion of tropical storms. This effect is absent at the equator, but it is stronger at lower latitudes.

So that explains why we don't get tropical storms at the equator, but why we do get them at the low latitudes near to the equator.

This map shows us sea surface temperatures, and the brownie orangey colours show us where the warmer sea surface temperatures are found.

Tropical storms occur over warm tropical oceans, and the water temperature needs to be 27 degrees Celsius or above.

And that's shown on the map now, highlighted on the map now.

The moist air above these warm seas rises rapidly, feeding into that low-pressure system and leading to the development of tropical storms. And for that reason, most tropical storms take place in late summer and autumn when ocean temperatures have reached 27 degrees Celsius or above.

Wind shear also affects the formation of tropical storms. Now, wind shear increases at higher altitudes, and a high wind shear can tear apart a storm structure and weaken it.

Wind shear decreases at lower altitudes, and tropical storms need low wind shear up into the higher altitudes.

That means warm, moist air can rise high into the atmosphere without being disrupted.

Another quick check for you now.

What are the three reasons tropical storms are found at low latitudes? Oceans have warmer temperatures, wind shear is lower, the Coriolis effect is stronger, or air is sinking.

Pause the video and come back when you think you know.

Did you say oceans have warmer temperatures, wind shear is lower, and the Coriolis effect is stronger? If so, well done.

If you remember, for a tropical storm to develop, it needs a low-pressure system, and air would be rising in a low-pressure system.

The first task for you now.

Using this map for question number one, describe the location of tropical storms. And for question number two, using what we've been learning, why is it tropical storms are found in these locations? Pause the video and come back when you are ready.

For your first answer, it might have looked something like this.

Tropical storms are found at low latitudes, generally between 5 to 30 degrees north and south of the equator.

They form over tropical oceans, the Atlantic, Pacific, and Indian oceans.

Number two, your answer might have looked something like this.

Tropical storms are found at low latitudes as they need tropical oceans with temperatures over 27 degrees Celsius.

They need the Coriolis effect to initiate the spinning movement of the storms, and this is strongest here.

They need low wind shear so that heat and moisture can rise without being disrupted and strengthen the storm.

Well done if your answers were similar to these.

That's really, really good.

Let's move on now to the second part of the lesson: how do tropical storms develop? The global atmospheric circulation system affects the formation of tropical storms. Warm air rising, cooling, and condensing within the Hadley cells creates an area of low pressure and unstable conditions.

The trade winds, the surface winds, flowing from high pressure to low pressure, control the storm's path.

Warm ocean water, 27 degrees Celsius and above, evaporates and rises, creating low pressure at the surface.

As the air rises, it cools and condenses, forming large thunderstorm clouds and releasing latent heat.

The released heat warms the surrounding air, causing it to rise further, further lowering the surface pressure.

This creates a stronger upward motion which draws in more warm, moist air from the ocean and fuels the formation of the storm.

Quick check for you.

True or false? Latent heat is important in the formation and strengthening of tropical storms. Pause the video and see what you think.

Did you say true? That's brilliant, but why? Can you explain why latent heat is so important? Pause the video and come back in a minute.

Did your answer look something like this? Latent heat is energy released as the water vapour cools and condenses.

This warms the surrounding air, which makes it rise faster, increasing the upward air movement that forms the storm and strengthening the winds and the rainfall in the storm.

If so, that's fantastic.

Well done.

So at the top of the storm, some of the air descends back down into the centre of the storm.

It sinks.

This creates an area of high pressure, sinking air, which provides a calm, clear region known as the eye.

The Coriolis effect causes the rising air to spin, and this means that the tropical storms rotate.

The tropical storm gets bigger as it gains heat and moisture, and surface winds control its direction of movement.

This is showing us the structure of a tropical storm.

Warm, moist air is rising.

Canopy of clouds leads to heavy rain.

So as the warm, moist air cools and condenses, it creates those clouds.

Those rain bands can be seen in that diagram.

The cooler air descends in the middle of the storm.

And the storm rotates due to the Coriolis effect.

Winds spiral outwards at the top of the storm.

The eye of the storm has clear, calm conditions.

The winds are strongest closest to the eye wall.

More warm, moist air is drawn into the centre, and sea level rises due to the low pressure and strong surface winds.

Tropical storms form in source areas.

They intensify as they move over warm oceans and then dissipate or lose energy when they make landfall or hit cooler water.

This map here shows us how this happens.

Using the key down the side, we can see the colours represent the strength of the storm and the winds.

That coloured line is changing colour from black to red to green as it moves towards cooler waters and makes landfall, showing that the wind speed is decreasing.

The wind speed is highest across the ocean.

So we can see there that the colours that represent the strongest winds, so hurricane five and four, are found out over the warm ocean waters.

Hurricanes continue to slow down over land as they lose their energy source, that warm ocean water.

And there's also more friction than over the ocean.

So you can see there that the wind speed continues to decrease as it makes landfall and then moves inland.

And eventually the storm is stopping due to the loss of energy source and increased friction.

Bit of a check for you now.

Why does the strength of a tropical storm dissipate when they make landfall? A, it has lost its main energy source, warm ocean water.

There is greater friction, reducing the wind speed.

The Coriolis effect has no impact, and there is no rotation.

Pause the video and have a think.

Come back when you're ready.

Did you say it's lost its main energy source, the warm ocean water, and there is greater friction, reducing the wind speed? Well done if you did, that's fantastic.

So over to you.

There's a practise task for you.

Can you annotate these diagrams to explain how a tropical storm develops? To annotate means to add text to a diagram that gives explanation, rather than just a simple label.

So try to add detail to your annotations around these diagrams. Pause the video and have a go.

Your annotations for diagram 1 might include warm, moist air rising and evaporating.

As the air rises, it cools and condenses, forming large thunderstorm clouds, which releases latent heat, and this heat encourages evaporation, fueling the formation and size of the storm.

Well done if that was what your labels looked similar to.

And for the second diagram, the labels might include, the Coriolis effect causes the tropical storm to rotate.

Surface winds control the direction of movement, and some air is sinking in the middle, which creates a calm zone of high pressure known as the eye of the storm.

Well done if your labels are similar to those on the slide.

Let's move on to the third part of today's lesson.

How is climate change affecting tropical storms? Climate change, which includes a rise in global temperatures, changing weather patterns, and sea level rise, is unlikely to affect the frequency of tropical storms. Scientists find it hard to prove that.

And we can see looking at this graph, which shows decades since 1851 to 2021, the number of hurricanes that were experienced in the USA.

Now that graph shows that there are fluctuations in the number of hurricanes reaching the USA.

There's no obvious upward trend, so we can't attribute the frequency of tropical storms to climate change.

Climate change may affect the distribution of tropical storms as global sea temperatures rise.

This map shows current sea surface temperatures are the highest between the tropics.

You use the key, you can see the colours match the higher sea surface temperatures between the tropics.

If sea surface temperatures increase further from the equator, or they stay warmer for longer, it could create conditions that allow more tropical storms to develop.

They may happen further north and south from the tropics, or they may happen at different times of the year.

Quick check for you now.

How might a rise in global sea temperatures affect tropical storms? A, they may happen in more of the world's oceans.

B, it will lead to fewer tropical storms. C, they could happen for a longer period of time.

Pause the video and chat to your partner.

Come back when you're ready.

Did you say they might happen in more of the world's oceans as sea surface temperatures rise, or they could happen for a longer period of time? Well done if you did.

Now, climate change may affect the intensity of tropical storms. Warmer atmosphere can hold more moisture, which leads to more intense rainfall.

And warmer ocean waters means storms can pick up more energy, leading to more intense storms. And we can see this trend on the graph below.

We've got the years between 1950 and 2020 and we've got two line graphs, each with their own scale.

On the left hand side shows storm intensity measured in PDI, which is the power dissipation index, and it takes into account the wind speed and duration of storms. So it's a way of measuring the storm intensity.

On the other side, we have sea surface temperature, and we can see that there is a general trend that as sea surface temperature increases, as does storm intensity.

Quick check for you now.

Climate change could affect tropical storms by increasing their frequency, increasing their intensity, changing their distribution.

Pause the video and see what you think.

Did you say increasing their intensity and changing their distribution? Well done.

Sofia says, "As sea surface temperatures get and stay warmer, tropical storms could happen at higher latitudes or during more of the year." Final task for you now.

Number one, describe the trend indicated by the graph.

So look carefully at the graph below.

What is the trend that we can see? Number two, suggest an explanation for the trend you have identified.

So why is it that that trend happens? Look carefully at the command words in the question to help structure your answer and use the labels on the graph to help you.

Pause the video and come back when you have an answer.

Your answer might have looked something like this for question number one.

The graph suggests that as sea surface temperatures increase, the intensity of tropical storms increases, though there are also exceptions to this trend.

Number two, your answer might look like this.

Tropical storms are fueled by warm ocean water.

The warmer the ocean surface water gets, the more intense the tropical storms should become.

Well done if your answers are very similar to those.

You've worked really hard today with a lot of very tricky geography.

Let's have a look at a little summary of the lesson.

Tropical storms, also known as tropical cyclones, are found between 5 degrees and 30 degrees north and south of the equator.

Tropical storms have specific conditions required for them to develop.

Sea surface temperatures of at least 27 degrees Celsius drives tropical storms, causing warm, moist air to evaporate.

The Coriolis effect causes tropical storms to rotate.

Tropical storms lose strength as they hit land or cooler waters.

Climate change is affecting the intensity and distribution of tropical storms, but has less impact on their frequency.

Well done today, geographers.

You've done really well.

I hope you've enjoyed it and understood it, and I look forward to seeing you soon.