Lesson video

Hi, geographers.

My name's Ms. Hoggett, and I'm really excited you're here to learn some geography with me today.

I'm sure you're going to be absolutely fantastic if you try your best.

Don't worry too much if you haven't studied this topic before.

I'm here to help you every step of the way.

Our title for today's lesson is, "Types of Rainfall", and this sits within the unit, "Weather and climate, How Do They Vary?" Our learning outcome for today's lesson is that I can describe how rain forms and understand the different reasons for its formation.

To help us through today's lesson, we have a number of keywords.

Evaporation, precipitation, sublimation and relief.

Evaporation, the process of something changing from a liquid state to a gaseous state.

Precipitation, water that falls from the sky as rain, snow, hail or sleet.

Sublimation, conversion of a substance from a solid to a gaseous form.

And relief, the physical shape of the land in an area including its height and steepness.

So our keywords for today include evaporation, precipitation, sublimation and relief.

Today's lesson is split into two learning cycles.

How is rain formed and is it all the same? And what are the patterns of rainfall in the UK? We're going to start with our first learning cycle now, how is rain formed and is it all the same? So to start thinking about rain, we need to think about water and water can exist in different forms. As a solid, it's ice.

As a liquid, it's water, and as a gas, it's water vapour.

And it's important to recognise that water can move between these different states depending on the temperature and pressure.

When heat is added to a liquid, like water, it turns into a gas.

This process is called evaporation.

The water molecules gain energy and spread out, becoming invisible water vapour in the air.

If we cool that gas down though, it turns back to a liquid.

This is called condensation, which is how clouds form in the sky or water droplets appear on a cold glass.

When a solid like ice is heated, it melts and turns into a liquid.

The particles gain energy and start moving more freely.

If we remove heat from the liquid, it freezes and becomes solid again, like when water turns into ice cubes in a freezer.

Sometimes a solid turns straight into a gas without becoming a liquid first.

This is called sublimation.

Dry ice, so frozen carbon dioxide, does this when it turns into fog-like vapour.

The opposite process happens when a gas turns directly into a solid, it's called deposition.

This happens when frost forms on a window on a cold day.

Now, all air contains water vapour.

Warm air can hold more water vapour than cold air.

Hot air rises, because it's less dense.

We can remember this by thinking about how hot air balloons work.

The role that rising warm air plays in the formation of rain can be understood by looking at the hydrological cycle.

This is the continuous movement of water on, above and below the Earth.

You might be more familiar with the term water cycle.

This is what you might have come across before, but it is the same thing.

So we know that energy from the sun heats up the land and that causes the air above the sea to rise and evaporate, forming clouds.

Eventually the water droplets become too heavy and the water falls as precipitation.

The water then can take a few different journeys.

It can either run off the surface or it can be absorbed into the ground or it might collect in lakes and rivers, but eventually, all of that water is going to end up draining back into the sea.

So that's the hydrological cycle.

Often, precipitation starts as solid ice crystals if the temperature of the air in the cloud is cold enough, but it may melt to form liquid rain before it hits the ground.

Only in winter is it cool enough within the lower levels of the atmosphere for snow to reach us.

And only when it's below freezing on the ground will that snow settle.

Let's check your understanding so far.

Which of the following is not an example of precipitation, A, rain, B, snow, C, heat, or D, sleet? Pause the video and have a go at answering this question.

Well done if you said heat.

Rain, snow and sleet are all examples of precipitation.

Good work, geographers.

So yes, we have the hydrological cycle, but there are three reasons why air rises.

The first one is convection, which is caused by the heat energy from the sun.

The second is relief and the third is weather fronts.

Now, ultimately that means we have three types of rainfall, convectional, relief and frontal, and we're going to learn about all three of those in today's lesson.

So let's start with convectional rain.

This occurs due to the sun heating the Earth's surface.

The Earth's surface in turn warms the air above it and the air starts to rise.

You can see that on the diagram, the warm air represented by the red arrows is starting to rise.

As the warm air rises, it cools and the water vapour condenses into clouds and you can see that because the arrows have turned blue.

When the water droplets become too heavy, precipitation occurs and remember, precipitation could be rain, snow, sleet or hail.

In the UK, we get convectional rain inland in summer where the ground gets hottest, usually inland away from the cooling effect of the sea.

Likewise, on or close to the equator, convectional rainfall happens every day.

Let's check your understanding.

Put the statements in order showing the correct formation of convectional rain.

A, as warm air rises, it condenses to form clouds.

B, the sun heats up Earth causing warm air to rise.

And C, when the water vapour becomes too heavy, it falls as precipitation.

Pause the video and have a go at putting those statements in the correct order.

Well done for giving that a really good go.

Did you put down the correct answer? So B comes first, the sun heats up the Earth, causing warm air to rise.

As warm air rises, it condenses to form clouds and when the water vapour becomes too heavy, it falls as precipitation.

So our next type of rainfall to consider is relief rainfall.

Now, relief is the physical shape of the land in an area, including its height and steepness and relief rainfall happens where moist air is forced to rise over the mountains or high terrain, and that's why we have a big hill in our diagram.

As you can see, the red arrow is representing that warm air starting to rise over that landscape.

Now, we know already that temperature decreases with altitude and therefore if that air is forced to rise, it will cool down.

As it does, water vapour will condense into water droplets.

Rain will then occur when the water droplets in the clouds become too heavy and on the leeward side of the mountain that's sheltered from the wind, air descends and warms. Here, less rain falls as warmer air can hold more moisture as vapour, and this is what we call the rain shadow effect.

So where that air started to rise happens on the windward side and then we can see that the precipitation is falling on that side too and then on the leeward side, after it goes over that high relief, we can start to see it's warming and it's much drier.

Now, the rain shadow effect explains why the Atacama desert is found west of the Andes Mountains and has become one of the driest places on Earth.

So on this satellite image, we can see where we have the Atacama desert.

Now firstly, warm moist air comes from the Atlantic Ocean.

We call these the southeast trade winds as they carry a lot of moisture as they move across South America.

When these winds reach the Andes Mountains, they start to rise.

As the air moves up, it cools down.

Cooler air can't hold as much moisture, so it condenses into cloud and we get rain on the eastern side of the mountains.

This means that the windward side of the Andes is lush, green and full of life.

But something different's happening on the other side.

Now that most of the moisture has already fallen as rain, the air moves down the leeward side of the mountain.

As it descends, it warms up and dries out.

This creates high pressure, meaning the air prevents clouds and rain from forming.

This is why the Atacama desert is one of the driest places on Earth, but there's also another factor that impacts this as well.

Cold ocean currents from the Pacific Ocean chill the air above them making it difficult for the air to pick up moisture.

So between this and the rain shadow effect, the Atacama gets almost no rain at all and it's become one of the driest places on Earth.

Let's check your understanding.

True or false.

Temperature increases with altitude.

Pause the video and have a go at answering this question.

Well done for having a go, the answer is false, why? Well, we know that temperature decreases with altitude.

This explains why we get snow-capped mountains.

Lastly, we have frontal rainfall and this occurs where a warm air mass meets a cold air mass as they do not mix easily.

Warm air is forced to rise over the cold air.

Now again, we can look at the diagram and see the arrows and we've got the red arrows moving up within that warm air mass over the cold air mass.

The warm air mass begins to cool as it rises and water vapour condenses forming clouds.

When the water droplets become too heavy, they then fall as rain.

Now, the UK experiences lots of frontal rain because of its location on Earth.

Lots of air masses meet above the UK and these air masses don't mix.

Let's check your understanding then of those three types of rainfall.

So which of the diagrams accurately represents frontal rainfall, A, B, or C? Pause the video and have a go at answering this question.

Well done if you said A.

We can see A is showing frontal rainfall.

B is showing convectional rainfall and C is showing relief rainfall.

Good work, geographers.

So let's have a go at practising what we've learned in learning cycle one.

I'd like you to fill in the table to explain which you think is the most common type of rain in each location.

You may want to use an atlas to help, but the locations are also labelled on the world map we can see on the screen.

Pause the video and have a go at completing that table.

Well done for giving that a really good go, everybody.

I'm sure you've done fantastic.

So your answer may include some of the following.

Manaus, convectional.

It's near the equator, so it's very hot.

Air rises due to the heating of the land.

Rising air cools, water vapour condenses to form clouds and rain.

Milton Keynes, frontal rainfall.

Warm air from the south and cold air from the north meets over the UK.

Warm air rises over cold air.

Rising air cools, so water vapour condenses to form clouds and rain.

And lastly, Mont Blanc, relief.

Mont Blanc is part of the European Alps where air is forced to rise.

Air is forced to rise over the mountains, air cools, so water vapour condenses to form clouds and rain.

What a fantastic end to learning cycle one.

We're now going to move on to learning cycle two.

What are the patterns of rainfall in the UK? So the UK experiences different amounts of rainfall in different places.

Pause the video and have a think, what do you notice about the pattern of rainfall across the UK? What did you come up with? Did you notice that the wettest places in the UK are in the west, whereas the UK is much drier in the east? We can see that, because this type of map is a choropleth map that uses colour to represent different values.

The darker the colour, the higher the value.

So it tells us that the darkest colours show over 500 millimetres of rainfall in January on average.

Let's check your understanding of that.

So across the UK, the total amount of rainfall received is the same, true or false? Pause the video and have a go at answering that question.

Well done if you said false.

Places in the west of the UK are wetter, so we know there is an uneven amount of rainfall across the UK.

Now, we can explain the pattern of the UK's rainfall by comparing it to a map of the UK's relief.

What patterns or similarities do you notice across these two maps? Pause the video and have a think.

Now, the UK experiences the highest amount of rainfall where the land is highest and we can see that when we compare these two maps.

Warm, moist air is carried across the Atlantic Ocean and forced to rise over high land when it reaches the UK, and so it rains.

Here, we can see we have the Northwest Highlands labelled.

We also have Snowdonia, the Lake district and Dartmoor and these are some of our areas of high land across the UK.

So what type of rainfall is found where mountains are forced to rise? Pause the video and tell the person next to you.

Well done, Jun, for saying relief, absolutely spot on.

Where air is forced to rise over high land and mountains, we know we're going to find relief rainfall.

So let's check your understanding so far.

I'd like you to complete Sophia's sentence using the correct type of rainfall.

In the west of the UK, there is higher land, such as the Scottish Highlands, Pennines, Eryri, which is in Snowdonia and Dartmoor, therefore, the west of the UK is affected by what? Pause the video and have a go at answering that question.

Well done if you said relief rainfall, that's absolutely spot on.

Good work, team.

We can also use temperature to suggest where convectional rainfall might occur.

Looking at these two maps, what region of the UK experiences the hottest temperatures? Pause the video and tell the person next to you.

Well done, Alex, it is the southeast.

We can see that the hottest temperatures are in the southeast just around London.

In June, on average, convectional rainfall is most likely to occur in the southeast of England.

Let's take a moment to check our understanding.

Which of these regions is most likely to experience convectional rainfall on a hot day in June? Is it the north, south, southeast or southwest? Pause the video and have a go at answering that question.

Well done if you said the southeast.

Good work, geographers.

Next one, true or false? Frontal rainfall only takes place in the southeast of England.

Pause the video and have a go answering that question.

Well done if you said false.

Frontal rainfall occurs where two different air masses meet.

So it occurs across the whole of the UK.

So we're going to do our final practise task now.

Suggest where different types of rainfall occur within the UK and why.

I want you to include west, southeast and Scottish Highlands in your paragraph.

Pause the video and have a go at completing that task.

Good work, geographers.

Well done for giving that a really good go.

So your answer may have included, "In the west of the country, especially areas of high land, such as the Scottish Highlands, relief rainfall occurs.

Moist air from the Atlantic is forced to rise by the mountains.

As a result, water vapour cools, condenses and it rains.

This part of the UK is much wetter than the neighbouring areas that lie on the leeward side of mountains.

This is the rain shadow effect.

In the summer, convectional rainfall occurs in places like London in the southeast of England where the sun heats the ground, which in turn warms the air above it.

Warm air rises and as a result, water vapour cools, condenses and forms rain clouds.

Frontal rainfall takes place anywhere in the UK, because it occurs where two different air masses meet." So to summarise today's lesson.

Relief rainfall occurs where warm moist air from the ocean is forced to rise over mountains.

Convectional rainfall occurs where the sun heats the Earth's surface, which in turn warms the air above it, causing the air to rise, cool and water vapour to condense.

Frontal rainfall occurs where there's a warm air mass meeting a cool air mass, forcing the warm air mass to rise.

And the wettest places in the UK are found in the west where high land is found.

Geographers, you've been absolutely fantastic today and you've done a really good job.

Keep up with the good work and I'll see you soon for our next lesson.