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 if you try your absolute best, you're going to be fantastic.

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

I am here to help you every step of the way.

So our title for today's lesson is Depressions and this sits within the unit Weather and climate, how do they vary? Our learning outcome for today is that, "I can explain the weather associated with a depression." In order to help us through today's lesson, we have a number of keywords, pressure, a force pushing or pressing on a given area, air mass, a massive air with the same levels of temperature, humidity and pressure.

Depression, an area of low pressure characterised by rain, wind and cloud.

And front, a weather front is the boundary between a warm air mass and a cold air mass.

So our keywords for today, pressure, air mass, depression and front are going to really help us through today's lesson.

Today's lesson is split into two learning cycles.

What is a depression and what conditions do depressions bring? We're going to start with our first learning cycle now, what is a depression? So to start thinking about depressions, we need to firstly understand pressure.

And air pressure is the force exerted by the weight or the mass of the column of air above us.

At ground level, we experience areas of high and low pressure.

Notice how in the diagrams, high pressure is where the air is pushing towards the earth.

And low pressure is where air is moving away from the earth.

I remember this because if I was to push hard onto a table, I know that I'm applying high pressure to that table.

Whereas if I take my hand away, I know that I'm releasing that pressure.

So there's low pressure on that surface.

The same is happening in the sky above us.

Now, pressure differences cause winds to blow.

Air moves from areas of higher pressure to areas of lower pressure.

The greater the pressure difference, the stronger the winds.

As we can see in this diagram, we start with the sun and the sun heats the surface causing warm air to rise, which leaves that low pressure behind.

Air cools as it rises to form clouds because it condenses.

Where we have that high pressure, we know that the cooler air is sinking.

And as air flows into the partial vacuum created, that's when we start to experience wind.

Now, low pressure is caused by air rising away from the Earth's surface as we've already established.

And depressions are large scale weather features where that air is rising.

So we know these are really large scale.

Aisha says, "Depressions are depressing, because it rains." But Jacob says, "What? The rain is great for the garden!" So some people have very different opinions on whether or not depressions are a good thing.

Now, depressions form where cold and warm air masses collide.

Now air masses are large masses of air with the same levels of temperature, humidity, and pressure.

They don't mix easily and that's why the UK experiences really unsettled weather.

Because we're located at a junction of cold and warm air masses.

The diagram shows us where those cold and warm air masses are, and that's represented by colour.

So the blue air masses are the cold air masses and the red-orange air masses are our warm air masses.

If they travel over water, then they're called maritime air masses.

And if they travel over land, they're called continental air masses.

So that explains their different names.

When a wedge of air pushes into a cold air mass, the warm air rises.

This causes air pressure to drop.

Cold air then rushes in, creating strong winds.

The warm air then cools and condenses into clouds.

This causes rain to fall along with strong winds.

This diagram here is showing a depression.

In the Northern Hemisphere, depressions move from west to east.

They track across the UK from the southwest, and they can take one to three days to cross the country.

Another way we can see depressions is looking at satellite images.

And this one here is from the Met Office, showing a depression starting to move across the UK.

And you can see the UK there outlined on that satellite image.

So let's check your understanding.

Which of the two statements are correct? A, low pressure systems are called depressions.

B, low pressure occurs where air is rising.

C, low pressure occurs where air is sinking.

Or D, low pressure systems are called anticyclones.

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

Well done for giving that a really good go.

Let's check your answers.

So the two statements that are correct are A, low pressure systems are called depressions, and B, low pressure occurs where air is rising.

Good work, geographers.

We have another check for understanding now.

So in what direction do depressions move in the Northern Hemisphere? Is it A, east to west, B, west to east, C, south to north, or D, north to west? Pause the video and have a go at answering this question.

Well done for having a go.

The correct answer was B, west to east.

Good job for getting that correct.

So synoptic or surface pressure charts show how air pressure varies across geographical space and we can use these to work out where depressions are found.

Surface pressure charts use isobars, which join areas of equal pressure.

And Izzy says here that they look a bit like contours on OS maps.

So if you've seen an OS map before with all the orange lines that match up the areas of the same heights above sea level, we know that it's a similar concept.

So the numbers on this chart show air pressure in millibars.

Every point along this isobar has a pressure measurement of 988 millibars, whereas every point along this isobar has a pressure measurement of 1,000 millibars.

So we can see how that varies across this synoptic chart.

The pattern of isobars shows us how steep or shallow the pressure gradient is and what the wind speed is like.

When the isobars are further apart, it shows a shallow gradient as we can see here.

And this means that the pressure is changing more gradually and wind speeds are more gentle.

Whereas when the isobars are closer together, it shows a steep pressure gradient and this is associated with higher wind speed, because we know that pressure is changing quite quickly.

Now, surface pressure charts also indicate areas of high and low pressure and provide the central pressure value.

Low pressure is represented with an L.

If we take a look at that in more detail, we can see that the central pressure value here is 977 millibars.

Where a depression occurs.

Isobars decrease in value towards the pressure low at the centre.

Standard pressure at sea level is 1,013 millibars.

Now if we take a look at this diagram here, we can see that around that low, we had 1,004 millibars, 992 millibars and then 988 millibars.

So it's decreasing towards the centre.

Depressions can be identified by the lower pressure value and tightly packed isobars.

And Jacob says here, "I can see depressions over land and sea on this map." So it's important we consider how they can occur in different places.

Let's check your understanding.

So which two things can help identify depressions on a surface pressure chart? A, isobars close together.

B, isobars far apart.

C, higher numbers.

Or D, lower numbers.

Pause the video and have a go at this check for understanding.

Well done for having a go at that, geographers.

Let's check your answers.

So the correct answers are A, isobars are close together and D, lower numbers.

Good work, team.

Second check for understanding here then.

So which of the following shows a depression? Is it A, B, or C? Pause the video now and have a go at completing this.

Well done if you said B.

Good work, geographers, you're doing a fantastic job.

So we are now gonna practise what you've learned so far.

So I would like you to complete the following sentences.

Air pressure refers to the force exerted by the blank of air above us.

Blank pressure is caused by sinking air.

Blank pressure is caused by rising air.

And low pressure systems are called blank.

At the bottom of the screen, you'll see that there are some words to use, depressions, high, low and weight.

That's part one.

Part two of this practise task is I'd like you to draw a circle around the areas of low pressure on the surface pressure charts, and then I'd like you to explain how you know these are areas of low pressure.

Pause the video and have a go at completing this practise task.

Fantastic work, geographers.

Let's see how you've got on with that task.

So for task A.

1, we need to have the air pressure refers to the force exerted by the weight of the air above us.

High pressure is caused by sinking air.

Low pressure is caused by rising air.

And low pressure systems are called depressions.

For part two, we have the areas of low pressure on the surface pressure charts outlined on the map.

And then for part three, we have explained how we know these areas of low pressure by saying these areas have isobars that are tightly packed, linked to lower values and are labelled with an L, indicating low pressure system.

The isobars reduce in value as you move towards the centre of the low.

Well done, geographers.

We're now ready to move onto learning cycle number two, what conditions do depressions bring? Now depressions often form over the North Atlantic Ocean.

At this location, warm air masses from the south meet cold air masses from the north.

You can see that identified on our image.

Now, a front is a boundary between two air masses, for example, between a warm moist air mass from the tropics and a cold, drier air mass from the polar regions.

Depressions have two main fronts, a warm front and a cold front.

And we can see that on this diagram with our cold front located on the left with our blue line, which has triangles on it, and our warm front located more towards the right with our red line that has semicircles on it.

At the warm front, warm air rises over cold air.

This causes drizzly frontal rain, followed by rising temperatures as the warm air mass moves over the land.

Andeep says, "We can remember this is a warm front, because the red circles look like half a sun." Now notice in the diagram, we can see that the warm air is forced to rise over the cold air.

As it rises, condensation forms clouds, and then eventually, warm fronts bring steady rainfall.

Whenever we see a surface pressure chart, we know that the warm front is going to be represented by a red line with red semicircles on it.

Likewise, at the cold front, cold air pushes the warm air upwards.

This causes heavy frontal rain followed by falling temperatures as the colder air mass moves in.

Again, in the diagram, we can see that warm air is undercut by the cold air.

The cold air is sinking.

And then we have our cold front.

And the cold front brings heavy rain and showers.

A cold front is always going to be represented by a blue line with blue triangles.

And Alex says, "We can remember this is a cold front, because the blue triangles look like icicles." Really helpful.

Thank you, Alex.

Now where the cold front catches up with the warm front, there are sudden downpours of heavy rain.

We call this an occluded front, and it's represented by a purple line with purple semicircles and purple triangles.

Now a depression can take one to three days to pass, and we can expect the weather to follow this pattern.

Sky clearing, high cloud, drizzle, steady rain, clear and dry, heavy rain and thunder, showers and sky clearing.

Notice how I read that from right to left.

Now as the warm front arrives first, this diagram should always be read from right to left, which feels different to normal, but it's important to remember.

Now fronts can be identified on surface pressure maps.

Here, they can be seen curling or swirling anti-clockwise around the centre of depressions.

Let's check your understanding so far.

So what sort of weather is associated with the arrival of the warm front? A, dry conditions with warmer temperatures, B, rain and colder temperatures, or C, rain and warmer temperatures? Pause the video and have a go at answering this question.

Well done for having a go.

The answer is C, rain and warmer temperatures.

Good work, geographers.

I've got another check for understanding for you here.

So which diagram shows only a warm front? Pause the video and have a go at answering this question.

Well done if you said B.

We can see the red line there with the red semicircles that look like half suns on them.

So well done for getting that spot-on.

Another one for you, what sorts of weather is associated with the arrival of a cold front? A, dry conditions and colder temperatures.

B, heavy rain and colder temperatures, or C, heavy rain and warmer temperatures? Pause the video and have a go at answering this question.

Well done if you said B, heavy rain and colder temperatures.

And because you're doing so brilliantly, I do have one final check for understanding for you here.

So which diagram shows only a cold front? Pause the video and have a go at answering this question.

Done if you said C.

We've got the blue line there with the blue triangles that look like icicles and therefore that shows a cold front.

Fantastic job, geographers.

So now we're ready for our final practise task of today.

For part one of this practise task, I would like you to order the statements to show how a depression is formed.

So the statements include A, a depression forms where warm air and cold air meet, B, air from around the area around it rushes in to fill the low, but because Earth is spinning, the air starts to swirl.

C, this creates an area of low pressure at the surface.

D, the rising air and swirling winds cause clouds, rain and wind, which leads to unsettled weather.

E, the warm air rises because it is lighter and as it rises, water vapour calls and forms clouds.

So for part one, I'd like you to order the statements to show how a depression is formed.

For part two of practise task B, I would like you to add the labels to the correct point on the diagram to show how the weather associated with a depression changes.

So you've got the seven boxes and your seven statements include A, showers, B, sky clearing, C, clear and dry, D, steady rain, E, heavy rain and thunder, F, sky clear, high cloud, and G, drizzle.

You can put the letter into the correct box.

Pause the video and have a go at completing both of those practise tasks.

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

Let's check your answer.

So for part one, explain how a depression is formed.

The statement should be in the following order.

A depression forms where warm air and cold air meet.

The warm air rises because it is lighter, and as it rises, water vapour cools and forms clouds.

C, this creates an area of low pressure at the surface.

B, air from around the area around it rushes in to fill the low, but because Earth is spinning, the air starts to swirl.

And D, the rising air and swirling winds cause clouds, rain, and wind, which leads to unsettled weather.

Well done if you've got that absolutely spot-on.

That's exactly how a depression is formed.

So to add the labels to the correct point in this diagram to show how the weather associated with the depression changes, we should start at number seven.

So seven is F, sky clear, high cloud.

Six is G, drizzle.

Five is D, steady rain.

Four is C, clear and dry.

Three is E, heavy rain and thunder.

Two is A, showers.

And one is B, sky clearing.

Good work, geographers, that was a tough one, but you did a really, really good job.

So to summarise today's lesson, we know that depressions are areas of low pressure where air is rising.

Depressions are formed where the warm air and the cold air masses meet.

As warm air is forced to rise, depressions bring changeable temperatures and rain at the fronts.

Strong winds are features of depressions.

The steep pressure gradient that drives these winds can be identified on a synoptic chart by tightly packed isobars.

And depressions usually move across the UK from west to east over one to three days.

You've been absolutely fantastic today, geographers.

I'm really, really impressed with your hard work.

See you soon for our next lesson.