3. Mid latitude weather systems

#TestThenCheck

Key concepts for your schema. Use these questions to test your knowledge before scrolling down!

Before you look down this page, test yourself on the following schema questions:

Formation of precipitation

- What are the three ways air can cool?
- What is adiabatic cooling?

Air masses

- What are the characteristics of the four main air masses affecting the British Isles and how are they modified as they approach us?

Jet stream

- How is it formed?
- What are the three key points about it?

Depressions

- How are they formed (link to air masses, the Polar Front, and the Jet Stream)?
- How does each of these weather elements vary as a depressions passes over - and why? Temperature, cloud, precipitation, wind direction and speed, pressure

Anticyclones

- How are they formed (link to the Jet Stream)?
- What is the difference in weather they bring in the summer and winter?

Synoptic charts and satellite images

- How confident are you that you can take your theoretical understanding of depressions and anticyclones and apply then to synoptic charts and satellite images?

The British Isles sits at around the mid 50s degrees north.

This is just north of the key division of surplus and deficit in terms of heat energy at 40 degrees,
So it is at the meeting point of the warm Tm air coming up from the south and the cold Pm air coming down from the north.
It also lies beneath the Polar Front Jet Stream.

As a result, the weather here is very changeable and often dramatic. This section will explore this in more detail.

This section will also draw significantly on your GCSE knowledge, so it would be a good idea to dig out your GCSE notes and have a look over them!

3.1 The formation of precipitation

Precipitation refers to ...

Precipitation occurs following adiabatic cooling. It is explained as follows:

As air rises, the air pressure falls.
As a result, the air expands.
As it expands it does work and uses up energy and the temperature falls.
Cold air cannot hold as much water vapour, and it becomes saturated.
Condensation occurs , forming cloud and --> possibility of rain.

Processes which cause air to rise can produce precipitation.

How then does air rise? Three ways.

Convectional uplift

When the air above an area of warmer ground is heated it will rise upwards by the process of convection.

This type of precipitation is common in the following locations:

in equatorial regions, at the rising part of the Hadley Cell, where the warm air rises due to heating from the overhead Sun.
after the cold front in a depression, as the lower layers of the Pm air are warmed as they travel south over the warmer Atlantic Ocean.

Frontal uplift

When a warm air mass meets a cold air mass, a boundary called a front forms between them and the warmer air rises over the colder air. At the warm front, the warm air rises over the cold air; at the cold front, the cold air undercuts the warm air, causing it to rise.

This type of precipitation is common in the mid latitudes, where depressions form as air rises between the Ferrel Cell and the Polar Cell.

Orographic Uplift (relief rainfall)

When moist air travelling over the sea meets mountain barriers when it comes ashore, the airflow is forced to rise up over the barrier.

How these processes result in precipitation is outlined below.

In each case, once the air starts to rise, this process can continue to be reinforced by the release of latent heat. The heat energy used for evaporation is released on condensation, further warming the air and causing further rising.

DEFINITION: Dew point is the temperature at which the air becomes saturated (full up with water vapour) and after which condensation will occur.

3.2 Mid latitude weather systems - air masses and the jet stream

A. Air masses

DEFINITION

Air mass - a large body of air with certain characteristics of temperature and humidity.

There are four main air masses which affect the British Isles.

Pm comes from the NW and is cold and moist. As it travels over the ocean towards the British Isles, its lower layers are heated and it becomes more unstable and will easily rise, cool and condense to give cloud and rain.
Tm comes from the SW and is mild and moist. As it is warm, it will tend to rise, cool and condense to give cloud and rain.
Pc comes from the E. Due to continentality, is it hot during the summer and cold during the winter. Although it is a dry air mass, it picks up moisture over the North Sea and can bring cloud to eastern England and snow to Eastern England during the winter.
Tc comes from the SE. It is hot and dry - not surprisingly then it's the least frequent air mass to affect the British Isles!

Link back to your GCSE notes on air masses to consolidate this. You don't need much more than what you had there for AS Level!

B. The Jet Stream

DEFINITIONS

Front - boundary between two air masses. There are often very steep gradients of change across from the fronts from warmer to colder temperatures.
Jet stream - a very fast moving ribbon of air in the upper atmosphere. It plays an important role in the formation of mid-latitude weather systems at 60 degrees N/S. It is found at approximately 10km in height at the tropopause (the boundary between the troposphere and stratosphere) and is at the boundary between the Polar and Ferrel Cells.

As we have already seen, as latitude increases, temperature increases.

Mid-latitude weather systems are largely driven by the fact that we are found at the meeting place of cold Pm and warm Tm air masses.

When these air masses meet, a front forms between them. called the the Polar Front. At this front, the warm air rises over the cold air.

As the air rises, it cools and condenses, releasing latent heat (jazz hands!) which further energises the air to rise.

The rising of the air along the Polar Front leads to the formation of the Polar Front Jet Stream, as shown in the diagram.

The PFJS is very important in the development and steering of mid-latitude weather systems, as we will see below.

There are a number of key points you need to know about the Jet Stream:

i. Steering depressions

The Polar Front Jet Stream is very important in determining the route and direction of the depressions. Over time, it tends to develop a meandering pattern of waves (known as Rossby Waves), looping far north and south in the mid-latitudes.

If it loops over the British Isles, then it tends to steer the depressions over us. If, however, it sits to our north or south, then it tends to steer them away from us, allowing high pressure to build over us instead.

ii. Steep temperature gradient

As the Polar Front Jet Stream forms at the boundary between the Pm and Tm air masses, there is a very sudden change in temperature from its north (cold) to its south (warm). This is known as a steep temperature gradient. Thus the Jet Stream marks the boundary between the warm air to the south and the cold air to the north.

The temperature gradient is stronger in the winter than the summer, and so the Jet Stream tends to be more energetic in the winter.

iii. Seasonal variation

Over the course of a year, the PFJS typically migrates north and south following the tilt of the Earth.

During the winter, it generally moves south over the British Isles. This draws down the cold air from the Poles towards us, and steers depressions over the top of us. This is why our winters are often wet, cold and stormy.
During the summer, it generally moves north towards Iceland. This allows the warmer tropical air to move up over the British Isles, and steers the depressions north of us. When these conditions occur, that's what gives us our hot, dry summers.

However, I said generally this is the case. Sometimes the jet stream doesn't move as far north during the summer. When this happens, sadly the warm air cannot move north over us, and we get depression after depression passing over us. These are our washout summers - and we're all too used to those, are we not...?!

Click here to see the forecast for the Jet Stream

Here's a summary video about the Jet Stream.

3.3 Depressions

A. Formation

The formation of depressions is linked to two things:

the meeting of Pm and Tm air and the formation of the Polar Front
the role of the Polar Front Jet Stream in the upper atmosphere.

i. The Polar Front

As the Pm from the north meets the Tm from the south, they don't mix and a front forms between them known as the Polar Front.
Due to the rotation of the Earth, a bulge forms in this front. The winds organise themselves into a rotational pattern and the Polar Front forms two main fronts in the depression: the warm front with Tm air behind it and the cold front with Pm air behind it.

ii. The Jet Stream

The Polar Front Jet Stream plays an important role in the formation of mid-latitude weather systems (depressions and anticyclones), and it is related to the Rossby Waves that tend to form in the Jet Stream.

As the Jet Stream meanders south, it slows down (more air arrives than leaves) resulting in upper air convergence. This creates an upper air surplus and the air spirals down from the Jet Stream to the surface, creating a high pressure system there (the anticyclone).
As the Jet Stream then meanders north again, it speeds up (more air leaves than arrives) resulting in upper air divergence. This creates an upper air deficit, and the air spirals upwards from the surface towards this deficit. This reinforces the development of the low pressure system below (the depression).

Upper air convergence

As the PRJS approaches a trough and flows towards the lower latitudes, it slows down.
→ upper air convergence (more air is arriving than leaving) i.e. surplus
→ air falls towards the surface, spiralling downwards in a clockwise direction
→ high pressure at the surface

Upper air divergence

As the PRJS moves out of the trough towards the higher latitudes, it speeds up.
→ upper air divergence (more air is leaving than arriving) i.e. deficit
→ this helps draw air up from the surface; it spirals upwards in an anticlockwise direction (this helps organise the air into the rotation pattern of the depression)
→ low pressure at the surface

Use this Slides presentation to explore this in more detail.

Formation of Depressions

Here's a great summary video of the link between the Jet Stream and surface pressure by a tremendously enthusiastic American forecaster!

B. Weather associated with a depression

Different parts of a depression have different types of weather associated with them. This is because a depression has a structure and different elements within it:

Depressions approach the British Isles from the West, so the first part of the depression we experience is the approach of the warm front, as shown in the table below.

EXAM TIP: This means you should read the following table from right to left. If you get an exam essay about this, you should do it in the order of the numbers shown in the table below. Learn this table in detail!

Use the Slides below to walk through the above table step by step and for additional guidance on how to learn this.

Weather in a depression

Additional notes

The cloud and rain in depressions are caused by frontal uplift (see above) which occurs when two different air masses meet but don't mix. In the case of the warm front, the warmer Tm air rises over the colder Pm air along the warm front. At the cold front, it's the colder Pm air circulating from the west that undercuts the warmer Tm air ahead of it, causing it to rise quite vigorously.
One of the reasons why wind speeds are stronger at fronts (and especially the cold front) is due to the release of latent heat on condensation. The more vigorous rising air at the cold front releases more latent heat as the towering cumuolnimbus clouds are formed and this explains the stronger, gusty wind speeds found there.

Click here to find out why you get showers after a cold front.

3.4 Anticyclones

A. Formation

Good news! We've kind of covered this before when making the link between the Jet Stream and surface weather systems.

The key points are:

There is upper air convergence in the Jet Stream.
This causes the air to spiral down in a clockwise direction to the surface.
This creates surface high pressure.

[See above for more details.]

B. Weather associated with anticyclones

What we covered at GCSE will be more than sufficient for AS Level, so this is mostly a recap. Hopefully you'll remember it well from those simpler times in your GCSE days...!

3.5 Synoptic weather charts and satellite images

In the exam, you may be given a synoptic weather chart showing either a depression or an anticyclone - or both.

Pupils sometimes struggle with these as the chart can - at least at the start - appear complex. But the approach is simple: the straightforward patterns the models would lead to to expect are there - they just tend to be distorted a bit as this is the real world. So you need to look not for the precise patterns to be exactly the same as the models, but for things that are close to what the models suggest.

This Slides presentation will hopefully help!

Interpreting Synoptic Charts

Now have a go at this quiz based on the map you've just been looking at. See how well you are able to take your knowledge of depressions and apply it to this map.

Summary video of this section

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