Weather Charts

By the end of the lesson, you will be able to:

  • Understand isobars and their relationship with wind speed
  • Identify various pressure systems and fronts on a weather chart
  • Interpret and produce plotted weather symbols.

Part A – Isobars, pressure and wind

Part B – Identifying pressure systems and fronts

Part C – Plotted weather charts

Teachers’ notes

Teachers’ Notes

Resources required

Computers with Internet access would be desirable. Alternatively if Internet access is not available, printed copies of student sheets and worksheets should be made.

Prior knowledge required

A basic background of weather and climate.

Teaching activities

Students can visit the following pages to gain a basic background into the topics covered:

Weather charts

Interpreting Weather Maps

The information on the student sheets can be delivered by the teacher and activities completed individually. Alternatively students can work through the whole lesson themselves.

Exercises

Three worksheets with exercises are provided to consolidate learning.

A series of additional exercises are provided for more able students, or those who have already studied pressure systems and fronts in more detail prior to this lesson.

Suggestions for homework

Any of the worksheet activities can be completed. Alternatively students can collect weather charts from the Internet or a newspaper and repeat the exercises using these.

 

Part A – Isobars, pressure and wind

Isobars are lines joining points of equal pressure, similar to contours, which are shown on weather charts. Charts showing isobars are useful because they can help to identify anticyclones and depressions. Pressure is measured in millibars and isobars are normally drawn at intervals of 4 millibars. Pressure values are corrected to Mean Sea Level Pressure (MSLP) before being plotted on a map, this ensures that altitude does not affect the mapping.

Isobars are also helpful because the help us to understand the direction and strength of the wind in a particular area. Where isobars are very close together, for example near a depression, they indicate strong winds. Where the isobars are more widely spaced, near an anticyclone for example, they indicate light winds.

The wind will blow almost parallel to the isobars. Around an anticyclone the winds will blow slightly across the isobars, away from the centre of the anticyclone. In depressions, the wind will blow slightly across the isobars towards the centre of the low pressure.

Buys’ Ballot’s Law states that if you stand with you back to the wind in the northern Hemisphere, low pressure will be on your left. This means that you can work out the wind direction at different locations on a weather chart.

What to do next

Using this information on isobars you should now be able to complete worksheet 1.

Then you can complete extension exercise 1 or go on to Part B – Identifying pressure systems and fronts.

Part B – Identifying pressure systems and fronts

Anticyclones

An anticyclone, also known as a ‘high’ can be identified on a weather chart as an often large area of widely spaced isobars, where pressure is higher than surrounding areas. In the Northern Hemisphere winds blow in a clockwise direction around high pressure. The highest pressure occurs at the centre and is known as the ‘high pressure centre’. Anticyclones can bring warm and sunny weather in summer, but cold and foggy weather in winter.

Depressions

A depression, also known as a ‘low’ can be recognised on a weather chart by an area of closely spaced isobars, often in a roughly circular shape, where pressure is lower than surrounding areas. They are often accompanied by fronts. In the Northern Hemisphere winds blow around depressions in an anticlockwise direction. The lowest pressure occurs at the middle of a depression, this is known as the ‘low pressure centre’. Depressions are often associated with strong winds and heavy rain and are nearly always accompanied by fronts.

depressions

Troughs

Troughs are elongated extensions of areas of low pressure. They bring similar weather to that associated with depressions.

Ridges

Ridges are elongated extensions of areas of high pressure. They bring similar weather to that associated with anticyclones.

 

Col

A col can be identified as an area of slack pressure between two anticyclones and two depressions.

The following diagram summarises the appearance on a weather chart of the main types of pressure systems.

Cold fronts and warm fronts

Cold fronts can be identified on weather charts as bold lines with triangles. These are blue when displayed on colour charts. The points of the triangle indicate the direction in which the front is moving. A cold front indicates a change in air mass, where warmer air is being replaced by colder air. They often bring short spells of heavy rainfall in the form of showers and squally winds, and are accompanied by a decrease in temperature, a veer in wind direction and a change to brighter showery conditions.

Warm fronts can be identified on weather charts as bold lines with semi-circles or humps. These are coloured red when displayed on colour charts. The direction of the humps indicates the direction in which the front is moving. A warm front indicates a change from a colder to a warmer air mass. They often bring spells of prolonged and sometimes heavy rainfall, with strong winds.

Occluded fronts

Occluded fronts can be identified on weather charts as bold lines with sets of triangles and semi-circles. These are coloured purple on coloured weather charts. The direction in which the symbols face indicates the direction in which the front is travelling. Occlusions are formed when the cold front overtakes the warm front, therefore they have similar characteristics to a cold front, but less intense.

Warm Sector

The warm sector of a depression is located behind the warm front and ahead of the cold front. It often brings mild temperatures but the weather can be overcast with drizzle.

What to do next

Using this information you should now be able to complete worksheet 2.

Then you can complete extension exercise 2 or go on to Part C.

 Part C – Plotted weather charts

The following image is an example of a UK plotted weather chart.

Plotted surface charts are made up of individual ‘station circles’. Each individual stations observation is put into graphical format so that it is simple to understand, can be put on a chart and be compared to its neighbours.

The diagram opposite shows the basic station circle, including temperature, pressure, weather, cloud cover, wind speed and direction. Some elements, e.g. weather and cloud cover, are put into a graphical code to make them more obvious. The Met Office uses a much more complex station circle but the one below is a simplified version using the main weather elements.

Wind direction is indicated by a line coming from the centre of the station circle. The line indicates the direction from which the wind is blowing.Wind speed and direction

On the end of the wind direction line are ‘feathers’, these indicate the wind speed. Half feathers represent 5 knots whilst whole feathers indicate 10 knots. A wind speed of 50 knots is indicated by a triangle. Combinations of these can be used to report wind speed to the nearest 5 knots.

The table below shows the symbols used to indicate different wind speeds.

Cloud cover

Cloud cover is indicated by the shading of the centre of the station circle. The table below shows the meanings of the different symbols.

Temperature

Temperature is plotted to the nearest degree and is located in the top left-hand corner of the station plot.

Pressure

Pressure is plotted in the top right-hand corner of the station plot.

Weather

In total the Met Office has 99 codes for recording the current weather at the time of the observation. However these can be simplified down. Different types of weather are represented using different weather symbols, a key to which can be found below.

What to do next

Using this information you should now be able to complete worksheet 3.

Then you can complete extension exercise 3 .

Worksheet 1 – Reading pressure values from a surface pressure chart

The image below is an extract from a surface pressure chart, on which isobars, fronts and pressure systems have already been drawn. Download a copy of the worksheet here. If working on a PC, print out a copy of this page. Then study the chart, and complete the table below, by entering the approximate pressure at each of the labelled locations.

Worksheet 2 – Surface chart analysis

The image below shows a surface pressure chart, on which isobars, fronts and pressure systems have already been drawn. Download a copy of the worksheet here. Then study the chart, and identify and label the following items:

Cold front Col
Warm front Trough
Occlusion High pressure centre
Trough of low pressure Low pressure centre
Ridge of High pressure Warm sector

If you need help refer back to Part B – Identifying pressure systems and fronts.

Worksheet 3 – Station circle plots

The following three questions contain examples of plotted station circles. Download a copy of the worksheet here. Then study each of these plots and complete the tables below with details of the temperature, weather, pressure, cloud cover, wind speed and wind direction.

The following three questions contain tables of weather data. Study each of these tables and plot the details of the temperature, weather, pressure, cloud cover, wind speed and wind direction on to the station circle provided.

If you need help refer back to Part C – Plotted weather charts.

Worksheet 1 – extension exercise

The following diagram shows a series of plotted pressure values. Download a copy of the worksheet here. Complete the diagram by drawing isobars at intervals of 4 millibars, including 992, 996, 1000, 1004 and 1008.

 

Worksheet 2 – extension exercise

Study the chart below. This chart is for mid-November. Download a copy of the worksheet here. Then using your knowledge of the characteristics of anticyclones, depressions and fronts in winter, complete the table below with approximate readings.

There is no one correct answer. Your values should simply indicate the typical values and the variations between each location. For example, should location A be warmer or colder than location B? Should location E have stronger or lighter winds than location D?


 

Worksheet 3 – extension exercise

Study the chart below. This chart is for mid November. Download a copy of the worksheet here. Then using your knowledge of the characteristics of anticyclones, depressions and fronts in winter, construct a station circle for each of the locations marked on the chart.

There is no one correct answer. You values should simply indicate the typical values and the variations between each location. For example should location A be warmer or colder than location B? Should E have stronger or lighter winds than location D?

If you have already completed extension exercise 2, you will simply need to convert your table of results into station circle plots.

Location A
Location B

 
Location C

 
Location D

 
Location E

 
Location F

 
Location G

 
Location H

 

Web page reproduced with the kind permission of the Met Office

In Depth – Understanding Weather

Understanding weather

Air masses

Atmosphere

Clouds

El Niño and La Niña

High pressure or anticyclone

Low pressure or depression

Temperature differences

Weather fronts

Weather systems

Wind

Air masses

Air masses are parcels of air that bring distinctive weather features to the country. An air mass is a body or ‘mass’ of air in which changes in temperature and humidity within them are relatively slight. That is to say the air making up the mass is very uniform. in temperature and humidity.

An air mass is separated from an adjacent body of air by a weather front. An air mass may cover several millions of square kilometres and extend vertically throughout the troposphere.

Atmosphere

A thin layer of mixed gases which covers the Earth and helps it from becoming too hot or too cold. Its circulation, the heat (terrestrial radiation) and light (solar radiation) which pass through it, and the processes which go on in it, all affect the climate. The atmosphere is about 800 km (500 miles) deep and is made up of 21% oxygen, 78% nitrogen, 0.037% carbon dioxide, and other gases including hydrogen, helium, neon, argon, krypton, xenon, and water vapour.

Clouds

A classification of clouds was introduced by Luke Howard (1772-1864) who used Latin words to describe their characteristics.

  • Cirrus – a tuft or filament (e.g. of hair)
  • Cumulus – a heap or pile
  • Stratus – a layer
  • Nimbus – rain bearing

There are now ten basic cloud types with names based on combinations of these words (the word ‘alto’, meaning high but now used to denote medium-level cloud, is also used).

Clouds form when moist air is cooled to such an extent it becomes saturated. The main mechanism for cooling air is to force it to rise. As air rises it expands – because the pressure decreases with height in the atmosphere – and this causes it to cool. Eventually it may become saturated and the water vapour then condenses into tiny water droplets, similar in size to those found in fog, and forms cloud. If the temperature falls below about minus 20 °C, many of the cloud droplets will have frozen so that the cloud is mainly composed of ice crystals.

The ten main types of cloud can be separated into three broad categories according to the height of their base above the ground: high clouds, medium clouds and low clouds.

High clouds are usually composed solely of ice crystals and have a base between 18,000 and 45,000 feet (5,500 and 14,000 metres).

  • Cirrus – white filaments
  • Cirrocumulus – small rippled elements
  • Cirrostratus – transparent sheet, often with a halo

Medium clouds are usually composed of water droplets or a mixture of water droplets and ice crystals, and have a base between 6,500 and 18,000 feet (2,000 and 5,500 metres).

  • Altocumulus – layered, rippled elements, generally white with some shading
  • Altostratus – thin layer, grey, allows sun to appear as if through ground glass
  • Nimbostratus – thick layer, low base, dark. Rain or snow falling from it may sometimes be heavy

Low clouds are usually composed of water droplets — though cumulonimbus clouds include ice crystals – and have a base below 6,500 feet (2,000 metres).

  • Stratocumulus – layered, series of rounded rolls, generally white with some shading
  • Stratus – layered, uniform base, grey
  • Cumulus – individual cells, vertical rolls or towers, flat base
  • Cumulonimbus – large cauliflower-shaped towers, often ‘anvil tops’, sometimes giving thunderstorms or showers of rain or snow

blue sky with clouds

 

El Niño and La Niña

The tropical Pacific Ocean has a warming and cooling cycle. This cycle is a completely natural event and usually lasts between three to seven years. When the waters become warmer it is called El Niño, and when they become cooler it is called La Niña. During the cycle, the temperature of the ocean can change by around 3 °C between the warmest and coolest times.

Fishermen off the South American coast have known about this natural event for hundreds of years. When it happens, they see a huge fall in the numbers of fish caught. But scientists are only just beginning to understand how the event affects Earth’s weather and climate.

El Niño and La Niña

High pressure or anticyclone

In an anticyclone (also referred to as a ‘high’) the winds tend to be light and blow in a clockwise direction. Also the air is descending, which inhibits the formation of cloud. The light winds and clear skies can lead to overnight fog or frost. If an anticyclone persists over northern Europe in winter, then much of the British Isles can be affected by very cold east winds from Siberia. However, in summer an anticyclone in the vicinity of the British Isles often brings fine, warm weather.

Low pressure or depression

In a depression (also referred to as a ‘low’), air is rising. As it rises and cools, water vapour condenses to form clouds and perhaps precipitation. Consequently, the weather in a depression is often cloudy, wet and windy (with winds blowing in an anticlockwise direction around the depression). There are usually frontal systems associated with depressions.

Temperature differences

Temperature affects other weather elements including air pressure, cloud formation, humidity and precipitation.

Factors affecting temperature:

  • Latitude – warmer closer to the equator and cooler moving away towards the poles
  • Altitude – getting colder as the land gets higher
  • Distance from the sea – temperatures inland are higher than the coast during the summer and lower than the coast during winter. This is because land heats up and cools down more quickly than the sea
  • North-facing slopes in the southern hemisphere and south-facing slopes in the northern hemisphere receive more sunlight than the opposite slopes and are warmer
  • Wind – generally makes the air feel cooler

Weather fronts

A weather front is simply the boundary between two air masses.

There are three different types of weather front. These are:

  1. Cold front
  2. Warm front
  3. Occluded front (also called an occlusion)

Cold front

This is the boundary between warm air and cold air and is indicative of cold air replacing warm air at a point on the Earth’s surface

On a synoptic chart a cold front appears blue

cold front symbol

 

The presence of a cold front means cold air is advancing and pushing underneath warmer air. This is because the cold air is ‘heavier’ or denser, than the warmer air. Cold air is thus replacing warm air at the surface. The symbols on the front indicate the direction the front is moving.

The passage of a cold front is normally marked at the earth’s surface by a rise of pressure, a fall of temperature and dew-point, and a veer of wind (in the northern hemisphere). Rain occurs in association with most cold fronts and may extend some 100 to 200 km ahead of or behind the front. Some cold fronts give only a shower at the front, while others give no precipitation. Thunder may occur at a cold front.

Warm front

This is the boundary between cold air and warm air and is indicative of warm air replacing cold air at a point on the Earth’s surface

On a synoptic chart a warm front appears red

warm front symbol

 

The presence of a warm front means warm air is advancing and rising up over cold air. This is because the warm air is ‘lighter’ or less dense, than the colder air. Warm air is thus replacing cold air at the surface. The symbols on the front indicate the direction the front is moving.

As a warm front approaches, temperature and dew-point within the cold air gradually rise and pressure falls at an increasing rate. Precipitation usually occurs within a wide belt some 400 km in advance of the front. Passage of the front is usually marked by a steadying of the barometer, a jump in temperature and dew-point, a veer of wind (in the northern hemisphere), and a cessation or near cessation of precipitation.

Occluded front

These are more complex than cold or warm fronts. An occlusion is formed when a cold front catches up with a warm front

When a cold front catches up with a warm front the warm air in the warm sector is forced up from the surface

On a synoptic chart an occluded front appears purple

occluded front symbol

 

Weather systems

Weather can change on a daily basis especially at middle to high latitudes where it is controlled by weather systems, depressions and anticyclones.
On a weather chart, lines joining places with equal sea-level pressures are called isobars. Charts showing isobars are useful because they identify features such as anticyclones (areas of high pressure), depressions (areas of low pressure), troughs and ridges which are associated with particular kinds of weather.

Wind

windsock

The movement of air around the earth from high pressure to low pressure is what brings about winds. The direction given for the wind refers to the direction from which it comes. For example, a westerly wind is blowing from the west towards the east.

Measurements of wind strength are made at 10 metres (33 feet) above the ground. A specified height has to be used because the wind speed decreases towards the ground. In this country winds are measured in knots (nautical miles per hour). However, forecast winds are often given in miles per hour (where 1 knot is equivalent to 1.15 mph) or in terms of the Beaufort Scale.

There are rapid variations in the wind – these are referred to as gusts. Gusts are higher inland than over the sea or windward coasts, although the mean wind speeds tend to be lower inland. Typically, gusts can be 60% higher than the mean speed, although in the middle of cities this can reach 100%. Northerly winds tend to be gustier than southerly ones. In general, the weather is strongly influenced by the wind direction, so information about the wind provides an indication of the type of weather likely to be experienced.

  • Northerly winds tend to bring relatively cold air from polar regions to the British Isles. Similarly, southerly winds tend to bring relatively warm air from the tropics
  • As cold polar air moves southwards over an increasingly warm sea, the heating of the air by the sea causes cumulus clouds to form. These clouds may grow sufficiently for showers to develop and, consequently, winds from the north-west, north or north-east usually bring cold, showery weather to the British Isles
  • Warm air from the tropics moving northwards over the sea is cooled from below. Sometimes the cooling is sufficient for sea fog or a thin layer of stratus to form. The cloud can become thick enough for drizzle, especially on windward coasts and over high ground. In general, winds from the west or south-west are associated with overcast, wet weather
  • Winds from the south and south-east mainly occur in summer and these bring warm, dry weather. However, southerly winds can sometimes bring hot, thundery weather
  • Easterly winds in winter bring very cold air to the British Isles. The characteristics and path of the air determine whether it is cloudy (with perhaps rain, sleet or snow) or fine and sunny. In summer, an easterly wind will mean it is cool on the east coast but warm elsewhere, usually with clear skies

windsock

Web page reproduced with the kind permission of the Met Office

In Depth – The Coriolis Effect

Coriolis Effect

As air blows from high to low pressure in the atmosphere, the Coriolis force diverts the air so that it follows the pressure contours. In the Northern Hemisphere, this means that air is blown around low pressure in an anticlockwise direction and around high pressure in a clockwise direction.

Think about a person standing at the Equator. In the course of a day, the planet rotates once, meaning that you travel a colossal 2π x R (the radius of the Earth – 6370km) = 40,000km through space – a speed of about 1700km/ hr. You don’t notice that you are travelling so fast, because the air around you is travelling at the same speed, so there is no wind. On the other hand, if you are standing at a Pole, all you do in the course of a day is turn around on the spot, you have no speed through space and similarly the air around you is stationary.

Now, think about really fast moving, Tropical air which is being pulled towards the poles by a pressure gradient. As it travels polewards, it moves over ground which is rotating more slowly, and so it overtakes the ground, and looks like it is moving from west to east. Similarly, slow moving polar air will be left behind by the rotating Earth and look like it is moving from east to west if it is pulled equatorward by a pressure difference.

In general, moving air in the Northern hemisphere is deflected to the right by the Coriolis Effect.

As the air blows from high to low pressure the Coriolis force acts on it, diverting it, and we end up with air following the pressure contours and blowing around low pressure in an anticlockwise direction and around high pressure in a clockwise direction (both true only for the Northern Hemisphere).

REPRESENTATION OF FLOW AROUND A LOW PRESSURE AREA.
FIGURE 1:SCHEMATIC REPRESENTATION OF FLOW AROUND A LOW PRESSURE AREA. PRESSURE GRADIENT FORCE REPRESENTED BY BLUE ARROWS. THE CORIOLIS FORCE, ALWAYS PERPENDICULAR TO THE VELOCITY, BY RED ARROWS. © SVG VERSION, ROLAND GEIDER (OGRE), OF THE ORIGINAL PNG, (CLEONTUNI)

In this diagram, the black arrows show the direction the air is moving in. The Coriolis force pulls the air to the right (red arrows). As the air is being pulled in to the depression by the pressure gradient (blue arrows), it is continuously deflected by the Coriolis Force. When the air moves in a circle around the depression, the Coriolis force (red arrows) is balanced by the pressure gradient force (blue arrows).

In summary, for the Northern Hemisphere:

  • Low pressure is called a cyclone and has anticlockwise winds blowing around it.
  • High pressure is called an anticyclone and has clockwise winds blowing around it.
  • The wind tends to blow along the pressure contours.
  • We name winds by the direction they are blowing from.
  • Buys Ballot’s Law states that “In the Northern Hemisphere, if you stand with your back to the wind then the lower pressure will be on your left”
  • Alternatively, some people find the rule ‘righty tighty, lefty loosey’ a useful reminder of the direction of rotation – high pressure is like tightening a screw (righty tighty) and low pressure like loosening a screw (lefty loosey) (Figure 2).

Figure 2: Air blows around a low pressure in an anticlockwise direction and around a high pressure in a clockwise direction in the Northern Hemisphere © RMetS

What about the Southern Hemisphere?

In the Southern Hemisphere, winds blow around a high pressure in an anticlockwise direction and around a low pressure in a clockwise direction.

The simplest way of visualising why this is the case is to take a ball (or an apple or orange, or anything spherical!). Mark on the poles and the equator, and then mark a spot in the ‘northern hemisphere’ and the ‘southern hemisphere’ of your sphere. Rotate your sphere. Keeping it rotating, tilt your sphere so that you are looking at it from the North Pole – your Northern Hemisphere spot should be going round in an anticlockwise direction. Now, making sure you keep rotating your sphere in the same direction, tilt it so that you are looking at the ‘south pole’. Your southern hemisphere spot should be rotating in a clockwise direction. This demonstration doesn’t explain the Coriolis effect, but it does show how things can be seen differently in the two hemispheres of the same planet.

Data and Image Sources

For an example of wind blowing along pressure contours, see the BBC website.

Useful Links:

Anticyclones, Depressions and Fronts

LESSON PLAN: Introduction to Anticyclones, Depressions and Fronts
Key Stage 4 – GCSE
Subject Geography


 

Length 1 lesson

Teaching Objectives/Learning Outcomes
By the end of the lesson, pupils will know and understand:
Characteristics of depressions and fronts and the sequence of associated weather
Characteristics of anticyclones and the contrast between those in summer and in winter.

Resources Required
None.

Prior Knowledge Required
A basic knowledge of weather and climate

Teaching Activities
The following web pages have related resources at a similar level:

Weather Systems

Student Charts

Weather Systems

Exercises
4 worksheets with exercises are provided to consolidate learning.
A series of extension exercises are provided for more able students, or those who have already studied the topics covered in more detail prior to this lesson.

Plenary – A quiz is available, which brings together all the topics covered. The can be used to examine whether the objectives of the lessons have been met.

Suggestions for Home Work
Any of the worksheet activities can be completed as homework.

PART A – ANTICYCLONES AND DEPRESSIONS

High Pressure Systems

A high pressure system, also known as an anticyclone occurs when the weather is dominated by stable conditions. Under an anticyclone air is descending, forming an area of higher pressure at the surface. Because of these stable conditions, cloud formation is inhibited, so the weather is usually settled with only small amounts of cloud cover. In the Northern Hemisphere winds blow in a clockwise direction around an anticyclone. As isobars are normally widely spaced around an anticyclone, winds are often quite light.
Anticyclones can be identified on weather charts as an often large area of widely spaced isobars, where pressure is higher than surrounding areas.

Winter Anticyclones

In winter the clear, settled conditions and light winds associated with anticyclones can lead to frost and fog. The clear skies allow heat to be lost from the surface of the earth by radiation, allowing temperatures to fall steadily overnight, leading to air or ground frosts. Light winds along with falling temperatures can encourage fog to form; this can linger well into the following morning and be slow to clear. If high pressure becomes established over Northern Europe during winter this can bring a spell of cold easterly winds to the UK.

Summer Anticyclones

In summer the clear settled conditions associated with anticyclones can bring long sunny days and warm temperatures. The weather is normally dry, although occasionally, very hot temperatures can trigger thunderstorms. An anticyclone situated over the UK or near continent usually brings warm, fine weather.

Low Pressure Systems

A low pressure system, also known as a depression occurs when the weather is dominated by unstable conditions. Under a depression air is rising, forming an area of low pressure at the surface. This rising air cools and condenses and helps encourage cloud formation, so the weather is often cloudy and wet. In the Northern Hemisphere winds blow in anticlockwise direction around a depression. Isobars are normally closely spaced around a depressions leading to strong winds.
Depressions can be identified on weather charts as an area of closely spaced isobars, often in a roughly circular shape, where pressure is lower than surrounding areas. They are often accompanied by fronts.

What to do next
Using this information on pressure systems you should now be able to complete worksheet 1. Then you can move on to extension exercise 1 or worksheet 2

  • PART B – FRONTS

A front is a boundary between two different types of air masses, these are normally warm moist air masses from the tropics and cooler drier air masses from polar regions. Fronts move with the wind so over the UK they normally move from west to east. The notes below provide information about the most common types of fronts. The descriptions given apply to active well developed fronts, weaker fronts may not display all the characteristics or they may be less well defined.

Warm Fronts

A warm front indicates that warm air is advancing and rising up over the colder air. This is because the warm air is ‘lighter’ or less dense, than the cold air. Therefore warm fronts occur where warmer air is replacing cooler air at the surface. As the warm front approaches there is a gradual deterioration in the weather. Clouds gradually lower from higher cirrus, through altostratus, to stratus and nimbostratus at the front. There is often a prolonged spell of rainfall which is often heavy. Behind the warm front the rain becomes lighter, turns to drizzle or ceases, but it remains cloudy.

Temperatures rise behind the warm front and winds turn clockwise, also known as a wind ‘veer’. Pressure falls steadily ahead of and during the passage of the warm front, but then rises slowly after its passage.

The following diagram shows the formation of a warm front in diagrammatic form.

PART B – FRONTS A front is a boundary between two different types of air masses, these are normally warm moist air masses from the tropics and cooler drier air masses from polar regions. Fronts move with the wind so over the UK they normally move from west to east. The notes below provide information about the most common types of fronts. The descriptions given apply to active well developed fronts, weaker fronts may not display all the characteristics or they may be less well defined. Warm Fronts A warm front indicates that warm air is advancing and rising up over the colder air. This is because the warm air is ‘lighter’ or less dense, than the cold air. Therefore warm fronts occur where warmer air is replacing cooler air at the surface. As the warm front approaches there is a gradual deterioration in the weather. Clouds gradually lower from higher cirrus, through altostratus, to stratus and nimbostratus at the front. There is often a prolonged spell of rainfall which is often heavy. Behind the warm front the rain becomes lighter, turns to drizzle or ceases, but it remains cloudy. Temperatures rise behind the warm front and winds turn clockwise, also known as a wind ‘veer’. Pressure falls steadily ahead of and during the passage of the warm front, but then rises slowly after its passage. The following diagram shows the formation of a warm front in diagrammatic form.

The following diagram shows a cross section through a warm front, with associated cloud, temperature and weather changes.

anticyclones_depressions_fronts

 

 

 

 

 

 

 

 

Cold Fronts

A cold front indicates that cold air is advancing and pushing underneath warmer air at the surface. This occurs because the cold air is ‘heavier’ or denser than the warm air. Therefore cold fronts occur where cooler air is replacing warmer air at the surface. The passage of weather associated with a cold front is much shorter lived than that with a warm front. As there is often a lot of cloud in the warmer air ahead of the cold front, there is often little indication of the approaching cold front. As the front passes temperatures fall and there is often a short spell of very heavy rain, sometimes with inbedded thunderstorms and cumulonimbus clouds. Behind the front the weather is much brighter with broken clouds but occasional showers. Winds veer with the passage of the cold front and are often strong and gusty, especially near showers. Pressure rises throughout the approach and passage of the cold front.

The following diagram shows the formation of a cold front in diagrammatic form.

anticyclones_depressions_fronts_3

 

 

 

 

 

 

 

 

 

 

 

The following diagram shows a cross section through a cold front, with associated cloud, temperature and weather changes.

anticyclones depressions fronts

 

 

 

 

 

 

Occlusions

In a mature depression the warm front normally precedes the cold front. Cold fronts generally travel much quicker than warm fronts, and eventually it will catch up with the warm front. Where the two fronts meet, warm air is lifted from the surface and an occlusion is formed. An occlusion can be thought of as having similar characteristics to both warm and cold fronts. The weather ahead of an occlusion is similar to that ahead of a warm front, whilst the weather behind is similar to that behind a cold front.

The following diagrams depict the formation of an occlusion

anticyclones_depressions_fronts_5

anticyclones_depressions_fronts_6

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

What to do next
You can now move on to Part C – Life Cycle of a Depression.

 

PART C – LIFE CYCLE OF A DEPRESSION

A Norwegian scientist called Vilhelm Bjerknes devised a simple model which described how depressions developed from the meeting of warm and cold air. The model had four stages which are detailed below.

Origin and Infancy

Initially a warm air mass such as one from the tropics, meets a cooler air mass, such as one from the polar regions. Depressions which affect the UK normally originate over the Atlantic Ocean.

anticyclones depressions

Maturity

The warm air rises up over the colder air which is sinking. A warm sector develops between the warm and cold fronts. The mature stage of a depression often occurs over the UK.

anticyclones depressions fronts

Occlusion

The cold front travels at around 40 to 50 miles per hour, compared to the warm front which travels at only 20 to 30 miles per hour. Therefore the cold front eventually catches up with the warm front. When this occurs an occlusion is formed.

anticyclones depressions fronts

Death

Eventually the frontal system dies as all the warm air has been pushed up from the surface and all that remains is cold air. The occlusion dies out as temperatures are similar on both sides. This stage normally occurs over Europe or Scandinavia.

What to do next
You can now move on to Part D – Depression cross-section and weather sequence

PART D – DEPRESSION CROSS SECTION AND WEATHER SEQUENCE

 

Cross-section through a Classic Depression

Most depressions have a warm and cold front, more mature depressions may also have an occluded front. The diagram below shows a cross-section through a depression, showing the warm and cold fronts and an indication of the associated weather.

anticyclones_depressions

table

What to do next

Using this information on the passage of depressions you should now be able to complete worksheet 3 and worksheet 4.

 

Web page reproduced with the kind permission of the Met Office

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