Weather systems

To get a complete picture of the weather around the world, weather observations are taken at agreed times at weather stations worldwide. They are then plotted onto a synoptic chart.

Synoptic charts

A synoptic chart is any map that summarises atmospheric conditions(temperature, precipitation, wind speed and direction, atmospheric pressure and cloud coverage) over a wide area at a given time. They display an overview of the weather conditions observed from many different weather stations, aeroplanes, balloons and satellites.

By collating the information over a wide area, meteorologists can observe the behaviour and movement of weather formations that might affect their local area in the future. This allows meteorologists to make more accurate weather forecasts. Charts are updated at least every six hours.

There are many different types of weather map, all drawn using internationally agreed standards and using accepted symbols.

Standard symbols used on weather charts

Symbol Precipitation Symbol Cloud cover Symbol Wind speed

Drizzle

Drizzle

10 knots

Clear sky

Sleet

Calm

Clear sky

Shower

Hail

One okta

Seven oktas

1-2 knots

Calm

Rain

Four oktas

Two oktas

Snow shower

5 knots

Shower

Snow

15 knots

Three oktas

Eight oktas

10 knots

One okta

Hail

Thunderstorm

Four oktas

Mist

15 knots

1-2 knots

Thunderstorm

Five oktas

Five oktas

Sky obscured

20 knots

Rain

Heavy rain

20 knots

Six oktas

Fog

50 knots or more

Two oktas

Sleet

Heavy rain

Seven oktas    

5 knots

Snow shower

Six oktas

Eight oktas    

Snow

Mist

50 knots or more

Sky obscured    

Three oktas

Fog        

The synoptic chart below shows complete cloud cover (eight oktas) over most of central and southern England and some drizzle or rain. Can you describe the weather conditions over the other parts of Europe shown on the map?

Synoptic chart

Synoptic chart

 

Weather symbols can seem confusing until you look for the patterns. Note how increasing cloud cover is shown by covering more and more eighths (oktas) of the circle. More wind speed is shown by adding a little tail for every extra 5 knots of wind (just under 5 mph/8 kmh) and a longer tail for 10 knots (just under 10 mph/16 kmh).

Similarly, once you know the symbol for rain is a black ball and the symbol for snow is a star, then you know sleet will be a ball plus a star, and two, three or four balls denotes heavier rainstorms.

Isobars and fronts

Isobars are lines on a weather map joining together places of equal atmospheric pressure. On the map the isobar marked 1004 represents an area of high pressure, while the isobar marked 976 represents an area of low pressure.

Isobars on a weather map

Isobars on a weather map

 

  • The numbers measure the atmospheric pressure in millibars.
  • Usually isobars are drawn at intervals of two or four millibars (one thousandth of a bar).
  • The closer the isobars are together the windier it is.
  • If the lowest number is in the middle circle this is a low pressure ordepression. Often low pressures mean wet and windy weather.
  • If the highest number is in the middle circle, this is a high pressure oranticyclone. Often high pressures mean dry, sunny weather.
  • Air pressure tends to range from 890 mb (a hurricane) to 1060 mb(an anticyclone).

The weather map below shows a low pressure, centred to the north of Scotland. As we move away from the centre the isobars increase in atmospheric pressure and become wider apart. There is a zone of high pressure over Italy in the southeast corner of the map.

Weather map of the UK

Weather map of the UK

 

Note the three different kinds of front shown on the map – a cold front, a warm front and an occluded front.

Fronts occur where two different air masses meet.

  1. Warm fronts are formed when warm air rises over a mass of cold air. As the air lifts into regions of lower pressure, it expands, cools and condenses the water vapour as wide, flat sheets of cloud.

    Warm fronts are shown on synoptic charts by a solid line with semicircles pointing towards the colder air and in the direction of movement. On coloured weather maps, a warm front is drawn with a solid red line with red semicircles.

  2. Cold fronts are usually associated with depressions. A cold front is the transition zone where a cold air mass is replacing the warmer air mass. The cold air is following the warm air and gradually moves underneath the warmer air. When the warm air is pushed upwards it will rain heavily. Often more rain will fall in the few minutes the cold front passes than it will during the whole passage of a warm front. As the cold front passes, the clouds roll by and the air temperature is cooler.

    Cold fronts are shown on synoptic charts by a solid line with triangles along the front pointing towards the warmer air and in the direction of movement. On coloured weather maps, a cold front is drawn with a solid blue line with blue triangles.

  3. Occluded fronts occur at the point where a cold front takes over a warm front or the other way around. If a cold front undercuts a warm front it is known as a cold occlusion and if the cold front rises over the warm front it is called a warm occlusion. Occluded fronts bring changeable weather conditions.

    On a synoptic chart occluded fronts are represented by semicircles and triangles positioned next to each other. The triangles are in blue and the semicircles are in red, or both are purple (mixing both red and blue colours together).

Isobars and fronts

Isobars are lines on a weather map joining together places of equal atmospheric pressure. On the map the isobar marked 1004 represents an area of high pressure, while the isobar marked 976 represents an area of low pressure.

Isobars on a weather map

Isobars on a weather map

 

  • The numbers measure the atmospheric pressure in millibars.
  • Usually isobars are drawn at intervals of two or four millibars (one thousandth of a bar).
  • The closer the isobars are together the windier it is.
  • If the lowest number is in the middle circle this is a low pressure ordepression. Often low pressures mean wet and windy weather.
  • If the highest number is in the middle circle, this is a high pressure oranticyclone. Often high pressures mean dry, sunny weather.
  • Air pressure tends to range from 890 mb (a hurricane) to 1060 mb(an anticyclone).

The weather map below shows a low pressure, centred to the north of Scotland. As we move away from the centre the isobars increase in atmospheric pressure and become wider apart. There is a zone of high pressure over Italy in the southeast corner of the map.

Weather map of the UK

Weather map of the UK

 

Note the three different kinds of front shown on the map – a cold front, a warm front and an occluded front.

Fronts occur where two different air masses meet.

  1. Warm fronts are formed when warm air rises over a mass of cold air. As the air lifts into regions of lower pressure, it expands, cools and condenses the water vapour as wide, flat sheets of cloud.

    Warm fronts are shown on synoptic charts by a solid line with semicircles pointing towards the colder air and in the direction of movement. On coloured weather maps, a warm front is drawn with a solid red line with red semicircles.

  2. Cold fronts are usually associated with depressions. A cold front is the transition zone where a cold air mass is replacing the warmer air mass. The cold air is following the warm air and gradually moves underneath the warmer air. When the warm air is pushed upwards it will rain heavily. Often more rain will fall in the few minutes the cold front passes than it will during the whole passage of a warm front. As the cold front passes, the clouds roll by and the air temperature is cooler.

    Cold fronts are shown on synoptic charts by a solid line with triangles along the front pointing towards the warmer air and in the direction of movement. On coloured weather maps, a cold front is drawn with a solid blue line with blue triangles.

  3. Occluded fronts occur at the point where a cold front takes over a warm front or the other way around. If a cold front undercuts a warm front it is known as a cold occlusion and if the cold front rises over the warm front it is called a warm occlusion. Occluded fronts bring changeable weather conditions.

    On a synoptic chart occluded fronts are represented by semicircles and triangles positioned next to each other. The triangles are in blue and the semicircles are in red, or both are purple (mixing both red and blue colours together).

Anticyclones

Anticyclones are the opposite of depressions – they are an area of highatmospheric pressure where the air is sinking.

  • As the air is sinking, not rising, no clouds or rain are formed. This is because as the air sinks it warms, meaning it can hold more water.
  • The absence of fronts means winds may be very light.
  • Consequently, high-pressure areas are often associated with settled, dry and bright conditions.
  • In summer, anticyclones bring dry, hot weather. In winter, clear skies may bring cold nights and frost.
  • In cold conditions, anticyclones may also bring fog and mist. This is because the cold forces moisture in the air to condense at low altitudes.
Synoptic chart of a typical summer anticyclone

Synoptic chart of a typical summer anticyclone

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