How to read

a weather map

What is a weather map?

The weather map is a chart showing the current climate while allowing us to predict future weather trends. It provides a simplified depiction of the current or predicted weather of an area. It displays a variety of meteorological features in a particular area at a particular moment in time. It uses a variety of symbols, which all have specific meanings, in order to indicate weather trends.

The most familiar map is the mean sea level analysis. This is compiled from hundreds of weather observations taken simultaneously. It is the map that is used daily on television and in the newspapers.

The dominant features of this map are the smooth, curving patterns of sea level isobars which show the central elements of our weather systems: highs, lows (including tropical cyclones) and cold fronts.

Isobars indicate lines of equal atmospheric pressure.

What do the symbols on the weather map mean?
Select a symbol

A crescent moon symbol means that it will be a clear night with no clouds so a good time to stargaze.

A sun symbol means a sunny day – a rarity in Ireland but always welcome.

A crescent moon covered by a fluffy cloud means we will have a partially cloudy night. These aren’t dark rain clouds so there is no need for your umbrella.

A sun partially covered by a cloud means we will have sunny intervals. Bring a jacket but you will be able to lose it here and there throughout the day.

A fluffy cloud with the word mist underneath it obviously means that we will have mist.

Clearly Fog is hard to draw, as the symbol for fog is simply the word with a frame around it.

A fluffy cloud means that it will be cloudy. However these are the nice white cotton candy clouds rather than horrible grey rain clouds.

Two fluffy clouds indicate scattered clouds.

A grey fluffy cloud image means that it will be overcast. Bring your brolly as it’s likely to at least drizzle.

A grey cloud with a drop of water or a broken blue line indicating water covering a crescent moon means light rain is due at night-time.

A grey cloud with a drop of water covering a yellow sun means light rain fall is due in the day time.

If the grey cloud with a droplet or a broken blue line indicating water coming from it has no sun or moon behind it then it just means that we will have light rain generally.

A grey cloud that has lots of small droplets coming out of it indicates that there will be drizzle generally throughout the day and night.

A dark grey cloud with two or three droplets of water or broken blue lines indicating water and a crescent moon behind it indicates a heavy night-time rainfall.

A dark grey cloud with two or three droplets of water or broken blue lines indicating water and a sun behind it means that we will have heavy rainfall during the day.

A dark grey cloud with two or three droplets of water or broken blue lines indicating water shows that we will have heavy rainfall generally.

A light grey cloud with a droplet of water and what resembles a white asterisk or a broken blue line and a broken white line with a crescent moon behind it means that we will have sleet showers at night time.

A light grey cloud with a droplet of water and what resembles a white asterisk or a broken blue line and a broken white line with a crescent moon behind it means that we will have sleet showers during the day.

A dark grey cloud with what looks like two white balls coming out of it and a crescent moon behind it shows that we will have hail showers at night-time.

A dark grey cloud with what looks like two white balls coming out of it and a yellow sun behind it shows that we will have hail showers during the day.

A dark grey cloud with what looks like two white balls coming out of it behind it shows that we will have hail showers.

A light grey cloud with what resembles an asterisk or a white dotted line coming out of it and a crescent moon behind it means that we will have light snowfall at night-time.

A light grey cloud with what resembles an asterisk or a white dotted line coming out of it and a sun behind it means that we will have light snowfall during the day.

A light grey cloud with what resembles an asterisk or a white dotted line coming out of it means that we will have light snowfall generally.

A dark grey cloud with what resembles two asterisk or a few white dotted lines coming out of it and a crescent moon behind it means that we will have heavy snowfall at night-time.

A dark grey cloud with what resembles two asterisk or a few white dotted lines coming out of it and a sun behind it means that we will have heavy snowfall during the day.

A dark grey cloud with what resembles two asterisk or a few white dotted lines coming out of it means that we will have heavy snowfall in general.

A dark grey cloud with a droplet of water or a broken blue line and a lightning bolt coming out of it and a crescent moon behind it indicates a thunder shower at night-time.

A dark grey cloud with a droplet of water or a broken blue line and a lightning bolt coming out of it and a sun behind it indicates a thunder shower during the day.

A dark grey cloud with a droplet of water or a broken blue line and a lightning bolt coming out of it indicates thunder showers generally.

Wind Symbols
Select a symbol

Wind speeds indicated by a white circle with an arrow at the top pointing in the wind direction. The arrow is always relative to true North. If the symbol is grey it means there will be gusts of wind.

Wind direction and mean speed

Maximum gust speed and mean wind direction

Wind direction and mean speed (Arrow size relative to wind speed)

UV Index
Select a symbol


UV Index 1 Low
No danger; you can stay safely outside, but you should still wear sunscreen.


UV Index 2 Low
No danger; you can stay safely outside, but you should still wear sunscreen.


UV Index 3 Moderate
You shouldn’t spend too much time in the sun unprotected, especially during midday hours when the rays are at their strongest.


UV Index 4 Moderate
You shouldn’t spend too much time in the sun unprotected, especially during midday hours when the rays are at their strongest.


UV Index 5 Moderate
You shouldn’t spend too much time in the sun unprotected, especially during midday hours when the rays are at their strongest.


UV Index 6 High
Seek shade, especially during midday hours. Cover up and wear sunscreen.


UV Index 7 High
Seek shade, especially during midday hours. Cover up and wear sunscreen.


UV Index 8 Very high
It is essential to cover up and spend as much time in the shade as possible, particularly between 11am and 3pm.


UV Index 9 Very high
It is essential to cover up and spend as much time in the shade as possible, particularly between 11am and 3pm.


UV Index 10 Very high
It is essential to cover up and spend as much time in the shade as possible, particularly between 11am and 3pm.


UV Index 11 Extreme
Avoid being outdoors as much as possible, especially during midday hours. Sunscreen, a hat and cover up is completely essential.

Air Pressure
Select a symbol

This indicates that the front is developing due to increase in temperature at the surface.

This represents the boundaries between air masses high above, but no change to the air mass on the surface.

This means that the front is weakening which is usually caused by rising pressure. There will be scattered clouds and rain.

Thickness lines measure the difference in height between two standard pressure levels in the atmosphere.

An elongated area of relatively low surface pressure, called a Trough.

A cold front, which is an advancing cold air mass. Its passage is marked by cloud and rain, followed by a drop in temperature and/or humidity.

This indicates that the front is developing due to increase in temperature at the surface.

This means a slow moving boundary between two air masses. This usually indicates cloud and rain.

An Occluded Front occur when a cold front catches up with a warm front. These fronts bring cloud and rain.

Isobars are lines which connect points with the same atmospheric pressure, measured in hectopascals (hPa). Anticyclones and depressions are marked by the letters H for high and L for low on weather charts.

Severe Weather Warnings

Each warning has a different colour. Green indicates no severe warnings. Yellow means you should be aware of the weather. Orange indicates you should be prepared for severe weather while red means you will have to protect yourself from extremely severe conditions.

They also sometimes use symbols beside the warnings as follows:


Snow


Fog


Rain


Wind

ISOBARS

  1. The movement of air from high to low pressures is measured on gradients. If the isobars are close together the difference between the areas of high and low pressure is greater and we have strong winds.
  2. When isobars form closed (but not always round) circles, the smallest circle in the center indicates a pressure center. This can be either a high-pressure or low pressure system. Air flows around these pressure gradients.
  3. Air moves around high pressure in a clockwise direction and low pressure in an anticlockwise direction, thus creating wind.

FRONTS

Fronts mark the boundaries between warmer air on one side and colder air on the other. If you are close to a front and it is moving towards you then you can expect a change in weather when the boundary of the front passes over you.

Cold front: This indicates potential torrential rain and high winds. It is illustrated by a blue line with blue triangles on one side. Whichever way the triangngles are pointing is the direction the cold front is moving.

Warm front: This often brings a gradual increase in rainfall which is then followed by prompt clearing and warming after it passes

Occluded front: This front usually brings drier air.

Stationary front: These types of fronts have long continuous rainy periods that linger for in one area for an extended period and move in waves.

Who relies on the weather forecast?
Select a symbol

SURFERS

  • Although you can surf in all weathers (within reason) they need to check what the waves will be like.
  • The harder the wind blows and the longer it blows for added to the distance it blows tells you what the waves will be like.
  • Slight will affect the waves and surfers study weather maps in order to decide if they should hit the surf on any day.

FARMERS

Weather is a hugely important for farmers, and accurate weather forecasting makes their lives so much easier, including

  • Planting
  • Harvesting
  • Irrigation
  • Tackling front (crops and livestock)

GARDNERS

  • Many seasonal plants require specific conditions to bloom and even the greenest fingered gardener can't save them if the weather goes awry.
  • If long periods of rain are forecast it will also affect the insect population so there will be less bees around which affects the pollination of crops.
  • Some plants specifically require good light and won't thrive in dull weather.

SAILORS

  • There are numerous elements that affect sailors including wind direction, wind speed, the state of the sea and also thunderstorms.
  • Getting caught out at sea in severe weather is extremely dangerous so getting all the advance information available is imperative for sailors.

FISHERMEN

  • Accurate forecasting can help make decisions and prepare before going out to sea.
  • Heading out in a boat during stormy weather leads to many lives lost every year.
  • High winds can also make it more difficult to cast nets. Long spells of bad weather seriously affect a fisherman's ability to make a living.

AVIATION STAFF

  • Bad weather can delay a landing & take-off.
  • Knowing in advance that there is bad weather on the route means that a pilot can re-route to avoid it.
  • In good weather they can take advantage of tailwinds to push the plane along, saving fuel.

OUTDOOR CONSTRUCTION WORKERS

  • Rain makes work impossible for road paving crews, roofers and painters.
  • High winds are extremely dangerous for anyone working on a height.
  • Foremen will plan the construction schedule around the long term forecast so they can finish the job as efficiently and cost effectively as possible.

How does the weather map affect you?

Knowing what the weather is like makes it far easier to get dressed for the day

UV rays play an important role in conditions such as premature skin aging, eye damage and skin cancers.

Knowing what weather to expect can help you adequately prepare so you get the most enjoyment from your pursuits.

A big rainstorm in the middle of tiling a roof could at best destroy your hard work and at worst flood your home.

A big problem for those living beside canals heavy rainfall can also cause drains to overflow flooding, even those homes that are miles from the water.

Look out for severe weather warnings and if possible change any plans you may have accordingly.

How is the weather map prepared and who does it?

The weather map is prepared by meteorologists who are basically weather scientists. The weather map is a simple representation of the vast amount of information they process.