25 July 2012 Last updated at 16:10

Understanding Forecasts

General information

Last updated: We receive hourly updates to forecasts for UK locations. For global and longer range UK 3 hourly information we receive updates every 6 hours. The time the last update was received is detailed at the top left of a forecast page.

Actual Forecast Location: We have recently increased the number of forecast locations to around 5000 in the UK. When you search for a location or UK postcode we provide the most meteorologically appropriate forecast.

In most cases it will be the closest geographical point, but sometimes the nearest forecast isn't the most accurate if you are searching for a coastal location or one at high altitude. In these instances we provide the most suitable alternative.

5 Day Forecast

Symbols indicate the predominant weather for the day or night in question, calculated based on a weighting of different types of weather. So if a day is forecast to be sunny with the possibility of a brief shower, then we will see a sunny or partly cloudy symbol rather than a rain cloud. For the purposes of this forecast, 'day' runs from 0400 to 1600 and 'night' from 1600 to 0400.

The maximum temperature is the highest temperature forecast between dawn and dusk, and the minimum temperature is the lowest temperature expected from dusk on the day in question to dawn the next day. Use the °C | °F buttons in Settings to switch the temperature between Celsius and Fahrenheit.

Wind speed and direction are the conditions expected at midday or midnight.

Wind direction is based on a 16 point compass: W, SW, SSW, etc. The wind direction states where the wind originates. Use the MPH | KM/H button in Settings to switch the wind speed between miles per hour and kilometres per hour.

Sunrise and sunset: There are many different ways to calculate the time of sunrise and sunset. In the algorithm we use, sunrise/set is considered to occur when the sun's upper limb is 35 arc minutes below the horizon.

Hourly and 3 hourly Forecast detail

Symbols indicate the weather for the stated time.

Temperatures are forecast for the stated time. Use the °C | °F button in Settings to switch the temperature between Celsius and Fahrenheit.

Wind speed and direction are the conditions expected at the time given.

Wind direction is based on a 16 point compass: W, SW, SSW, etc. The wind direction states where the wind originates. Use the MPH | KM/H button in My Weather to switch the wind speed between miles per hour and kilometres per hour.

Humidity levels indicates how much water vapour the air contains compared to the maximum it could contain at that temperature.

Pressure is measured in millibars (mb)

Visibility: Visibility based on whether the human eye can see the following distances:

Visibility description	Distance
Very poor	less than 1km
Poor	between 1km and 4km
Moderate	between 4km and 10 km
Good	between 10km and 20km
Very good	between 20km and 40km
Excellent	more than 40km

Environmental Summary

All indices are available for UK towns and cities, and the UV index is also available for some International cities.

UV Index

The UV Index shows the risk level posed by ultraviolet radiation from the Sun. It ranges from one, the lowest risk level, to eleven, the highest. The risk from UV is greatest when the Sun reaches its highest point in the sky around midday. Thick cloud can greatly reduce UV levels but certain types of thin cloud, surprisingly, can magnify the effect.

UV Index	Risk Level	Action Required
1 to 2	Low	None. You can safely stay outside.
3 to 5	Moderate	Take care during midday hours and do not spend too much time in the sun unprotected.
6 to 7	High	Seek shade during midday hours, cover up and wear sunscreen.
8 to 10	Very High	Spend time in the shade between 11 and 3. Shirt, sunscreen and hat essential
11+	Extreme	Avoid being outside during midday hours. Shirt, sunscreen and hat essential

Pollen Index

Our pollen index shows the risk from tree, grass and weed pollen and fungal spores. Weed pollens affect a small amount of sufferers from June to September with a peak in August. The pollen index changes emphasis slightly during the season to reflect the predominant types of pollen:

Tree pollen forecasts - late March until mid-May

Grass pollen forecasts - mid-May until July

Weed pollen forecasts - late June until September

Fungal spore forecasts - late August until end of November

Pollen Index	Risk Level
1 to 3	Low
4 to 5	Moderate
6 to 7	High
8 +	Very High

Air Pollution Index

The air pollution index uses a complex chemistry model to produce a forecast for five main pollutants: sulphur dioxide, carbon monoxide, nitrogen dioxide, ozone and PM10 (particulate matter of less than 10 micrometers). The index shows the combined risk from these pollutants.

Air Pollution Index	Risk Level
1 to 3	Low
4 to 6	Moderate
7 to 9	High
10	Very High

Our Pollen and Pollution Index systems use a four-point scale:

Index Level	Action required
Low Risk	Effects are unlikely to be noticed, even by those sensitive to air pollution.
Moderate Risk	Sensitive people may notice mild effects but are unlikely to need to take any action.
High Risk	Sensitive people may notice significant effects and may need action to reduce or avoid them, for example by spending less time outdoors.
Very High	Sensitive people may notice significant effects and should avoid spending to much time outdoors.

Maps

Time

Times on the UK maps are shown in GMT (or BST when applicable).

World maps are shown in an appropriate time zone for the area:

Continent	Time Zone
Australasia	GMT +10 hours
Asia - East	GMT +8 hours
Asia - South East	GMT +7 hours
Asia - South	GMT +5 hours
Middle East	GMT +3 hours
Europe - East	GMT +2 hours
Africa	GMT +1 hour
Europe - North	GMT +1 hour
Europe - West	GMT + 0
America - South	GMT -4 hours
America - Central	GMT -6 hours
America - North	GMT -6 hours

What do the maps show?

We have a combination of infra red satellite and radar images for observed charts, and rain, cloud, snow, fog and frost for forecast charts. The charts show 12 hours of weather prior to the current time and also the weather to be expected for the next 5 days.

What is the difference between a 'forecast' chart and the 'observations' chart?

The 'forecast rain', or precipitation, chart shows where rainfall or snowfall is expected to fall at the time shown. Rain is represented by the blue areas, while the white areas show snow.

The observation maps tells you about weather that has already happened. Radar is used to detect where there is moisture in the atmosphere (which could be rain, snow, hail and very occasionally a large cumulonimbus cloud). The 'echoes' received by the radar are shown on the 'observation' charts.

This gives an excellent indication of where rain (or snow or hail) is falling at a given time. The brighter the colours, the heavier the rainfall. This data is quality controlled, however there are some circumstances where the radar may indicate that rain is falling, but it may be drying up before it reaches the ground, and very occasionally it may still show a very large moist cloud!

What do 'pressure charts' show?

The white lines on the charts (isobars) join together places of equal pressure. The isobars are shown at intervals of 4 millibars. Isobar labels are currently only available on the Atlantic pressure chart at intervals of every three isobars. 'Low' and 'high' might be written at the centre of areas of low and high pressure.

The forecast information you see on our weather maps is updated four times a day (for the forecast for the next two days) in the UK or twice a day (for days 3-5 in the UK and all five days for non-UK locations).

Quality control of the data is carried out by the Duty Forecaster at BBC Weather Centre. Any changes made are sent to all of our outputs, so the data you see on the website is the same as that you see on TV.

Observations

Observed: We receive hourly observations for the UK and every three hours for the rest of the world. The time the last update was received is detailed here. Observations are made on the hour, so the weather information reflects the conditions at that time.

Temperature Use the °C | °F button in Settings to switch the temperature between Celsius and Fahrenheit.

Wind direction is based on a 16 point compass: W, SW, SSW, etc. The wind direction states where the wind originates. Wind speed is measured in miles per hour. Use the MPH | KM/H button in Settings to switch the wind speed between miles per hour and kilometres per hour.

Humidity levels indicates how much water vapour the air contains compared to the maximum it could contain at that temperature.

Pressure is measured in millibars (mb)

Coastal Forecast

This is a Met Office commissioned weather forecast for the inshore waters around the UK coast.

Detailed weather and sea state forecasts, for the next 24 hours in 4 time periods, are available for 24 sailing areas around the coast.

The data is sourced from the Met Office Mesoscale model and the UK Water Model.

The file is updated twice daily at 0500 and 1700. The figure in milliBars for pressure is followed by R (Rising), F (Falling) or S (Steady).

Shipping Forecast

Overview

This file is updated 4 times a day at or after approximately 0015, 0505, 1130 and 1725.

It gives a summary of gale warnings in force, a general synopsis and area forecasts for specified sea areas around the UK.

Area Forecast

Area forecasts for the waters around the United Kingdom are disseminated via NAVTEX transmitters at Cullercoats, Niton and Portpatrick and we publish these here.

They are broadcast (and published on our site) at approximately 0620/1820 (Portpatrick), 0840/2040 (Niton) and 0900/2100 (Cullercoats). These give a summary of gale warnings in force, a general synopsis and 24 hour forecasts plus and outlook for the following 24 hours.

Extended Outlook

Once a day an Extended Outlook is broadcast via each of the UK NAVTEX stations on 518 kHz. The extended outlook aims to signpost expected hazards for the Cullercoats, Niton and Portpatrick areas during a three-day outlook period beyond the period of the 24 hour forecast. We publish this file at 2200 each day.

High Seas Forecast

We publish at 0800 and 2000 GMT the text of these forecasts for the high seas Met Area 1 which are broadcast by the GMDSS Inmarsat EGC SafetyNET service twice a day at 0930 and 2130 GMT.

The bulletin is in three parts: storm warnings, general synopsis and forecasts for the sea areas. Storm warnings are broadcast at other times when necessary but these are not currently published on our site.

Warnings

We currently publish ad hoc Warnings and their associated cancellation messages as soon as we receive them on our site.

Previous gale or other warnings for other areas may still be in force. Old cancellations will only be over-copied when a new file arrives and a blank space will be left if we have received nothing.

Please always refer to the latest Shipping and Inshore Waters forecast, or other sources, including Coast Radio, NAVTEX and Coastguards, for more complete information.

In the Shipping Forecast, Gale Warnings (Force 8 and above) are issued whenever mean wind speeds are expected to reach or exceed 34 knots (Force 8), or gusts (other than in isolated thunderstorms) are expected to reach or exceed 43 knots.

Strong Wind Warnings (Force 6 and above) are issued when required for the Inshore Waters of the UK.

In the forecasts for the High Seas of the North Atlantic, Storm Warnings are issued when the mean wind speed is expected to reach or exceed 48 knots (Force 10 or above). These are not currently published on our site.