• Units
• Weather symbols
• Wind symbols
• Temperature symbols
• Solar UV symbols
• Rainfall radar imagery
• Satellite imagery
• Visibility definitions
• Pollen count
• Daily air quality index
• Severe weather warnings
• Pressure chart symbols

Units

Weather symbols

Weather symbols

Clear night	Sunny day	Partly cloudy (night)
Sunny intervals	Dust	Mist
Fog	Cloudy	Overcast
Light rain shower (night)	Light rain shower (day)	Drizzle
Light rain	Heavy rain shower (night)	Heavy rain shower (day)
Heavy rain	Sleet shower (night)	Sleet shower (day)
Sleet	Hail shower (night)	Hail shower (day)
Hail	Light snow shower (night)	Light snow shower (day)
Light snow	Heavy snow shower (night)	Heavy snow shower (day)
Heavy snow	Thunder shower (night)	Thunder shower (day)
Thunder	Tropical storm	Haze
No data

Wind symbols

Mean wind speeds are shown in a white symbol. The numbers in the circle indicate the mean wind speed in the units you have chosen (Miles per hour, Kilometres per hour or knots). The arrow points in the direction in which the wind is blowing. Wind gusts are shown in a grey symbol.

E.g.  indicates a mean wind of 7 mph, coming from the east.

Temperature symbols

The number in the square indicates the temperature in the units you have selected (Celsius or Fahrenheit). The squares are coloured depending on the temperature with lowest being blue and the warmest being red.

Solar UV Symbols

The strength of the sun's ultraviolet (UV) radiation is expressed as a 'Solar UV Index', a system developed by the  World Health Organization . These Met Office forecasts include the effects of:

• the position of the sun in the sky;
• forecast cloud cover;
• ozone amounts in the stratosphere.

The solar index does not exceed 8 in the UK (8 is rare; 7 may occur on exceptional days, mostly in the two weeks around the summer solstice). Indices of 9 and 10 are common in the Mediterranean area.

UV symbols

UV index	Exposure category	Protection required
	Low	None. You can safely stay outside.
	Moderate	Take care during midday hours and do not spend too much time in the sun unprotected.
	High	Seek shade during midday hours, cover up and wear sunscreen.
	Very high	Spend time in the shade between 11 and 3. Shirt, sunscreen and hat essential
	Extreme	Avoid being outside during midday hours. Shirt, sunscreen and hat essential

More about enjoying the sun safely

Rainfall radar

Satellite imagery

Visible images

Visible images record visible light from the sun reflected back to the satellite by cloud tops and land and sea surfaces. They are equivalent to a black and white photograph from space. They are better able to show low cloud than infrared images (low cloud is more reflective than the underlying land or sea surface). However, visible pictures can only be made during daylight hours.

Infrared images

The infrared image shows the invisible infrared radiation emitted directly by cloud tops and land or ocean surfaces. The warmer an object is, the more intensely it emits radiation, thus allowing us to determine its temperature. These intensities can be converted into greyscale tones, with cooler temperatures showing as lighter tones and warmer as darker.

Lighter areas of cloud show where the cloud tops are cooler and therefore where weather features like fronts and shower clouds are. The advantage of infrared images is that they can be recorded 24 hours a day. However, low cloud, having similar temperatures to the underlying surface, are less easily discernable.

Visibility definitions

Chance of rain/snow (probability of precipitation)

This is an indication of the likelihood of rain, sleet, snow, hail, drizzle etc (precipitation) falling.

PoP is given in the 5-day forecasts for specific locations. Each line of the table in the forecast represents a 3-hour period of the day, and the Precipitation Probability is given as a percentage (%) to the nearest 10% - which indicates how likely it is that any precipitation will fall during that 3-hour period at the selected location. More precisely, by "any precipitation" we mean at least 0.1mm, which is about the smallest amount that we can measure. Note that this does not mean the probability that it will be raining, snowing, hailing etc for the whole of the 3-hour period, only the probability that some precipitation will fall during that period.

So what does a PoP of 10% mean? This means that there is a 1 in 10 chance that precipitation will fall during this period. Another way of looking at this probability is that there is a 9 in 10 chance that it will stay dry. Similarly, a PoP of 80% means an 8 in 10 chance that precipitation will fall, and only a 2 in 10 chance that it will remain dry.

More about the science of Probability of Precipitation

Feels like temperature

Feels like temperature takes into account wind speed to give a more representative indication of how the temperature will feel. For example if the temperature is 0 °C it will feel much colder with a wind speed of 25 than with a wind speed of 10.

Feels like temperature is displayed in the units you have selected (Celsius or Fahrenheit). The squares are coloured depending on the temperature with lowest being blue and the warmest being red.

Pollen count

Pollen count forecasts are displayed using the following key:

More about pollen forecasts

Daily air quality index

^* Adults and children with heart or lung problems are at greater risk of symptoms. Follow your doctor's usual advice about exercising and managing your condition. It is possible that very sensitive individuals may experience health effects even on Low air pollution days. Anyone experiencing symptoms should follow the guidance provided in the 'Additional information on the effects of air pollution' section.

More about the Daily Air Quality Index

Severe weather warnings

Warnings are colour coded red, amber, yellow depending on the likelihood and potential impact of severe weather. Green is used to indicate no warnings are in force.

Symbol	Warning
	Rain
	Wind
	Ice
	Snow
	Fog

A detailed guide to severe weather warnings is available.

Surface pressure charts

	Cold front The leading edge of an advancing colder air mass. Its passage is usually marked by cloud and precipitation, followed by a drop in temperature and/or humidity.
	Warm front The leading edge of an advancing warmer air mass, the passage of which commonly brings cloud and precipitation followed by increasing temperature and/or humidity.
	Occluded front (or 'occlusion') Occlusions form when the cold front of a depression catches up with the warm front, lifting the warm air between the fronts into a narrow wedge above the surface. Occluded fronts bring cloud and precipitation.
	Developing cold/warm front (frontogenesis) Represents a front that is forming due to increase in temperature gradient at the surface.
	Weakening cold/warm front (frontolysis) Represents a front that is losing its identity, usually due to rising pressure. Cloud and precipitation becomes increasingly fragmented.
	Upper cold/warm front Upper fronts represent the boundaries between air masses at levels above the surface. For instance, the passage of an upper warm front may bring warmer air at an altitude of 10,000 ft, without bringing a change of air mass at the surface.
	Quasi-stationary front A stationary or slow-moving boundary between two air masses. Cloud and precipitation are usually associated.
	Isobars Contours of equal mean sea-level pressure (MSLP), measured in hectopascals (hPa). MSLP maxima (anticyclones) and minima (depressions) are marked by the letters H (High) and L (Low) on weather charts.
	Thickness lines Pressure decreases with altitude, and thickness measures the difference in height between two standard pressure levels in the atmosphere. It is proportional to the mean temperature of this layer of air, so is a useful way of describing the temperature of an airmass. Weather charts commonly show contour lines of 1,000-500 hPa thickness, which represent the depth (in decametres, where 1 dam = 10 m) of the layer between the 1,000 hPa and 500 hPa pressure levels. Cold, polar air has low thickness, and values of 528 dam or less frequently bring snow to the UK. Conversely, warm, tropical air has high thickness, and values in excess of 564 dam across the UK often indicate a heatwave.
	Trough An elongated area of relatively low surface pressure. The troughs marked on weather charts may also represent an area of low thickness (thickness trough), or a perturbation in the upper troposphere (upper trough). All are associated with increasing cloud and risk of precipitation.
	Convergence line A slow-moving trough, which is parallel to the isobars and tends to be persistent over many hours or days. They are quite common in cold northerly outbreaks down the Irish Sea, affecting west Wales, Devon and Cornwall in particular, but can be found in other areas also. This convergence line can give hours of persistent precipitation over very localised areas, whilst a few miles down the road it is relatively dry, leading to some heavy snowfall/rainfall. In summer the convergence lines are not as easy to forecast, but then can still occur due to sea-breeze convergence, and are over the land, whilst in winter they are over the sea.