Key
Units
Weather symbols
Wind symbols
Temperature symbols
Solar UV symbols
Rainfall radar imagery
Satellite imagery
Visibility definitions
Pressure chart symbols
| Units |
| Wind |
Miles per hour (m.p.h.) |
| Direction |
16-point compass
(S, SSW, SW, etc.) |
| Temperature |
Degrees Celsius (°C) |
| Pressure |
Hectopascal
(hPa) |
| Visibility |
Kilometre (km) |
| Height |
Metre (m) |
| Rainfall |
Millimetre (mm) |
Snow
accumulation
|
Centimetre (cm) |
| Sunshine |
Hour (hr) |
| Humidity |
Percentage (%) |
| Weather symbols |
Clear sky (night) |
 |
Low-level cloud |
 |
Sleet shower (night) |
 |
Light snow |
 |
Sunny |
 |
Light rain shower (night) |
 |
Sleet shower (day) |
 |
Heavy snow shower (night) |
 |
Partly cloudy (night) |
 |
Light rain shower (day) |
 |
Sleet |
 |
Heavy snow shower (day) |
 |
Sunny intervals |
 |
Drizzle |
 |
Hail shower (night) |
 |
Heavy snow |
 |
Dust |
 |
Light rain |
 |
Hail shower (day) |
 |
Thundery shower (night) |
 |
Mist |
 |
Heavy rain shower (night) |
 |
Hail |
 |
Thundery shower (day) |
 |
Fog |
 |
Heavy rain shower (day) |
 |
Light snow shower (night) |
 |
Thunder storm |
 |
Haze |
 |
Heavy rain |
 |
Light snow shower (day) |
 |
Tropical storm |
 |
Medium-level cloud |
 |
Not available |
 |
|
|
|
|
| Wind symbols |
On the wind maps, the number in the circle indicates the wind
speed in miles per hour, and the arrow points in the direction
the wind is blowing. Gusts in miles per hour are shown in the
red box beside the wind arrow.
E.g.  indicates a mean wind of 12 m.p.h., coming from the south, gusting
24 m.p.h. |
| 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 Index |
|
|
|
|
|
| Exposure category |
Low |
Moderate |
High |
Very high |
Extreme |
| Protection required |
None. You can safely stay outside. |
Seek shade during midday hours, cover up and wear sunscreen. |
Avoid being outside during midday hours. Shirt, sunscreen and hat essential. |
More about enjoying the sun safely |
|
| 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 |
| Description |
Range |
| Unknown |
- |
| Very poor |
Less than 1 km |
| Poor |
Between 1-4
km |
| Moderate |
Between 4-10 km |
| Good |
Between 10-20
km |
| Very good |
Between 20-40 km |
| Excellent |
More than 40 km |
| Pressure chart symbols |
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 gives 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. |
|
|
Downpour (more than 16 mm per hour)
Very
heavy (8 to 16 mm per hour)
Heavy (4 to 8 mm per hour)
Moderate (2 to 4 mm per hour)
Moderate (1 to 2 mm per hour)
Slight (0.5 to 1 mm per hour)
Very
slight (less then 0.5 mm per hour)
