Glossary
- 1. A Guide to the Systems Graphics
- 2. A Guide to Meteorological Terms used in the system.
- 3. Met Office Services
1. A Guide to the Systems Graphics
(a) Bespoke Regional Observed and Forecast
This map is updated every 30 minutes and shows the observed weather state during the last hour followed by the latest forecast out to 5 hours beyond the current time, each at 15 minute intervals, for your club and immediate vicinity. If any rain or snow is expected you will see various colours indicating the likely intensity. Light blue would infer a few drops or very light drizzle, blue to green show moderate rainfall, while the more prominent colours of yellow, red up to white indicates much heavier rainfall likely to disrupt your match.
Cloud is also forecast and where the underlying map is clear and bright then there will be minimal cloud cover. Thicker cloud will make the detail progressively less clear and darker. Major towns and your Golf Club are overlaid to ensure a point of reference is maintained.
Next to the map there is a “dashboard”. Here, details of the forecast weather elements and symbols, at your Golf Club’s exact location, will be constantly updated, also at 15 minute intervals. So you will see the likely cloud cover, intensity of rain, temperature, wind speed and direction, humidity and other weather phenomena in a simplified format.
Warnings of Lightning and Torrential Rain will flash up in a warning triangle for the forecasts. This is an advisory as the precise location of thunderstorms is very difficult to predict ahead of time but is a good guide to potential activity. Actual lightning strikes are detected in real-time by our system and are plotted on the regional observed maps, shown during the first hour of the regional map sequences. It is therefore very useful to look at the regional observed sequence when thunderstorm activity is forecast. Remember lightning causes thunder so whenever you hear a rumble you can be certain there is lightning present.
Other warnings, for Fog and Frost, will be developed in time for the winter.
(b) UK Observed
The observed rainfall and cloud conditions over the UK at the time shown are animated over the last 6 hours in 15-minute steps.
Satellite data indicating the cloud amount, thickness and cloud-top temperatures are combined to show the observed cloud state over the British Isles.
Clear skies are indicated where the surface topography or sea can be seen clearly. During the daytime, the surface here is bright and illuminated by sunshine (parts of eastern Wales and western Ireland in this example). Low cloud is shown with dark shading which partly obscures the surface (much of central and north-east England). Thin high cloud is shown with light grey-white shading which weakly obscures the ground (eastern Ireland). Areas of deep, thick cloud which reach high into the troposphere and which are typically associated with heavier rainfall are shown with light-grey-white shading which almost completely obscure the background surface (parts of Scotland).
Observed regions of radar-detected rainfall are plotted on top of the cloud shading using a colour scale which indicates rainfall intensity (sky blue to blue for light rain, green to yellow for heavier rain and red to white for torrential and cloudburst rainfall). The UK and Ireland radar network has a geographically limited coverage, the edges of which are indicated by the light grey circular outlines, which may occasionally change in cases when one or more of the radar stations fails to transmit data at a specific time. The Observed UK images always show the current radar coverage at each image time.
(c) Three and Five day Forecasts
UK 3-day Forecast
This forecast animation is unique because it provides the forecast in hourly segments throughout the forecast period (see bar on screen). This is intended to capture the general dynamic of the weather developments over the British Isles, but not to make specific predictions such as “there will be a shower at exactly 2pm in three days time at a given location”. Experience of the system will enable users to predict with accuracy the likely weather regime over a particular location.
In this example forecast chart, a deep low pressure system (986 mb) is centred over Eastern England. Low pressure is indicative of rising air and that generally means unsettled weather with showers or longer periods of rain, often heavy. The chart shows areas of cloud and showery rain circulating around it in an anti-clockwise direction.
The yellow and green dots represent a region of heavy and persistent rain affecting Eastern Scotland with a shallow arc extending south through the lake district to South Wales where the small blue spots indicate light rain.
The lines around the Low centre (Isobars) indicate wind speed. The closer they are the stronger the wind. Here light to moderate SE winds (light blue arrows) to the north of Newcastle increase to become strong E’ly winds over Aberdeen and northwards (purple and dark blue arrows). Over the Southwest of England it is dry with a fresh north-westerly wind and in the Southeast another area of light rain with light to moderate SSW’ly winds.
The wind direction and speed are indicated by the arrows, which move with the wind. The arrows are distributed evenly throughout the map. When arrows get too close to others in some region, they are faded out and placed into a different region which is arrow-sparse. New arrows are grown into the animation smoothly after which their size and colour indicates the strength of the wind - sky blues for light winds, dark blues for moderate winds, moving through purples for strong to gale force winds and finally red for storm force winds.
Temperatures are shown in the coloured circles, dark blue is cold, and red, hot. On this map, for example, the temperature over Stornoway is forecast to be 9 degrees C.
For an explanation of the cloud, raindrops, pressure contours and temperature disks in this animation, see the Euro 5 day glossary below.
Euro 5 day Forecast
Like the three day chart, this is a dynamic representation of the likely weather regime on a larger European scale. This example shows High Pressure (designated by the letter H) dominating from the Baltic to the North Sea. High pressure generally means descending air and largely prevents convection. Therefore it is likely to be dry. Consider the atmosphere as having a lid on it, that means that if the situation persists it can get very hazy and encourage poor air quality. Any precipitation is likely to be very light and more likely in the winter when the lid encourages low level cloud to form that is not burnt off by the sun.
The lines are isobars, joining places of equal pressure. The colours indicate the pressure, ranging from darker to lighter blues for low pressures, through white then yellow, orange and reds for increasingly high pressures. The closer these lines are the windier it is. In the Northern Hemisphere winds always blow clockwise around an area of High pressure. In this example the winds over Stockholm are therefore N’ly, and over Dublin SSE’ly. The centres of specific pressure systems are indicated by the symbols "L" or "H", together with the central pressure values. These symbols track smoothly with the moving systems and shrink away when a system decays and can no longer be tracked.
Rainfall is depicted through coloured dots (like miniature billiard balls) which fall slowly downwards and leftwards with time, simulating raindrops. Small light blue drops represent light rain or widely dispersed showery rain (less than 1 mm per hour). Regions with heavier or more persistent rain use larger dots whose colour changes to greens, yellows, oranges and finally reds for rain rates of up to 12mm per hour or more. Precipitation falling as snow is coloured white (North of St Petersburg and Northern Scandinavia), while sleet is shown with a smaller white dot within the coloured rain dot. The chart therefore shows an area of heavy rain over the Balkans and Iceland and another off Iberia. A ribbon of mainly light rain extends from Kiev to Vienna and light showery rain over France and Northern Spain.
The background map shows the underlying topography and is brightly coloured during the day, representing the effect of sunlight, and darkly coloured at night. An accurate solar zenith angle calculation is performed to show the progression of night and day across the map at the different times. A number of major cities are also plotted on the map.
Cloud amounts are represented by dark and light patches of shading over the background map. Cloudy conditions, including low cloud, are shown as dark patches which partially obscure the background. Regions where conditions are bright, but where there is a lot of high cloud in the sky are shown with light grey shading over the background. In the example, the UK and most of Northern Europe is forecast to be sunny, with thick cloud over Russia, Northern Scandinavia, and to the West of Ireland and hazy high cloud over Morocco and Spain.
(d) The Graphic Tab
This graph, updated twice per day (early morning and early evening), shows the expected hourly progression of various key weather elements over the next 24 hours at your club. Hourly rainfall totals (bar chart elements), wind speed and direction (circles with windspeed in mph and a direction arrow) and temperature (coloured circles with a value in degC) are shown for each hour of the forecast. The data comes from the Met Office’s very high resolution NAE model, designed specially to provide a very accurate forecast for the British Isles
2. A Guide to Meteorological Terms used in the system.
Areas of Pressure. Within the atmosphere air pressure is judged to be high or low relative to the surrounding air. As a rule of thumb High Pressure brings stable conditions and depending upon position can bring long, warm, dry periods in the summer, and cold temperatures in the winter. Low pressure brings instability, typically Thunderstorms and heavy rain in the summer, and periods of prolonged and heavy rain in the winter. The more intense (deeper) the Low the worse the weather, particularly the winds.
The strength of the wind is generally proportional to the spacing of the isobars. The closer together they are the stronger the wind, although thermal effects can also cause local increases in wind speed, such as daytime sea-breezes near the coast.
Cloud. Clouds are water droplets that can be either liquid or solid (ice crystals). Liquid water droplets can occur at temperatures much lower than zero. On the observed chart the cloud is detected through its temperature. Very cold cloud temperatures indicate frozen droplets and means cloud can extend to perhaps 30,000 ft or more. This is shown as white, whereas warmer (lower) cloud is shown as grey. On the regional forecast charts we have simplified the cloud by making it dark grey where there is going to be significant cloud cover.
Fog. See Humidity. Fog is caused by fine droplets of moisture condensing out in a layer above the ground surface, defined as having a horizontal visibility below 1000m. When the visibility is above 1000m it is referred to as mist. Haze is when visibility is less than 7km but the air is relatively dry (lower than 95% humidity) and this may be caused by fine particles from industrial and transport pollution.
Frost. Although apparently simple to define it is in fact very complicated. There are two principle types, Hoar and Rime, and both an air and ground frost. An air frost occurs when the temperature falls below freezing at head-height (a few feet above the ground). A ground frost occurs when the temperature directly over the ground surface falls below freezing. For the golf club the latter is important as it can damage greens and fairways if walked or driven over.
After calm and clear nights the air temperature on the ground can sometimes become several degrees colder than at head-height. So, even if the air temperature a few feet over the ground is above freezing, a surface- or rime frost can occur. This generally disappears quickly after daybreak when thermal air movements break down the vertical temperature contrasts. A hoar frost, on the other hand, which occurs when the air is generally below freezing, even at head-height, may last much longer due to the frost penetrating into the ground surface.
Humidity. This is effectively a measure of the dryness of the air. We show the relative humidity which is the proportion of the air with saturated moisture. Relative humidity generally drops as the temperature rises during the day and rises when it becomes colder at night, since warmer air can hold more moisture than colder air. 100% humidity means that the air is holding as much moisture as possible; any further moisture then condenses out as droplets. Thus, saturated air can lead to mist and fog when the temperature falls. High humidity also means there is less opportunity for evaporation and that is why you might sweat more on warm and humid summer days. High humidity in summer may also indicate the development of thunderstorms.
Lightning. The Met Office uses the Arrival Time Difference to determine the location of lightning strikes. There are seven ATD location points, from Iceland to Cyprus and by timing the radio waves associated with lightning it is possible to determine the location to within 2½ km
Radar. There are currently 17 weather radar installations in the UK and Ireland with two more proposed. Their positioning means the whole of the UK is covered, and they can detect rainfall, intensity and precipitation type (drizzle, rain, hail, sleet and snow). The Weatherfor system uses a composite of all these radars, while the Met Office can provide the underlying data down to 1km resolution for each site.
Rain. Commonly described as frontal or convective, frontal rain is associated with a front which is a line separating two different air masses. Generally a large scale feature it can produce persistent rain for several hours from thick layered clouds. Convective rainfall is showery in nature and comes from “cumulus” type clouds, that vary in appearance from fluffy pieces of “cotton wool” to towering CB’s that create torrential rainfall, thunderstorms and even tornadoes.
On the Weatherfor system we have created special graphics that merge the radar imagery and provide detailed information relating to timing, location and intensity. Forecast rainfall is produced by the unique Met Office NowCast model and we apply the same graphics to provide the detail for your club.
Temperatures. A temperature forecast indicates the likely air temperatures at approximately head-height (a few feet above the ground) in the shade at the given location. How warm or cold it actually feels depends of course on several factors and not just on the air temperature. It may feel much warmer than the pure air temperature on calm and sunny days, especially above a dark, tarmac surface, or when the humidity is high. On the other hand, strong winds with rainfall and cloudy skies may make it feel much colder than it really is. The Weatherfor system includes these important extra factors in its forecast to help users gauge the total likely weather experience.
Thunderstorms. Their violence is equal to their complexity, often bringing periods of torrential rain or perhaps hail with damaging squally wind gusts in some cases. They develop in unstable air and from either towering convective cloud (Cumulonimbus) or unstable medium cloud (often produces cloud to cloud lightning strikes). The Weatherfor system uses the ATD system (see lightning) and this mainly detects cloud to ground strikes.
Wind. In the Northern Hemisphere wind blows anticlockwise around a low pressure centre and clockwise around a high pressure centre, its strength determined by pressure gradient (hence often called the “gradient wind”). Air movements try to reduce the pressure gradients, but the earth’s rotation deflects the wind so that it blows along the isobars. However, they do not blow exactly along the lines of the isobars due to surface friction and point slightly away from High pressure and slightly towards Low pressure. On the Weatherfor system we have allowed for this to provide an accurate wind forecast.
Thermal effects can also cause local winds, which are less easily predicted accurately. Sea breezes mainly occur when there is a large temperature difference between land and sea and the gradient wind is light. Severe gusts can occur near to thunderstorms due to rapid vertical air movements as hot and cold airmasses mix in the developing convective clouds. In extreme cases, violent winds may occur and tornados may be spawned.
3. Met Office Services
(a) SynopVis
The SynopVis meteorological visualisation software is a new and sophisticated display system, developed by Dr. Paul James, a research- and product development scientist at the Met Office. It can produce graphical images and smooth video animations with a very high rendering quality from a wide-range of data sources and has several specialized and unique features.
Meteorological fields can be plotted over clear and detailed topographical surface maps for any global region, with subtle layering techniques being deployed for a high visual impact. Flexible feature tracking and wind-vector animation methodologies enable smoothly flowing video sequences to be created which are visually pleasing and powerful for educational and communicative goals.
The Weatherfor system is the very first commercial system to exploit the power of SynopVis. The software was extended specially to enable the display of observed and forecast rainfall and cloud fields on the regional and UK maps, while the 3- and 5-day forecast sequences deploy SynopVis’ most advanced refinements to yield one of the most stunning renderings of any weather forecast yet seen.
SynopVis 2008