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Help for Newbies with Charts etc !
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16-12-2010 12:05pm#1
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The Highs and Lows of UK Weather
The UK's mid latitude position between 50 - 60°N leaves it between the warm south and the cold north. The atmosphere is driven by a need to reduce this temperature gradient by sending warm air north and cold air south using cyclones and anticyclones, or the highs and lows which make up our 'synoptic-scale' weather, and which we see on the pressure charts.
When low pressure systems form in the western Atlantic, they pick up warm moist air which is then mixed with cooler air from the north as they move east across the top of the Azores high. The boundaries between these air masses are marked by fronts.
A typical anticlockwise rotating low centre will reach western Ireland and then curve north up the western Scottish coast. The warm front moves across the UK first, with warm moist air behind it bringing spells of rain and often reduced visibility with south to south westerly winds. The cold front chasing along behind is the boundary between this warm moist air, and the cooler more unstable brighter and more showery weather behind, with winds typically from the south west to north west.
The transition of these fronts is something the sailor has to take note of as the passage of these fronts over head will bring strong winds and occasionally heavy rain which can quickly turn a pleasant day's sail into a dash for shore.
High pressure usually follows these low pressure systems. These anticyclones are clockwise rotating sinking air masses which we associate in the summer season with light winds and sunny skies, and often building afternoon onshore or sea breezes.
The pressure imbalance created between the high and low pressure areas is what drives the wind as it tries to reduce this pressure gradient. Wind which we see on the water is, in the northern hemisphere, more backed (further left looking upwind) than this 500m pressure gradient wind due to the effect of friction or drag on the wind by the surface. The rougher the surface the more drag on the wind, i.e. winds are more backed over the land (20 - 40°) than they are over the smoother sea (10 - 20°). This is something to look for on the water when winds are blowing off the shore, as they will veer (clockwise, or shift right looking upwind) downwind of the shore.
Heat from the sun has a noticeable effect of the surface winds. How many mornings have you looked out the window to see calm conditions but by lunchtime the winds have picked up nicely? During the night the earth cools and so in turn it cools the air directly above it, which reduces its 'energy' or mixing and so reduces the wind speed. As the sun heats the earth, the air next to the surface begins to warm and so begins to rise and mix as cooler air sinks to the surface to replace it, which results in an increased wind speed.
Friction and heating are just two examples of the many factors which we must take into account when on the water. The forecasts we receive are often for a much wider area and there are small scale processes which affect the local winds. We will look at the forecasts themselves in part 2 and their strengths and limitations before we move on to have a closer look as to how the sailor can adapt them to their local shores.
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"And now the shipping forecast..."
There are many forecasts available for sailors before they set to sea. All these forecasts are written by the meteorologist using the most recent data available and is the best picture they can give at that time. It is always worth noting that most forecasts are often very general as they have to cover such a wide area, particularly the shipping forecast. Also land-based forecasts will often give wind speeds less than those that you will see on the water, due to many things, including friction as we discussed in the previous article, and will discuss in more depth later.
Reporting Wind Speed
Wind speed in mariners' terms is measured using the Beaufort Scale, each force corresponding to a wind speed range and in turn a water state.
Wind is also reported in miles per hour or kilometres per hour on many land based forecasts. So when looking at television and newspaper forecasts make sure you check the unit of measurement. Miles per hour is similar to knots (1 knot = 1.15 mph), but a forecast of 30 kph may sound strong but it would actually be a nice sail in 16 knots of breeze (1 knot = 1.85 kph). You can use the calculators on the BBC Weather site to convert these.
The BBC also use arrows, which are range from thin to thick as wind speed increases, and they also change in colour depicting warm or cool winds. Green or blue arrows depict cooler air which when coming down from the north flow over increasingly warm sea surface temperatures. The lower level air is heated which rises and is then replaced by cooler air aloft as it sinks which, on the water gives gusty conditions as the air is 'unstable' and 'turbulent'.
Rain later. Good.
In 1853 Captain Robert FitzRoy was put in charge of forming the Met Office to provide meteorological and sea current data to mariners. The Met Office continues to provide this information in the form of the shipping forecast and the inshore waters forecasts that you can listen to on BBC Radio 4 or read on this website. At first these seem quite daunting, especially when heard being read at pace on Radio 4, however once understood and read at one's leisure they can give you a good overall picture of what is currently happening and what to expect.
The shipping forecast gives a general synopsis which explains where the highs and lows are and where they are expected to travel to over the next twenty four hours. It then proceeds to go around British waters and give the wind direction and strength followed by weather and visibility. There are a few things which you need to remember; backing is anticlockwise and veering is clockwise, or left and right looking upwind. When wind directions vary cyclonically it means that the low centre is coming close by and wind directions will vary by a large amount in a cyclonic or anticlockwise direction.
Following the shipping forecast in the radio bulletins is the coastal forecast. This brings the synoptic scale down to a more local level. The areas are much smaller but the format is the same; wind direction and strength, weather and visibility. This forecast will take into account more of the effects of the land, such as where winds funnel and increase through channels, such as Dover in a westerly, and where winds will increase as they are forced around headlands, St Davids Heads in a south to south westerly, or decrease if in the lee of high ground such as Thames Estuary in a light north westerly.
Tides - When sailing in coastal waters, particularly those such as the Solent or Bristol Channel, you should always remember that a tide pushing you into the wind will add up to 3 or 4 knots onto the present wind speed, and subtract it when it is wind with tide. Check UK Tides on this site.
In addition to the shipping forecast there are many other sources of information. Next week we will look at where to find them, which are the most useful and also how to read the synoptic charts in more detail.
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Reading Between the Lines
You can use maps and charts to help you understand the weather and therefore to plan your sailing. These can be found on the almost endless internet sites providing weather forecasts, charts and observations from around the world. They range from giving a basic picture-based outlook for your area, to the actual current observations, to the model output itself. You are already reading one of the most organised and informative sites!
A good place to start is with the synoptic scale chart, or the pressure chart, which will give you an idea what the broad scale weather is and how it is likely to change over the next few days. Watch the weather on TV and follow the pressure charts through the week to see how fast things are changing; where are the fronts? How fast are they moving?
How strong will the winds be?
A mean sea level pressure chart shows the direct relationship between isobar spacing (pressure gradient) and orientation, and the strength and direction of surface winds. The general rule is that winds are strongest where the isobars are closest together. Thus the strongest winds are usually experienced near cold fronts and low pressure systems. Winds are normally light near high pressure systems where the isobars are widely spaced.
The synoptic chart shows lines of equal pressure to which the surface wind blows about 15 - 30° backed from parallel. To infer a wind speed from these lines, take your dividers or mark off on a piece of paper the distance between the two closet isobars to your sailing area. Then read off the wind speed at your latitude from the scale in the corner of the chart. This is the actual (if taken from an analysis chart) or the predicted wind speed at about 500m, the wind on the water is around 20 or 30% less.- The Met Office animates the charts but these do not have the geostrophic scale on.
- weather.noaa.gov - then click on SIGWX for the required time period
- westwind.ch - If you click on ALL it will take a while to download but you will be able to animate all the forecast charts.
Warm fronts and cold fronts are marked by red semicircles and blue triangles respectively on the synoptic weather charts. As the cold front catches up with the warm front it pushes it's way underneath the rising warm air and the air masses begin to mix, this is marked by an occluded front on the weather map, a semi circle next to a triangle.
One thing to look for when studying the synoptic chart is not only how close the isobars are which gives an idea of wind strength, but also by how much does the direction of the isobars change around the front. A strong 'V' around the front will indicate an active front, however if the isobars bend little around the front it is likely to be less active with lighter winds, less rain and not large veer behind the front.
One very good site to trawl round is westwind.ch, it's a collection of all the weather sites you could wish for, from written forecasts to the actual model output. The AVN model will give you a more detailed forecast of the wind over the course of the day. Wind speeds are displayed using wind barbs. The angle shows the direction and the speed is shown in 5 or 10 knot barbs, short or long respectively. The synoptic charts can also be found under UKMO and Brack.
As well as searching on the internet and watching the weather on TV, newspapers show the weather charts and radio, especially BBC Radio 4, give regular weather reports. Local radio stations often give the sea forecast and sea conditions particularly over the weekend or holiday periods. Once out on the water the coastguard relays the Met Office coastal forecasts, with an announcement on Channel 16 that will advise you which channel to tune into. All times and other sources of national and local forecasts and tides can be found in your almanac.
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Surface weather analysis
Weather map symbols:
1. cold front;
2. warm front;
3. stationary front;
4. occluded front;
5. surface trough;
6. squall/shear line;
7. dry line;
8. tropical wave
Main articles: Surface weather analysis, Low pressure area, and High pressure area
A surface weather analysis is a special type of weather map which provides a view of weather elements over a geographical area at a specified time based on information from ground–based weather stations.[4] Weather maps are created by plotting or tracing the values of relevant quantities such as sea-level pressure, temperature, and cloud cover onto a geographical map to help find synoptic scale features such as weather fronts. Surface weather analyses have special symbols which show frontal systems, cloud cover, precipitation, or other important information. For example, an H may represent high pressure, implying fair weather. An L on the other hand may represent low pressure, which frequently accompanies precipitation. Various symbols are used not just for frontal zones and other surface boundaries on weather maps, but also to depict the present weather at various locations on the weather map. In addition, areas of precipitation help determine the frontal type and location.[4]
URL="http://en.wikipedia.org/w/index.php?title=Weather_front&action=edit§ion=3"]edit[/URL Front types
URL="http://en.wikipedia.org/w/index.php?title=Weather_front&action=edit§ion=4"]edit[/URL Cold front
Main article: Cold front
Illustration of a cold front
A cold front is located at the leading edge of the temperature drop off, which in an isotherm analysis shows up as the leading edge of the isotherm gradient, and it normally lies within a sharp surface trough. Cold fronts can move up to twice as fast and produce sharper changes in weather than warm fronts, since cold air is denser than warm air and rapidly replaces the warm air preceding the boundary. On weather maps, the surface position of the cold front is marked with the symbol of a blue line of triangle-shaped pips pointing in the direction of travel, and it is placed at the leading edge of the cooler air mass.[2] Cold fronts come in association with a high pressure area. The concept of colder, dense air "wedging" under the less dense warmer air is often used to depict how air is lifted along a frontal boundary. This lift would then form a narrow line of showers and thunderstorms if enough moisture were present. However, this concept isn't an accurate description of the physical processes[5]; upward motion is not produced because of warm air "ramping up" cold, dense air, rather, frontogenetical circulation is behind the upward forcing.
URL="http://en.wikipedia.org/w/index.php?title=Weather_front&action=edit§ion=5"]edit[/URL Warm front
Main article: Warm front
File:Example of a warm front.avg Illustration of a warm front
Warm fronts are at the leading edge of a homogeneous warm air mass, which is located on the equator ward edge of the gradient in isotherms, and lie within broader troughs of low pressure than cold fronts. A warm front moves more slowly than the cold front which usually follows because cold air is denser and harder to remove from the Earth's surface.[2] This also forces temperature differences across warm fronts to be broader in scale. Clouds ahead of the warm front are mostly strati form, and rainfall gradually increases as the front approaches. Fog can also occur preceding a warm frontal passage. Clearing and warming is usually rapid after frontal passage. If the warm air mass is unstable, thunderstorms may be embedded among the strati form clouds ahead of the front, and after frontal passage thundershowers may continue. On weather maps, the surface location of a warm front is marked with a red line of semicircles pointing in the direction of travel.[2]
URL="http://en.wikipedia.org/w/index.php?title=Weather_front&action=edit§ion=6"]edit[/URL Occluded front
Main article: Occluded front
Occluded front depiction
An occluded front is formed when a cold front overtakes a warm front.[6] The cold and warm fronts curve naturally poleward into the point of occlusion, which is also known as the triple point.[7] It lies within a sharp trough, but the air mass behind the boundary can be either warm or cold. In a cold occlusion, the air mass overtaking the warm front is cooler than the cool air ahead of the warm front and plows under both air masses. In a warm occlusion, the air mass overtaking the warm front is warmer than the cold air ahead of the warm front and rides over the colder air mass while lifting the warm air.[2]
A wide variety of weather can be found along an occluded front, with thunderstorms possible, but usually their passage is associated with a drying of the air mass. Occluded fronts are indicated on a weather map by a purple line with alternating half-circles and triangles pointing in direction of travel.[2] Occluded fronts usually form around mature low-pressure areas.[2]
URL="http://en.wikipedia.org/w/index.php?title=Weather_front&action=edit§ion=7"]edit[/URL Stationary front and shearline
A stationary front is a non-moving (or stalled) boundary between two air masses, neither of which is strong enough to replace the other. They tend to remain essentially in the same area for extended periods of time, usually moving in waves.[8] There is normally a broad temperature gradient behind the boundary with more widely spaced isotherm packing.
A wide variety of weather can be found along a stationary front, but usually clouds and prolonged precipitation are found there. Stationary fronts either dissipate after several days or devolve into shear lines, but they can transform into a cold or warm front if conditions aloft change. Stationary fronts are marked on weather maps with alternating red half-circles and blue spikes pointing in opposite directions, indicating no significant movement.
When stationary fronts become smaller in scale, degenerating to a narrow zone where wind direction changes significantly over a relatively short distance, they become known as shearlines.[9] A shearline is depicted as a line of red dots and dashes.[2]
URL="http://en.wikipedia.org/w/index.php?title=Weather_front&action=edit§ion=8"]edit[/URL Dry line
Main article: Dry line
A similar phenomenon to a weather front is the dry line, which is the boundary between air masses with significant moisture differences. When the westerlies increase on the north side of surface highs, areas of lowered pressure will form downwind of north–south oriented mountain chains, leading to the formation of a lee trough. Near the surface during daylight hours, warm moist air is denser than dry air of greater temperature, and thus the warm moist air wedges under the drier air like a cold front. At higher altitudes, the warm moist air is less dense than the dry air and the boundary slope reverses. In the vicinity of the reversal aloft, severe weather is possible, especially when a triple point is formed with a cold front.[10] A weaker form of the dry line seen more commonly is the lee trough, which displays weaker differences in moisture. When moisture pools along the boundary during the warm season, it can be the focus of diurnal thunderstorms.[11]
The dry line may occur anywhere on earth in regions intermediate between desert areas and warm seas. The southern plains west of the Mississippi River in the United States are a particularly favored location. The dry line normally moves eastward during the day and westward at night. A dry line is depicted on National Weather Service (NWS) surface analyses as an orange line with scallops facing into the moist sector. Dry lines are one of the few surface fronts where the pips indicated do not necessarily reflect the direction of motion.[12]
URL="http://en.wikipedia.org/w/index.php?title=Weather_front&action=edit§ion=9"]edit[/URL Squall line
Main article: Squall line
A shelf cloud such as this one can be a sign that a squall is imminent
Organized areas of thunderstorm activity not only reinforce pre-existing frontal zones, but can outrun cold fronts in a pattern where the upper level jet splits apart into two streams, with the resultant Mesoscale Convective System (MCS) forming at the point of the upper level split in the wind pattern running southeast into the warm sector parallel to low-level thickness lines. When the convection is strong and linear or curved, the MCS is called a squall line, with the feature placed at the leading edge of the significant wind shift and pressure rise.[13] Even weaker and less organized areas of thunderstorms lead to locally cooler air and higher pressures, and outflow boundaries exist ahead of this type of activity, which can act as foci for additional thunderstorm activity later in the day.[14]
These features are often depicted in the warm season across the United States on surface analyses and lie within surface troughs. If outflow boundaries or squall lines form over arid regions, a haboob may result.[15] Squall lines are depicted on NWS surface analyses as an alternating pattern of two red dots and a dash labelled SQLN or SQUALL LINE, while outflow boundaries are depicted as troughs with a label of OUTFLOW BNDRY.
URL="http://en.wikipedia.org/w/index.php?title=Weather_front&action=edit§ion=10"]edit[/URL Precipitation produced
Main article: Precipitation (meteorology)
Convective precipitation
Fronts are the principal cause of significant weather. Convective precipitation (showers, thundershowers, and related unstable weather) is caused by air being lifted and condensing into clouds by the movement of the cold front or cold occlusion under a mass of warmer, moist air. If the temperature differences of the two air masses involved are large and the turbulence is extreme because of wind shear and the presence of a strong jet stream, "roll clouds" and tornadoes may occur.[16]
In the warm season, lee troughs, breezes, outflow boundaries and occlusions can lead to convection if enough moisture is available. Orographic precipitation is precipitation created through the lifting action of air moving over terrain such as mountains and hills, which is most common behind cold fronts that move into mountainous areas. It may sometimes occur in advance of warm fronts moving northward to the east of mountainous terrain. However, precipitation along warm fronts is relatively steady, as in rain or drizzle. Fog, sometimes extensive and dense, often occurs in pre-warm-frontal areas.[17] Although, not all fronts produce precipitation or even clouds because moisture must be present in the air mass which is being lifted.[1]
URL="http://en.wikipedia.org/w/index.php?title=Weather_front&action=edit§ion=11"]edit[/URL Movement
Fronts are generally guided by winds aloft, but do not move as quickly. Cold fronts and occluded fronts in the Northern Hemisphere usually travel from the northwest to southeast, while warm fronts move more poleward with time. In the Northern Hemisphere a warm front moves from southwest to northeast. In the Southern Hemisphere, the reverse is true; a cold front usually moves from southwest to northeast, and a warm front moves from northwest to southeast. Movement is largely caused by the pressure gradient force (horizontal differences in atmospheric pressure) and the Coriolis effect, which is caused by Earth's spinning about its axis. Frontal zones can be slowed down by geographic features like mountains and large bodies of warm water.[2]0 -
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homer_simpson wrote: »When you guys refer to charts as in Z6, Z12 ect, could someone explain this for me please?
00z = midnight
06z = 6 in the morning
12z = noon
18z = 6 in the evening
With regard to the model charts they refer to what time the computer models start a new updated run. That time can usually be seen on the top left corner of the chart and the time it is refering to on the top right.
for example the 18z GFS model started computing at 18:00 and was available on the various sites from about 21:30 ..
In the chart below the model run was started at 18:00 today and refers to Tue 21st at 6am
Or short version the Z means time lol
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