Portsmouth and District Canoe Club Weather Reports

The articles on this page were originally published in the P&DCC newsletters between October 2004 and October 2005 (inclusive). They were all written by Dave R and are designed to break down some of the mystery in understanding why we get the weather we do. At the time of writing Dave was a Level 5 Sea Coach and had over 25 years in the sport. He has also been very much involved in surfing and inland waters. The information has been gained from a large number of sources as well as Dave's own findings and involvement in the weather service. Dave continues to enjoy sea kayaking throughout the UK and has almost managed to cover all of the Scottish Coast and highlands.

I thought I would put a series of short articles together on the weather because a number of club members have said to me that they find it difficult to understand weather forecasts and how they affect us as kayakers. My aim is to use the requirements of the 5* syllabus as a basis and to use some of the material I prepared for the short talks I have given to the club over the last few years. Each article will build on the information in the previous articles and I shall aim to keep each article to one and a half pages so that they are "digestible". So here goes…

Weather occurs because of the movement of air masses around the globe. They move because of the different temperatures of the air masses. The sun heats the earth and the hot air in contact with the earth expands and becomes less heavy. All air contains moisture; the hotter the air the more water vapour it can hold. But the heating of the earth is uneven so the hot air at the equator rises and flows towards the poles, where the air is cold. This circulation is not straight forward as the first figure shows. Some of the warm air descends at the sub-tropics and some of it continues to the poles. The colder, denser air at the poles tends to flow southwards (in the northern hemisphere). [I shall now confine the articles to what happens in the northern hemisphere.]

This movement of the air is further complicated by the spinning action of the earth [the Coriolis Force]. The rotation of the earth causes the air to flow in a north easterly / southwesterly direction. Remember that the direction of the wind is the direction from which it blows. In the temperate latitudes, where the UK lies, the polar air meets the warmer tropical air and this boundary is called the Polar Front. I shall say more about this later.

We are all well aware that the weather of the UK is very changeable. This is because of the position of the UK in relation to the large landmass of Europe. [Yes you guessed, it’s Europe’s fault!] To the west of the UK there is a large mass of water, and to the east of the UK there is a large land mass. Air that comes from the west, southwest or northwest, tends to be moist because it has been in contact with the sea. Air coming from the east, northeast or south east tends to be drier because it has been in contact with the land. The other important point is that air coming from the north tends to be cold whereas that from the south is warm.

So, if the air is blowing from the southwest it tends to be warm and moist, whereas air from the northeast tends to be colder and dry. These, of course, are generalizations and there are various complications to be superimposed on this generalization. Nevertheless, you can deduce the general weather pattern if you know the origin of the air blowing across the UK.

You need to remember that air blowing from Europe tends to be hot during the summer and cold during the winter. So an easterly air stream would be cold and dry in the winter and hot and dry during the summer.

In the next article I shall talk about the Polar Front and the formation of depressions.

In the previous article I referred to the Polar Front, which is the boundary between the warm air flowing from the equator and the cold air flowing from the polar region. This mixing of the air causes variable weather. It just so happens that the UK lies roughly at the same latitude as the Polar Front. I say roughly because the Polar Front lies between 40^o and 70^o N. During the summer it tends to move northwards and during the winter it moves southwards, in line with the UK. weather2-1.bmp (169474 bytes)

Because of the spinning of the earth the clash of these air masses is not "head on", but at an angle. This causes ripples in the Polar Front, which cause "depressions", which are centres of low pressure. The figure below shows the generation of a depression. The warm air tends to rise over the cold air in front, and cold air creeps below the back of the warm air.

It is important to realise that a depression is not a surface effect. The whole "body" of air in the depression can extend to a height of 12km above sea level. There are three kinds of motion in a depression; the cyclonic motion of the air in an anticlockwise direction around the centre of the low-pressure area; the upward movement of the warm air as it is drawn in at low levels; and the general eastwards movement of the whole system.

Depressions can travel at speeds of between 20km and 80km per hour. The upward movement of the warm air causes it to cool, become more humid and possibly cause precipitation [this could be rain, drizzle or snow] and the rotational movement causes a change in temperature as the depressions moves over a stationary position, namely you!

The boundaries between the different air masses, associated with a depression, are called "warm" and "cold" fronts. I shall say more about these in the next article. Each type of air mass brings different types of weather but it is probably the weather at the fronts themselves that is more important. weather2-2.bmp (98022 bytes)

One would expect that, generally, a wind would blow from high pressure to low pressure. Near to a depression this is not so. The depression is pushed along by the upper winds but at the surface with the ground or sea friction causes the wind direction to turn inwards and rotate around the depression. The figure illustrates this. When the depression comes across land the increased friction causes the wind to turn more into the depression. If you have your back to the [surface] wind then the center of the depression is to your left. If you are at B, facing south then you know the weather will improve. But if you are at A, looking north, then look out for worst weather.

A depression comprises an area of low pressure with a warm front closely followed by a cold front, and possibly an occlusion. What are warm and cold fronts, and what is an occlusion? On a weather chart they look like this. weather3-1.jpg (35824 bytes)

A warm front is the boundary between cold air and a following mass of warm air. It is shown on a weather chart as a solid line with small semi-circles on it; I think of these as melting icicles. The figure below shows a cross-section through a warm front.

The warm air rises above the cold air, which it pushes ahead. At the junction of the two air masses, the warm air is cooled, water vapour condenses, clouds develop and rain results. The first clouds heralding the warm front may be several hundreds of miles ahead of the front. Gradually the upper [cirrus] clouds thicken, medium levels clouds [alto stratus] appear and then the lower [cumulus] clouds fill the sky. By the time the front has obscured the sky rain is usually imminent.

A typical rain belt for a warm front may last 4 to 6 hours. As the front passes over there is an increase in the temperature, the wind may increase for a short time and it veers in direction [that is swings more to clockwise in direction].

A cold front closely follows on the footsteps of a warm front. As its name implies it brings cold air behind it. The cold air is pushing the warm air ahead of it and the cold air forces itself under the arm air. Once again, there is cooling of the warm air at the boundary of the two air masses, which causes clouds to form and rain. The cold air pushes the warm air so vigorously that tall cumulonimbus [thunder] clouds form. These can cause violent up-currents of air giving rise to heavy showers, thunder, hail and very squally winds. As the front passes the wind turns anti-clockwise [it backs].

Cold fronts have a very steep boundary and they are faster than warm fronts, so the squally weather that precedes them is usually short-lived, maybe an hour for it to pass over. As the front passes over it gives a sudden improvement in the weather. On a chart a cold front is a solid line with small triangles on it [like icicles]

When the cold front catches up with the warm front, a continuous belt of cold air lies under the warm air. There is the same sequence of clouds, with possibly some rain, but there is no warm break as the occlusion passes over. Occlusions indicate that the fronts are several days old and are becoming blurred and mixed. They have less "power" than the separate fronts and the weather changes are less sudden. They are shown on the chart as lines with alternate triangles and semi-circles on them.

Sea Kayaking and the Weather: Part 4 Next Item Back to Top
By Dave R
The Passage of a Depression and Anticyclones
In the previous articles I explained how a depression is formed and the characteristics of the warm and cold fronts that follow it. In the figure below I show a cross-section through a depression. Imagine you are to the right of the depression as it approaches. As it moves to the right the weather you will experience is shown in the chart. The upper chart shows a "synoptic" chart; the solid lines with numbers are the lines of equal pressure [isobars]. The closer the isobars are together, the stronger the wind. The warm and cold fronts are shown by the "icicles" and "melted icicles".

Some time later the cloud thickens, heavy rain comes in, possibly with hail and the wind can increase suddenly. Then the cold front passes over; the temperature drops suddenly and the wind also reduces and turns anticlockwise [backs] in direction.

	Ahead of the Warm Front	Passage of the Warm Front	Warm Sector	Passage of the Cold Front	Cold Sector
Pressure	Starts to fall steadily	Continues to fall	Steadies	Starts to rise	Continues to rise
Temperature	Quite cold, starts to rise	Continues to rise	Quite mild	Sudden drop	Remains cold
Cloud Cover	Cloud base drops and thickens	Cloud base is low and thick	Cloud base may thin and break	Cloud thickens (sometimes with large thunder clouds)	Clouds thin with some cumulus
Wind speed and Direction	Speed increases and direction backs	Veers and becomes blustery with strong gusts	Remains steady, backs slightly	Speed increases, sometimes to gale force, sharp veer	Winds are squally
Precipitation	None at first, rain closer to front, sometimes snow	Continues and sometimes heavy	Rain turns to drizzle or stops	Heavy rain, sometimes hail, thunder or sleet	showers

An anticylcone is a region of relatively high pressure producing light winds. In the summer the anticyclone brings good, stable, predictable weather; this is what we want for our expeditions. The cause is usually the "Azores High", which extends north eastwards to embrace our shores. In winter anticyclones bring cold air, usually from Europe [Yes its Europes fault again !] . The cold air also brings light winds and clear skies.

Sometimes we can have a short-lived "ridge" of high pressure which brings stable weather and clear skies for only a day or so, in between unstable weather from depressions. These are useful for those quick dashes across extended open water when you are trying to escape being pinned down on deserted Scottish islands !

The wind probably has more influence on the kayaker than any other aspect of the weather. The wind can slow you down, push you along, turn you sideways and it also causes the waves.

Waves are generally caused by the friction of the wind blowing over the water. The longer the stretch of water over which the wind blows [the fetch] then the bigger are the waves. Add to this the effect of the tides and the waves can become even bigger if the wind is blowing in the opposite direction to the tide. Wind strength is described in several ways; the table below relates the Beaufort wind scale to various other descriptors.

Predicting the effect of the wind on your paddling speed is not easy. I have reproduced below two sets of figures relating the kayak speed to the headwind speed, assuming an unassisted kayak speed of 3 knots.

With a tail wind the increase in speed depends on the design of kayak. As a rule of thumb expect your speed to increase by about 0.5 to 1 knot with a reasonably strong tail wind.

Winds can occur due to local effects. On a sunny day the convection of the air over the land can cause strong thermal upcurrents over the land. Air is drawn in from above the sea, causing on onshore wind. The reverse can occur later in the day or evening, as the land cools under a cloudless sky. As the side of a hill cools, so does the air in contact with it. The dense air slides down the hillside causing an offshore breeze.

To understand about the types of fog we first need to know a little about how heat is lost and gained. The sun heats both the sea and the land, but the land warms up quicker than the water. At night, when there is no sun light, the ground gives off heat. If there is a covering of cloud then the amount of heat lost from the ground is less than when there is a clear sky.

We also need to know about the dew point. Air can carry water vapour. The amount of water vapour it can hold depends on, amongst other things, the temperature of the air. The higher the temperature, the more water vapour it can support. The dew point is the temperature at which the air is holding the maximum amount of water vapour. If the air temperature goes below the dew point then some water vapour condenses as water droplets.

There are two types of fog we need to consider: radiation fog and advection fog.

When there is very little wind then the air in contact with the ground cools to the same temperature as the ground. If the ground cools at night, then so does the air in contact with it. When the air reaches its dew point then water droplets are formed. If there is little or no wind then the droplets form on any cold object, such as kayaks, tents and the ground. This is dew. If there is some slight air movement then the air is stirred up and the water droplets are retained in the air as small droplets – forming fog - as the air is moved upwards. Warmer air then moves close to the ground and the process is repeated. This way a considerable depth of radiation fog can develop.

Next morning the sun cannot penetrate the fog straight away. It warms the upper layers of the fog bank, gradually thinning the fog until it can penetrate to the ground. The ground then warms up quickly and, hey presto, the fog clears.

The cause of this type of fog is slightly different, but the effect is the same. Instead of calm air stagnating over a colder surface, travelling air [due to the wind] moves the warmer moist air to a surface that is already colder than the dew point of the travelling air. This surface could be land or it could be a water current coming from a colder region. Examples of this are the fog banks off Newfoundland or off California.

When the sea surface over which the air is travelling is large and consistently colder then thick fog banks occur. This type of sea fog is slow to clear and usually requires a radical change of wind direction to make it disperse.

Waves are caused by the wind. The stronger the wind, the bigger the waves and the greater the separation between wave peaks. Near to the atmospheric disturbance that causes them, the waves are confused and jumbled. As they move away from the disturbance the waves become more orderly and create regular parallel lines of swell. The distance between wave peaks [the wavelength] is about 10 to 20 times the height of the wave. It is important to realise that the wave is a disturbance moving through the water and not a mass of water moving away quickly.

The wave disturbance is not confined to the surface of the sea. The disturbance extends down to a depth about one half of the wavelength. So, if the wave is 2m high, the wavelength would be about 20 to 40m and the depth of the disturbance is 10 to 20m.

This applies to the open sea but what happened when the wave reaches the shore ? As the wave reaches shallower water the resistance at the sea floor causes the wave to steepen and the tip of the wave begins to overtake the main body of the wave. Eventually the wave becomes unstable and breaks. If the shore is gentle shelving then a line of regular breakers occurs. If, however, the shore is steep, then "dumping" waves can occur. These are waves that break suddenly on the beach with tremendous impact. They are very powerful, can smash kayaks and have a strong back tow that can hold the kayak, and the paddler, in the area of breaking water. BEWARE this can be a killer.

The Shipping Forecast is broadcast 4 times a day to assist commercial shipping. Radio broadcasts are at 0048hrs and 0535hrs [long wave and FM], 1200hrs and 1754hrs [long wave only]. The forecasts give details of: gale warnings; the general synopsis; sea-are forecast; and coastal station reports. The figure to the right shows the shipping areas.

The really conscientious expedition paddler might record the details of the shipping forecast on special pads, and then transfer the information to blank charts. After a few sessions this is not too difficult and it gives you an on-going record of the trends. The most important factor in taking down a weather bulletin from the radio is to keep a calm and "un-thinking" state of mind. Prepare yourself beforehand then when the forecast begins write without thinking; do not try to correct previous entries.

The coastal reports that accompany the shipping forecast give valuable information on the current weather. Note the trend of the pressure. If it is rising then there is the likelihood of good weather to follow. If it is falling then the weather is likely to deteriorate. It helps to use symbols rather than words for some parts of the forecasts. For example, pressure trends are signified by a line such as / or \. Wind direction can be represented by an arrow; rain is � ; showers are abbreviated to a small inverted triangle.

For the local paddler the Inshore Forecast is more useful. This comes out each day, following the shiping forecast at 0053hrs and 0540hrs local time on Radio 4 (198kHz, 1515m) and VHF. The forecast covers the weather for up to 12 miles offshore for areas around the UK shores. The areas are different to the Shipping Forecast Areas. The forecasts give details of : wind; weather; visibility; sea state; and the outlook for the following 24 hours. Actual weather reports from selected coastal stations are included with the 0540hr broadcast.

If you are on an extended expedition do not forget to use the local library at any town you visit. They now have internet access, even in Scotland, so you can gain access to all the above web sites during your expedition.

The Global Circulation of Air	Part 1
The Polar Front and Depressions	Part 2
Warm Fronts and Occlusions	Part 3
The Passage of a Depression	Part 4
Wind, How we describe it and its effects on the paddler	Part 5
Fog and the effects of the shore on waves	Part 6
The Shipping Forecast and Sources of Weather Forecasts	Part 7

Beaufort Scale	Speed		Description	Kayakers Guide
Beaufort Scale	Knots	mph	Description	Kayakers Guide
0	< 1	< 1	Calm
1	1 – 3	1 – 3	Light air
2	4 – 6	4 – 7	Light breeze	Suitable for beginners under instruction
3	7 – 10	8 – 12	Gentle breeze
4	11 – 16	13 – 18	Moderate breeze	Proficiency level kayakers should be OK
5	17 – 21	19 – 24	Fresh breeze	Above proficiency standard
6	22 – 27	25 – 31	Strong breeze	Advanced kayakers only
7	28 – 33	32 – 38	Near gale	Advanced paddlers wish they had got a weather forecast
8	34 - 40	39 - 46	Gale	Rely on the power of prayer !

Beaufort Scale	Kayak Speed in knots
	Dowd Scale	Ferrero Scale
0 –1	3.00 – 2.75	3
2 – 3	2.75 – 2.00	2.50 –2.00
4	2.5 – 2.25	1.5
5	2.25 – 1.5	0.5
6	1.5 – 0.75	-