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Sea & Weather (7)

Predicting imminent local weather

 

Shipwreck at low tide

 

"Mr Simple, I think we shall have some bad weather. The moon looks greasy and the stars want snuffing. You'll have two reefs in the topsails afore morning."
Peter Simple, by Captain Frederick Marryat, 1834

 

Sea kayakers don't really need to predict sunshine, rain, temperature or humidity. Even the sea state and ocean swell are seldom important except at an exposed beach. But we do need to know the likely strength and direction of the wind. See Beaufort Scale For Kayakers.

A professional weather forecast gives accurate regional predictions but weather can vary quite a lot over a distance of just a few kilometres, especially when a cold front is passing overhead. There is a lot to be said for "nowcasting" which means using what you can personally see, hear and feel to predict what is about to happen locally. If you get into the habit of keeping an eye on the sky you can draw useful conclusions from local wind, cloud and atmospheric pressure. Especially if you're willing to use.an altimeter / barometer to record atmospheric pressure every three hours.

Prediction from local observations

Look to windward for a good idea what weather you will experience in the next 30-60 minutes. Dark clouds and rain usually mean strong winds. However strong winds also come under cloudless skies so it is worth knowing more.

The origin of wind

Global winds
Some of the winds we experience are part of a planetary pattern which includes the Roaring Forties and the Trade Winds. The prevailing south-westerly winds in Britain are part of this pattern. See Global Circulation.

Low and high pressure systems
Convection currents over candleWhen air is heated, it expands and rises. At the bottom of any column of rising warm air is a zone of low pressure. From all around, cooler, denser air flows towards it and is sucked upwards. This familiar process of convection happens over a candle flame. It happens over a field of wheat on a sunny day, creating small fluffy cumulus clouds and thermals for glider pilots.

It may happen over half an ocean if cold water is penetrated by a warmer current. Air picks up warmth from the warmer water, expands, rises in a column, and cools at altitude until the moisture within it condenses into clouds. The Coriolis effect of the Earth's rotation makes the rising column of air rotate like water going down a plughole. Surface winds are created as air is sucked into the base of the column. A weather system arises which is called a low or, in Europe, a depression. Technically it's one example of a cyclone. Low pressure systems are strongly associated with clouds and rain.

A high pressure system (air descending from high altitude, high pressure, usually fine weather) is known as a high or anticyclone.

Local winds
Some winds are caused by local geography. In temperate climates with no nearby mountains they seldom exceed 5 knots, so on their own they are not enough to affect a kayak trip but they can turn a Force 3 wind into a Force 2 or 4.

Sea winds, also known as inflow winds, arise when the land heats up during the day. The air over it starts to rise, drawing in cooler air from the sea. At night the land temperature falls below that of the sea and the wind reverses.

In temperate Britain, the clear, calm weather of a summer anticyclone often encourages the development of a sea breeze of 5 knots by mid-afternoon.

Sea winds are strongest in the tropics but they can be powerful in places along the coast of any large land mass. According to the US Pilot Guide to the west coast "heating of the North American continent helps draw air into the Strait of Juan de Fuca [between Vancouver Island and the USA]. This sea breeze helps reinforce the prevailing [south-westerly] flow and results in winds up to 30 knots in the late afternoon".

Valley winds occur where a range of mountains stands alongside a valley. On a hot summer morning, the sun rapidly heats the air above the upper slopes of the mountain which becomes warmer than the valley. An anabatic wind starts to blow up the valley and up the sides of the mountains. Anabatic winds are usually quite gentle.

Later in the day, air temperatures in the valley increase above those on the mountain tops. Relatively cold, dense air sinks into the valley and carries on descending, down the valley towards the sea as a katabatic or fall wind. Especially if they come from a glacier or snow-covered mountains, katabatic winds can be powerful, causing sudden fierce winds far out to sea and making life interesting for sea kayakers in a river mouth.

In Britain, katabatic winds seldom exceed 5 knots. On the coast of British Columbia, Squamish winds sweep down fjords to the sea. In winter they arrive at the sea doing 35 knots and sometimes a lot more. In the area of Juneau, Alaska, the cold descending Taku wind can be powerful.

Probably the best-known katabatic wind in Europe is the Mistral, which is created by cold air from the Alps sinking into the Rhone valley, racing down the Rhone, over Marseille and south out to sea. Other similar winds are the Bora which blows from the Balkans south-west over the Adriatic, and the Meltemi which blows south over the Greek islands. They tend to start in the early afternoon and blow at Force 4-7 until sunset. The Sailing Directions for the North Atlantic & Mediterranean published by the National Geospatial-Intelligence Agency list thirty-seven other winds of this sort in the Mediterranean alone. Last time we looked, the Sailing Directions could be downloaded from this page on the www.nga.mil site.

Föhn winds. Not a problem for sea kayakers, but worth mentioning. Where mountains stand by the sea, it is well known that an onshore wind will be forced upwards until it sheds its moisture as cloud and rain. If the prevailing wind is onshore, the usual weather on a mountainous coast is overcast, wet and cool and the natural landscape is forest, boulders shaggy with moss, frequent steep rivers and patches of bog. However if the wind blows offshore the coast may be warm and dry because of föhn winds. Having already been forced up over the mountains from inland, wind reaching the sea is dry and perhaps as much as 10°C (18°F) warmer than on the other side of the mountains.

Atmospheric pressure

We live at the bottom of a deep sea of air which imposes on us a pressure of about 1 kilogram per square centimetre (14 pounds per square inch).

Atmospheric pressure is measured with a barometer and recorded in millibars (mb), otherwise known as hectoPascals (hPa). The first barometer was a long glass tube sealed at one end, filled with mercury and turned upside down. Its inventor, Torricelli, found that average atmospheric pressure at sea level would support a column of mercury nearly 30 inches high in the tube, and atmospheric pressure is still occasionally stated in inches of mercury (inHg).

Average pressure at sea level is defined as 1 atmosphere which is 1013 mb or 29.92 inHg.

Depressions are areas of low pressure which arise when warm air starts to rise above colder air alongside. Air moves constantly into any depression from neighbouring areas of higher pressure, in the form of wind. The greater the pressure difference, the stronger the wind.

The pressure at the centre of a deep depression may be 940 mb, a pressure difference of 70 mb which will create strong winds or gales. If the atmospheric pressure where you are is falling rapidly, it is a warning of imminent bad weather. Weather forecasts for shipping state the rate of pressure change, usually as the amount by which the pressure has fallen or risen over the last three hours. A pressure change of 0.1 to 1.5 mb in that period means isobars quite far apart, winds gentle or moderate. A decrease in pressure of more than 6 mb in three hours means isobars close together, winds strong and possibly getting much stronger. Some expedition sea kayakers keep an eye on the rate of pressure change by wearing a wrist altimeter, which is just a digital barometer.

High pressure systems also create wind but the greatest pressure is seldom more than 1030 mb, a pressure difference of only 15 mb. See The Classic Anticyclone below.

Just as contour lines on an ordinary map indicate a hill or valley by joining points of equal height, so the isobars on a meteorological map indicate depressions and anticyclones by joining points where the pressure is the same. If contour lines on a map are very close together they indicate a steep hill, with the slope descending at a right-angle to the contour. Isobars very close together indicate an intense pressure system with strong winds.

One might expect the wind to blow at a right-angle to the isobars, directly towards the lowest pressure. In fact it blows nearly parallel to the isobars. This is caused by the rotation of the Earth, and can be seen in the swirling spiral shape of clouds on satellite photos of a depression. In the northern hemisphere, the wind blows anticlockwise round the centre of a low and clockwise round the centre of a high.

Global circulation

Heated by the sun, air at the Equator constantly rises and spreads out at high altitude. It cools, descends at the Poles and blows back towards the Equator.

The convection pattern resembles that over a candle flame, but the Earth's rotation and differential heating of land and sea mean that it is complex and has seasonal changes. A lot of air from the Equator cools and descends long before it reaches the Poles, along parallel lines about 35 degrees north and south of the Equator. The pattern is further distorted by differential heating of central Asia in summer, resulting in monsoons. The result looks something like this. High pressure is shown in red, low pressure in blue, and continuous winds in black. The Poles are not shown.

Global circulation

There is always low atmospheric pressure at the Equator. There is always high atmospheric pressure at the Poles and in northern Canada. There are belts of high atmospheric pressure along the 35 degree lines or Horse Latitudes. Constant high pressure exists in the Atlantic around the Azores and St Helena; in the Pacific west of California and Chile; and west of Australia.

Winds blow constantly from high pressure to low pressure. Because the Earth rotates, these winds do not blow directly north-south but at an angle. This accounts for the westerly winds of the Roaring Forties, north Atlantic and north Pacific, and the easterly Trade Winds each side of the Equator.

Air masses

High altitude air descending to the Arctic is cold and will still be cold when it reaches Europe or Canada. Unless it passed over an ocean on the way, it will also be dry. That makes it a "polar continental" air mass.

High altitude air descending into the Azores High or the Pacific High gets warm and moist when it reaches sea level, which makes it a "tropical maritime" air mass.

In the northern hemisphere, polar air masses spread out southward until they meet a northbound tropical air mass. Because of their different heat and humidity, they don't easily mix. Warm tropical air tends to ride up over cold, dense polar air. The line where they collide is parallel to the Equator at about 50 degrees north, about the latitude of England and the US/Canadian border. It is called the polar front, and it is unstable.

Frontal waveRather than stay as a straight line, it often develops notches or "frontal waves". In the northern hemisphere (illustrated) a swirl of cold air penetrates towards the Equator and warm air is carried north. The black arrows show the direction of local winds at the front.

Where the tropical air meets cold Arctic air it rises, cools down and its moisture condenses into clouds. The lines where the warm and cold air masses meet are called fronts and there you will find cloud and wind. At a warm front, warm air is rising gently over cold air. On a weather map, a warm front is conventionally marked as a line with semicircles. At the cold front, cold dense air is wedging itself vigorously under warm air. This is marked as a line with triangles. If the cold air mass slides underneath and lifts the warm air right off the ground you have an occluded front, shown on weather maps as a line of alternate triangles and semicircles.

For most of us in temperate regions, the weather consists of one front after another. If a frontal wave deepens, the warm moist air may become the "warm sector"of a classic depression. The Earth's rotation causes depressions in the temperate latitudes of the northern hemisphere to spin anticlockwise and move slowly to the east (blue arrows on the synoptic map below) producing wind, cloud and rain.

Coastal weather in Europe

Climate is what you expect for the season, weather is what you get on the day.

Climate. The Atlantic coast of Europe is a cool, wet, windy place. The prevailing wind is south westerly (it blows from the south west). By the time it reaches the coast, it has picked up a lot of moisture which it drops when it reaches land and is forced upwards.

Unlike central and eastern Europe which have a continental climate with hot summers and cold winters, the Atlantic coast has a temperate climate with cool wet summers and warm wet winters.

Scotland is further north than Moscow, on the same latitude as Hudson Bay and Labrador. One might expect the sea to freeze in winter but in fact it is unusual for there to be even snow at sea level. This is one effect of a northern swirl of the Gulf Stream known as the North Atlantic Drift.

Air temperatures at sea level in Britain are generally within the range 3 to 23 degrees centigrade / Celsius, although they can spend a week as low as -2 or as high as 35 degrees. The north of Scotland is typically 3-5 degrees cooler than the south of England.

Sea temperatures are, in summer, 14 to 16 degrees centigrade in England and Ireland and about 13 degrees in Scotland. Winter sea temperatures around Britain are usually between 6 and 9 degrees.

Weather. The warmth has a price. The presence of the North Atlantic Drift in an otherwise cold ocean powers rising columns of relatively warm air.

Isobar chart of Atlantic depressionIn other words, depressions up to 5000 km wide. These rotate anticlockwise while moving slowly to the east (blue arrows), producing wind (red arrows), cloud and rain. They arrive on the coast of Europe one after the other.

Especially in high latitudes and on coast which is directly exposed to the Atlantic, the weather can change a lot in a short time.

Because northern Europe lies on the polar front it gets winds both from the Arctic and from the Azores. The origin of the wind, and whether it passed mainly over land ("continental") or sea ("maritime") on the way, determines its warmth and its moisture content. That makes the difference between a nice day and a bad one. An airstream reaching Britain from the south-west is tropical maritime. It is warm and laden with moisture picked up over the Atlantic. This means grey skies, mist and rain as it cools down. An airstream reaching Britain from the north east is polar continental. It brings cold dry air from Scandinavia and arctic Russia.

In winter, the Mediterranean is dominated by the temperate low pressure belt. Winter temperatures in the Mediterranean are not far off those of a British summer and there is frequent wind and rain. The Mediterranean summer climate is very different to that of northern Europe because the sun brings the subtropical high pressure belt northwards, which means constant warm dry weather.

The classic depression

Isobar chart of British depressionDepressions in temperate latitudes of the northern hemisphere slowly rotate anticlockwise while moving east.

The speed and direction of wind in the warm sector gives a good indication of the speed and direction of travel of the whole depression. It can move east at anything from zero to 60 knots but usually travels at 15 to 20 knots. So the weather you have today is more or less the weather they had yesterday 500 km to the west.

A depression is a complex three-dimensional shape. Surface winds are just the bottom layer of it. The isobars on a weather map show what is happening at sea level but above that is a column of rising air. Into the base of this is drawn a swirl of warm, damp air from the south. This makes up the warm sector of a depression. On a weather map, the warm sector classically takes the shape of a rose thorn. In the northern hemisphere the eastern edge of the warm sector is called the warm front, and the western edge is the cold front.

Each front forms a line from the centre to the outside edge, usually extending east and south from the centre of the depression. The centre of a classic depression passes to the north of Britain, so its warm and cold fronts bring cloud, rain and wind to the whole country. If the centre of a depression passes to the south of you, you may still experience cloud and rain but you will not get the strong wind and rapid change of wind direction associated with a cold front.

What do you feel and see as a depression passes over your location? If its centre is between you and the nearest Pole you will see a characteristic succession of clouds. On this diagram the warm sector is shown in pink; the warm front is the shallow gradient on the right; the cold front is the steep gradient on the left with the thunderstorm; and the blue arrow shows the direction in which the depression is travelling. The horizontal scale is 2500 km and the exaggerated vertical scale is 8000 metres.

Cross-section through depression

The classic signs of an approaching warm front are:

• Cirrus clouds (abbreviation Ci) and pressure starting to fall. Cirrus are high-altitude clouds of ice crystals, appearing as white wisps and streaks ("mares tails") against a blue sky. They are up in the jet stream, probably 750 km or more ahead of the warm front so on the weather map above, that is what you would see in southern Norway. Depressions travel in the same direction as the jet stream, and the streaks of cirrus point back to the centre of the depression. In the northern hemisphere, if you stand with your back to the surface wind and the cirrus clouds show that the centre of the approaching depression is to your left, the weather is about to deteriorate. This is the "crossed winds rule". Over the next 24 hours you are likely to see a continued steady fall in atmospheric pressure accompanied by...

• Cirrostratus (abbreviation Cs). Below cirrus but still quite high. Cirrostratus covers most or all of the sky in a very thin layer through which the sun can often be seen. On the weather map above, that's the weather in the North Sea. Then...

• Altostratus (abbreviation As). Below cirrostratus. Grey, overcast weather, perhaps drizzle. Then...

• Nimbostratus (Ns). Below altostratus. Low or very low rain clouds covering the sky in a thick grey or dark layer. On a television weather forecast, you will often see a north-south belt of continuous light rain 75 km wide, slowly heading east. This is the warm front. On the image above, it is over north-east Scotland. As it goes over you, the wind veers (shifts in a clockwise direction). Typically, a south wind becomes a west wind.

Between the warm front and the approaching cold front is the warm sector. This is a mass of warm moist air, steadily losing its heat to the surface and to the cold air on each side. As it does so, it cools down and sheds its moisture as haze, drizzle, low flat grey cloud (stratus or St), and mist or fog which is the same thing but at ground level.

After 1 to 12 hours the cold front arrives. You may see it coming, as a wall of dark cloud on the western horizon. The higher and darker the wall, the more violent the wind and rain. On the image above, it has just reached the west coast of Ireland where they can expect:

• Cumulonimbus (Cn). Tall, thick clouds which are low and often dark at the base. Expect an hour or so of heavy rain and strong winds. There may be tall, black clouds with thunder and very strong, gusty line squalls at sea level. As the front passes overhead, the atmospheric pressure starts to rise again, sometimes sharply. The air temperature drops. You will probably notice a sharp change of wind direction. Again, the wind veers, typically from south-west to north-west. Except for the rain, visibility improves. Then...

• Cumulus (Cu), cumulonimbus, stratocumulus (Sc). After the cold front there is often an unstable period of low thick cloud, wind and showery rain. Then...

• Altostratus. Grey, overcast, showery weather with an increase in atmospheric pressure. Then...

• Cirrostratus. And maybe blue skies.

Classically, atmospheric pressure will steadily rise for 24 hours after the cold front passes and then settle at a little more than 1000 mb. If it rises only slightly and then starts to go down, probably another depression is in the way.

The classic anticyclone

On a weather map, a high may be as wide as a depression but its isobars are usually further apart, indicating a smaller pressure difference and less wind. There is no warm sector, no fronts, and wind created by an anticyclone blows the opposite way to that in a depression - outwards. In the northern hemisphere, winds in an anticyclone blow clockwise. In the image above, an anticyclone is sitting over central Europe.

High pressure is often associated with long periods of calm dry weather. In winter, this classically means grey and overcast weather but in summer it means clear, sunny skies with a slight haze. Occasionally it brings fog in coastal waters.

A high pressure area established in a particular location will affect the weather by deflecting depressions so that they fade out or go elsewhere. There is almost always a high pressure area in the Pacific west of California. A high pressure area often establishes itself temporarily over Britain, Scandinavia or central or southern Europe.

Fog

Fog consists of water droplets condensing out of the air, which happens when relatively warm, moist air is chilled. It can be defined as cloud at ground level. Mist reduces visibility to between 1000 and 5000 metres; fog reduces it to less than 1000 metres, and thick fog may reduce it to less than 5 metres. Fog is common where a cold ocean current runs along a coast, as in Newfoundland, or where seasonal winds cause an upwelling of cold water as on the California coast.

On the European side of the Atlantic sea temperatures are much warmer and kayakers seldom encounter thick fog, although your editor recalls several occasions in south Devon when it came on in ten minutes, under a clear autumn sky. On one occasion it was so thick he could not see the front of his own kayak and all sounds were muffled. He kept on going through the cold grey and suddenly emerged into the brilliant sunshine of a warm September afternoon. Looking back, there was a boiling white wall of stationary fog, 15 metres high.

Fog happens when particularly warm moist air heads away from the tropics and meets cooler air or water. More often, it happens when air at an ordinary temperature and humidity is chilled. This can happen in many ways. Hill fog is common in the mountains, where wind is chilled when it is forced upwards by the rising ground. Fog in the lowlands occurs mainly in autumn and winter when clear skies allow the land to cool overnight. This "radiation fog" may blow a kilometre or so out to sea. However it tends to dissipate in contact with the sea, so there may well be good visibility a few metres offshore, and in any case it generally burns off by mid-morning.

True sea fog happens when moist air is suddenly chilled by contact with cooler water. It can happen at any time of year, but mainly in winter and early spring. In Britain it is particularly associated with the arrival in the English Channel and Irish Sea of warm, moist air brought by a south-westerly airstream, within the warm sector of a depression. A fog bank can arrive at 30 mph on a warm, still day under a blue sky.

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