Kayarchy - the sea kayaker's online handbook and reference

 

Sea & Weather (7)

Predicting imminent local weather

Sea kayakers planning a trip 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 miles, 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. 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. This weather system is called a low. 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 an 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 climates during clear, calm summer weather weather there is often 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. 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.

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.

In Europe, probably the best-known katabatic wind 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 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 about 14 psi, or 1 kg/cm2, or 1 bar. It is defined as 1 atmosphere which is 1013 mb or 29.92 inHg.

Low pressure systems arise when warm air starts to rise above colder air alongside. Air moves constantly into any low pressure system from neighboring areas of higher pressure, in the form of wind. The greater the pressure difference, the stronger the wind.

The pressure at the center of a deep low 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 high and low pressure systems 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 low. In the northern hemisphere, the wind blows counterclockwise round the center of a low and clockwise round the center of an anticyclone.

Global circulation

Heated by the sun, air at the Equator constantly rises and spreads out towards the Poles. It cools at high altitude, 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 Canada or Europe. Unless it passed over an ocean on the way, it will also be dry, making 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 the latitude of the US/Canadian border, Newfoundland and England. 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 low. The Earth's rotation causes lows in the temperate latitudes of the northern hemisphere to spin anticlockwise and move slowly to the east producing wind, cloud and rain.

Coastal weather in North America

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

Ocean currents in North America

Climate

Cool currents on the west coast and mainly warm currents on the east mean there is a marked difference in sea temperatures in similar latitudes.

Thanks to the cool Alaska and California currents, most of the west coast has a temperate climate with cool summers and warm winters. Even north of Vancouver, snow seldom lies at sea level in winter.

The Great Lakes have a fairly dry "continental" climate with hot summers and cold winters.

The eastern seaboard has a range of climate from sub-tropical in Florida where the Gulf Stream runs alongside and onshore winds are very often "tropical maritime" from the Azores High, to sub-arctic in Newfoundland which is dominated by the cold Labrador current.

Weather

There are free authoritative downloads about regional weather patterns on the Internet. For the USA, you can download NOAA Coast Pilots and for Canada there are NAV CANADA Local Area Weather Manuals. We think it's worth giving a very basic introduction to weather systems, starting with the fact that in North America they travel west to east.

S. Alaska & British Columbia. See NAV CANADA's Weather In British Columbia which you can download here, and the Guide to Interpreting Environment Canada's West Coast Marine Forecasts which you can download courtesy of the Oregon Ocean Paddling Society. Last time we looked the link was here.

The whole of the Inside Passage from Seattle to Skagway gets some shelter from winds and swell behind a chain of islands. The coast in the lee of mountainous Vancouver Island is particularly sheltered. The island also creates a rain shadow so that annual rainfall of 3 meters in Tofino on the west coast becomes only 1.15 meters at Nanaimo on the east.

Coastal low in British ColumbiaThere is almost constant low atmospheric pressure over the Pacific ocean west of BC and Alaska, known as the Aleutian Low.

A succession of low pressure system crosses the ocean and reaches the coast. Lows are deeper in the Gulf of Alaska. Coastal lows reaching the southern and central BC coast are less predictable and often deepen suddenly into bombs as they reach the coast. They can change in as little as 9 hours from a very weak system to a severe storm.

Winters are stormy, with swell running more than 12 feet high for 20% of the time and 20 feet for 4% if the time. However in summer the Pacific High extends north to the latitude of Vancouver Island, resulting in stable weather for southern BC and sending low pressure systems and their strong winds north into the Gulf of Alaska and northern BC.

On summer afternoons, the land heats up more than the sea and there are significant inflow winds up most inlets and valleys. Inflow winds can be strong in the Strait of Juan de Fuca, Portland Inlet, Howe Sound and in the Hope area.

Inland temperatures are high in summer and low in winter but at the coast they are moderated by the ocean. Surface sea temperatures around Vancouver range between about 60°F (16°C) in summer and 40°F (4°C) in winter. Even in midwinter, air temperatures in southern BC seldom drop far enough to give more than a few days of snow at sea level.

Washington to California. Detailed weather description in US Coast Pilot here. There is almost constant high atmospheric pressure over the ocean between Hawaii and California (the Pacific High) which means a stable coastal climate. It shelters the California coast from the wind, but the entire west coast is the lee shore for the Pacific ocean so expect big waves on exposed sections. The Sea of Cortez / Gulf of California is sheltered by the Baja Peninsula.

Cool currents run along most of the Pacific coast, keeping surface sea temperatures generally within the range 45 - 68 °F (07 - 20°C) all year. Surface sea temperatures in parts of southern California can reach 72°F (22°C) in summer.

Winter gales and rough seas are common on the north and central sections. Strong winds and high seas can be expected several times a month in winter in northern California but are rare south of Los Angeles.

North from Oregon the coastal climate is wet, especially in winter, as the prevailing winds have passed over the ocean and picked up a high moisture content. The Pacific high deflects eastbound low pressure systems north to British Columbia and Alaska.

Great Lakes. Detailed weather description in US Coast Pilot here and NAV CANADA's The Weather In Ontario & Quebec here. The climate is continental (hot summers, cold winters) but moderated by the presence of the Great Lakes themselves.

This is one of the most active areas for migratory weather systems, and the weather changes very fast with the arrival of lows originating anywhere from the Gulf of Mexico to Alberta. See The Classic Low, below. The Great Lakes basin lies very near the polar front so lows import air masses alternately from the Arctic and the Gulf of Mexico. The origin of the wind determines its warmth and its moisture content and makes the difference between a good day and a bad one.

Thunderstorms are quite common in July and August, and for much of the winter the Lakes get fog at warm fronts and strong winds at cold fronts. Lows particularly affect the lower lakes in spring and the upper lakes in the fall, when there may be intense storms and low temperatures with the arrival of a polar air mass. The US Coast Pilot comments "Autumn is dangerous; clear crisp days are often interrupted by rapidly intensifying low pressure systems whose gale-force winds can whip tumultuous seas..." There is surfing around the shores of the Lakes on waves generated by local winds, especially in the colder months.

Water temperatures in summer are highly variable. In Lake Superior, except for shallow bays and beaches, the surface water temperature is usually below 57°F (14°C) even in summer. Lake Michigan is a few degrees warmer. Apparently the western end of the shallow Lake Erie can reach 75°F (24°C).

In mid-winter, water temperatures are 32-39°F (0-4°C). Polar air masses frequently reach the Lakes, causing very low air temperatures and sometimes a low layer of "sea smoke" fog. Shore ice can be expected around all the lakes and Lake Erie can freeze over altogether.

Eastern seaboard. Detailed weather description in US Coast Pilot here. As with the Great Lakes, much of the east coast is on the track of low pressure systems moving west to east and producing changeable weather.

Temperatures on land are moderated by the presence of the ocean. Sea temperatures at Miami beach are 70°F (21°C) in winter and 86°F (30°C) in summer. Off New York they are 36°F (2°C) and 72°F (22°C). In Maine they are 34°F (1°C) to 62°F (17°C) or higher in sheltered areas. The cold Labrador current causes extensive sea ice as far south as the Gulf of St Lawrence from the first half of December until the middle of April. Some midwinter shore ice can be found in some sheltered inlets as far south as Chesapeake Bay.

The classic low

Isobar chart of British depressionWhere air masses of different temperature and humidity meet, fronts arise.

Over north America there is usually a chaotic pattern of warm and cold fronts, all travelling east. Sometimes there is a fully-developed low pressure system like the one shown approaching the Great Lakes on this weather map.

As the earth rotates, low pressure systems in the northern hemisphere spin counterclockwise and move slowly to the east (blue arrows), producing wind (red arrows), cloud and rain.

The speed and direction of wind in the warm sector gives a good indication of the speed and direction of travel of the whole system. It can move east at anything from zero to 60 knots but usually travels at 15 to 20 knots.

A low pressure system 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 low pressure system. 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 center to the outside edge, usually extending east and south from the center of the low pressure system. At a front you find cloud, rain and wind. If the center of a low 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 the fronts of a low pressure system pass by? If its center is between you and the 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 low is travelling. The horizontal scale is 1500 miles (2500 km) and the exaggerated vertical scale is 25,000 feet (7600 meters).

Cross-section through low

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 500 miles or more ahead of the warm front. Low pressure systems travel in the same direction as the jet stream, and the streaks of cirrus point back to the center of the system. In the northern hemisphere, if you stand with your back to the surface wind and the cirrus clouds show that the center of the approaching low pressure system 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. Then...

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

• Nimbostratus (Ns). Below altostratus. Low or very low rain clouds covering the sky in a thick gray or dark layer. On a television weather forecast, you will often see a north-south belt of continuous light rain 50 miles wide, slowly heading east. This is the warm front. 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 gray 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 Minneapolis 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. Gray, 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 low pressure system is in the way.

The classic anticyclone

On a weather map, a high may be as wide as a low pressure system 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 low pressure system - outwards. In the northern hemisphere, winds in an anticyclone blow clockwise. In the image above, an anticyclone is sitting over Florida.

High pressure is often associated with long periods of calm dry weather. In winter, this classically means gray 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 low pressure systems so that they fade out or go elsewhere. A high pressure area often establishes itself temporarily over one part of the country. There is almost always a high pressure area in the Pacific west of California which dominates the weather for the whole west coast.

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 metres (0.62 miles) and 5000 metres (3 miles); fog reduces it to less than 1000 metres, and thick fog may reduce it to less than 15 feet.

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 mile or so out to sea. However it tends to dissipate in contact with the sea, so there may well be good visibility a few yards 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 is common where a cold ocean current runs along a coast, as in Newfoundland and right along the Pacific coast. On the California coast it occurs when seasonal winds cause an upwelling of cold water. Sea fog can happen at any time of year and a fog bank can arrive at 30 mph on a warm, still day under a blue sky.

 

 

 

 

 

 

Go to next page for:
• Professional Weather Forecasts

go to previous page go to top of page go to next page
About Kayarchy | Full Contents | Legal Notice & Privacy Policy | Get In Touch | ©2010-2017 N Waller