Marine Weather (from a Sailor's Perspective)

I am not a meteorologist, but as a sailor, I do recognize the importance of having an appreciation for the science.

Red Sky at Night, Sailor's Delight

In the mid latitudes, weather systems generally move from west to east. A red sky in the west, as the sun sets, indicates that the atmosphere between you and the sun is essentially clear.

If you can see through the atmosphere that is west of you, you are likely looking at high pressure and a stable air mass.

What you are seeing is very likely tomorrow’s weather.

The sky appears red because the shorter blue wavelengths are filtered out by the thicker atmosphere, while the longer red wavelengths are able to get through.

Red Sky in the Morning, Sailor take Warning

In the mid latitudes, weather systems generally move from west to east. A red sky in the west, as the sun sets, indicates a clear atmosphere between you and the sun.

On the other hand, a red sky in the east at sunrise suggests that the clear weather has passed and the odds are, that the not so nice weather is moving in from the west.

Just as with the sunset, the sky appears red because the shorter blue wavelengths are filtered out by the thicker atmosphere, while the longer red wavelengths are able to get through.

The Atmosphere

Life on earth depends on earth’s atmosphere, a thin layer of gases that envelop the planet. Weather occurs in the troposphere, which extends from mean sea level to an altitude of about 12 kilometers. To fully appreciate the thinness of the troposphere, think of some landmark, i.e. a town, a bridge, a river that is located about 12 km from your location and imagine tipping that distance up vertical. Mount Everest, the world’s highest peak is extends 8.8 kilometers into the troposphere. Even though the troposphere represents less than half of earth’s atmosphere, it contains 99% of the atmosphere’s water.

Solar radiation heats earth’s atmosphere and the temperatures at the earth’s surface are maintained by the atmosphere. The mean temperature on the surface of planet earth is 140 celsius.

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We live in the troposphere. Weather happens in the troposphere.

Solar Heating

We have all felt solar radiation. While the ozone layer of the upper atmosphere absorbs most of the harmful ultra-violet radiation, we do receive enough solar energy at the surface to heat the land and the water on the planet.

Most of the energy arriving to earth’s atmosphere is reflected back into space. “Albedo” latin for “whiteness” is a measure of the reflection of solar radiation, where a black surface (0) absorbs all incident radiation and a white surface (1) reflects all incident radiation. The surface of the earth will heat at different rates depending on it’s colour. Freshly fallen snow could have an albedo as high as 0.9, while forests range from 0.1 to 0.2.

Land heats up more quickly than water.

Besides the earth’s reflectivity, there are other factors that impact the absorption of solar heat at the surface. The earth’s rotation means that earth loses heat during the night time. The area around the equator receives the most direct radiation, while the poles receive the most indirect. In our annual trip around the sun, the earth’s tilt of 23.5 degrees produces the seasons, which in turn heats and cools the hemispheres seasonally.

The uneven heating of the earth’s surface produces a couple of other important variables. Firstly, the earth’s atmosphere behaves like a liquid and flows. Secondly, the ocean is a liquid and flows. The horizontal movement of air masses and ocean currents has a significant impact on local weather.

The Hydrological Cycle

Source: National Oceanic and Atmospheric Administration, U.S. Department of Commerce

More that 70% of the earth’s surface is covered by water and approximately 97% of the earth’s water sits in the oceans.

When we think of the “air”, we don’t always consider that it contains water in some form. Less than o.5% of the earth’s water is in the atmosphere at any given point in time.

That brings us to the water cycle or the hydrological cycle, which describes the interaction of  the atmosphere with the surface water. Evaporation and transpiration puts water vapour into the atmosphere. This water vapour can move within the atmosphere, both horizontally and vertically. When the vapour is condensed into droplets, gravity will bring bring it back to earth in the form of liquid water, solid water or any other combination.

Water in “the air” (the atmosphere) exists in three different forms;

Water Vapour

Humidity refers to the level of microscopic suspended water droplets in “the air”. There is always some level of water vapour in “the air”. Warm air can hold more water vapour than cold air.

Relative humidity measures the amount of water vapour in the air as a percentage of the maximum amount that it could hold at a given air temperature. If relative humidity reaches 100% for a given air temperature, then that temperature is known as the dew point temperature. When the relative humidity reaches 100%, any cooling of the air will cause some of the water vapour to condense into liquid water.

Liquid Water

Water, in it’s liquid form is much more obvious. It is the ocean, it flows in the rivers and it sometimes falls as rain. Evaporation and transpiration puts water into the atmosphere, while the condensation of water vapour in the atmosphere forms water droplets which comes down as rain.

Ice

Water, in it’s solid state is also obvious, especially for those of us who live in the mid or high latitudes. As air temperatures drop below 00 celsius, water vapour becomes ice crystals and water droplets become snow.

Precpitation

Source: National Oceanic and Atmospheric Administration, U.S. Department of Commerce

Most of the stuff that falls out of the sky can be called precipitation;

Rain

We are all familiar with rain. It is the drops of water that falls out of the sky from time to time. To become water droplets, the water vapour in the air must merge together (coalesce). When the relative humidity reaches 100%, any cooling of the air will cause some of the water vapour to condense into liquid water. 

Drizzle

Somewhere between rain and fog. Very small drops of water that are barely affected by gravity. Yes, the drizzle is coming down, but very slowly.

Fog

Even smaller drops of water that are in suspension. Kind of like being inside a cloud on the ground. From a boating perspective, fog means reduced visibility. Heavy fog can reduce visibility to near zero. If fog is in the forecast, the visibility due to fog is generally part of a marine forecast. Fog happens when air containing water vapour is cooled to it’s dew point.

Snow

When water vapour freezes, ice crystals are formed. As these ice crystals float around through different temperature regions, they can merge together, become snowflakes and fall to earth.

Freezing Rain

Liquid water droplets (rain) falling on a surface that has a temperature that is below freezing will cause the droplets to freeze. Under certain conditions, water can be cooled to temperatures well below freezing and still remain in it’s liquid form. In this case, the “supercooled” water droplets will instantly freeze, when it hits a surface that is below freezing. Freezing rain over time could result in a buildup of ice.

Freezing Drizzle

Much like freezing rain. Ice buildup tends to be on the windward side of objects.

Hail

Hail is a product of thunderstorms. When a water droplet 

More about Fog

Most of the stuff that falls out of the sky can be called precipitation;

Rain

We are all familiar with rain. It is the drops of water that falls out of the sky from time to time. To become water droplets, the water vapour in the air must merge together (coalesce). When the relative humidity reaches 100%, any cooling of the air will cause some of the water vapour to condense into liquid water. 

Drizzle

Somewhere between rain and fog. Very small drops of water that are barely affected by gravity. Yes, the drizzle is coming down, but very slowly.

Fog

Even smaller drops of water that are in suspension. Kind of like being inside a cloud on the ground. From a boating perspective, fog means reduced visibility. Heavy fog can reduce visibility to near zero. If fog is in the forecast, the visibility due to fog is generally part of a marine forecast. Fog happens when air containing water vapour is cooled to it’s dew point.

Snow

When water vapour freezes, ice crystals are formed. As these ice crystals float around through different temperature regions, they can merge together, become snowflakes and fall to earth.

Freezing Rain

Liquid water droplets (rain) falling on a surface that has a temperature that is below freezing will cause the droplets to freeze. Under certain conditions, water can be cooled to temperatures well below freezing and still remain in it’s liquid form. In this case, the “supercooled” water droplets will instantly freeze, when it hits a surface that is below freezing. Freezing rain over time could result in a buildup of ice.

Freezing Drizzle

Much like freezing rain. Ice buildup tends to be on the windward side of objects.

Marine Weather Services in Canada

The primary source of Marine Weather Services in Canada is the Department of Environment and natural resources. Marine forecasts are available for all of Canada’s coastlines and waterways at: https://weather.gc.ca/marine/index_e.html.

In addition to marine weather forecasts, the department also publishes a National Marine Weather Guide as well as Regional Marine Weather Guides geared to the different regions of Canada. These guides provide very valuable information about conditions that are unique to certain parts of the country.

Global Weather

 

The wind and waves are always on the mind of the mariner. Wind and waves are generated by the weather. Earth’s weather occurs in the troposphere, which extends from the earth’s surface to an altitude of about 17 kilometers (in the mid-latitudes). Weather in the mid-latitudes is results from the interaction between the cold polar air mass which normally sits over the North pole and the warmer sub-tropical air found in the mid-latitudes. The colder polar air is drier and heavier, while the sub-tropical air is moist and lighter.

Now, let’s throw in a few more variables like daytime solar heating, nighttime cooling, the spin of the earth, just to name a few.

It is essential that the mariner has a good understanding of some basic meteorological concepts.

Atmospheric Circulation

Like the ocean, the earth’s atmosphere is a fluid (it flows). And like the ocean, the atmosphere has currents.

Currents in the atmosphere are effected by two main factors.

The first is solar radiation. The solar heating of the atmosphere is uneven for many reasons, but the two main reasons are the angle of the earth’s surface to the sun’s radiation and  earth’s rotation. The heat absorbed by the earth’s surface is most significant at the equator and least significant at the poles. The earth’s rotation means there is daytime heating and nighttime cooling. As fluids, both the oceans and the atmosphere are continuously compensating for the resulting temperature differentials. As a result, we have the wind, the ocean currents and a rivers of air in the atmosphere called the jet streams.

 

The Tropics

As the atmosphere heats up at the equator, the air becomes less dense and rises. The less dense air is replaced by denser air moving in from the south and from the north. The resulting circulation in the atmosphere is called the “Hadley Cell”.

The resulting surface winds in the tropics (northern hemisphere) are typically northerly and are deflected to the west by the Coriolis effect.

Image courtesy of Tinka Sloss

Source: https://www.windows2universe.org/earth/Atmosphere/hadley_cell.html

The earth’s rotation (Coriolis effect) deflects the Hadley cell upper winds to the East and surface winds to West. These surface winds are commonly referred to as the “Trade Winds” and the “Northeast Trade Winds” in the Northern Hemisphere. They are also known as the tropical easterlies and are the prevailing winds in the tropical regions.

The Mid-latitudes

As we move north (or south) of the “Hadley Cell”, we find a another circulating cell called the “Ferrel Cell”. The main distinction is that the “Ferrel Cell” circulates in the opposite direction. Generally speaking, the “Hadley Cell” is positioned between Equator and the 30th parrallel, while the “Ferrel Cell” sits between the 30th parrallel and the 60th parrallel (mid-latitudes).

The resulting surface winds in the mid-latitudes (northern hemisphere) are typically southerly. but the earth’s rotation (Coriolis effect) deflects the Ferrel cell upper winds to the west and surface winds to the east. These surface winds are commonly referred to as the “Prevailing Westerlies” in both hemispheres. 

Source: https://www.internetgeography.net/topics/what-is-global-atmospheric-circulation/

The Polar Air Mass

 

Between the 60th parrallel and the poles, we have the coldest air on the planet. The air mass in these regions is referred to as the “polar cell” or the “Polar Air Mass”.

At both the North and South pole, the air is cold, dense and heavy. Basically, the air sinks and pushes the cold air out and into the mid-latitudes (both north and south).

The above diagram suggests a well defined pattern, which is never the case. The boundary between the polar air mass and the sub-tropical air undulates north and south of the 60th parrallel as the result of the “Jet Stream”. 

Source: https://en.wikipedia.org/wiki/Jet_stream

The Jet Stream or should it be Jet Streams?

Jet Streams are rivers of air in the upper atmosphere that flows west to east. The more significant one is the Polar Jet which can flow at speeds that can exceed 100 miles per hour. The polar jet is a meandering river of air that defines the boundary between the Polar Air Mass and the sub-tropical air in the mid-latitudes.

Mid-Latitude Weather and the Polar Jet

The Polar Jet stream flows at a relatively constant speed in a straight line. If it has to make a sharp turn , it slows or bunches up. Once back on the straight away, it speeds up. The term used for the slowdown is convergence and the speeding up is divergence.

Convergence

In the case of convergence (the bunching up), the air is compressed, resulting in denser and heavier air. This heavier air sinks to the surface resulting in a high pressure area at the surface.

Divergence

Divergence, the speeding up results in an upper level low pressure. The air at the surface rises to fill the depletion, resulting in the development of a low pressure area at the surface.

The life of a mid-latitude cyclone

The polar jet stream marks the boundary between the polar air mass (cold and dry)and the sub-tropical air (warm and moist). The divergence (speeding up) of the polar jet results in the development of a low pressure area at the surface. The temperature differential together with the Coriolis effect puts the low pressure area in motion, spinning counterclockwise. If the cold air is moving towards the warm air, then it is cold front. If the warm air is moving towards the cold air, then it is cold front. 

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