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"Captain, it's a bit nippy today, isn't it?" |
| The ship left Nagoya on July 5, a warm summer day, but the weather has since gotten cooler with each passing day. This morning was downright chilly. |
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"Kawasaki-San, where do you think we are now?" |
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"l have no idea." |
| The Captain
took out a chart and unfurled it on the chart table. He then marked the ship's current position on the chart. The
location that the Captain marked was the same latitude as the Tsugaru straits at the
northernmost tip of Japan's
main island. That explained the chill in the air, but Kawasaki-san wondered why
the ship was so far north. |
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"Captain, according
to this chart, Nagoya and Long Beach are at almost the same latitude. If we
head due east from Nagoya, we should arrive at Long Beach. So why go so far
north?" |
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"Of the routes that go
from Nagaya to Long Beach via the waters offshore Nojimazaki, the shortest one is
called the Great Circle, which looks like this when drawn on this chart(Chart 1). Because the earth is spherical, the shortest are connecting two points on the surface of the sphere is the one that lies on the plane bisecting the sphere's center. This arc is the Great Circle. As you said, if we head due east from Nojimazaki, we will arrive in Long Beach, but that route is longer than the Great Circle by 270 nautical miles, or about 500 kilometers. Still, that's not to say that the Great Circle is always the best route. As captain, I always try to choose the speediest route and minimize fuel costs, which are influenced by a number of factors, including ocean currents and the weather the ship encounters. Waves and swells generated by winds play a particularly important factor. Taking into consideration the effects of such natural forces, l select the ship's route." |
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"So that's why you were just looking at the
weather chart. What route will we be taking this time?" |
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"This time we'll veer a
little farther north, though we might run into some rough seas and then we'll make a
beeline for our destination. Pretty soon we might have to turn on the heat." |
| True to the
Captain's words, the
next day the ship was rocked by high waves. From the bridge, the sea looked
slightly frothy and the wind was blowing from the starboard aft. On the bridge, the Captain was reading a telex from a weather routing service. It contained a weather forecast and advice to the Captain about what course to take. |
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"Kawasaki-San, yesterday's forecast was right
on target. For the next three days, we'll be sailing together with a
low-pressure system that has been approaching from behind, so we'll probably be
tossed around a bit. By the way, Kawasaki-san, what kind of weather information do you think we receive on
board?" |
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"The
same as on land?" |
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"Not quite.
Surface weather charts are often used at sea as well as on land, but on board we also
get upper air charts. They're useful for forecasting one to two days in advance
because the upper air weather precedes the weather on earth. We can get forecasts
for periods of up to ten days from weather routing services like this one. The
weather routing service uses computer simulation to estimate how much the forecasted
weather will slow down the ship. Then it forecasts which route will be the most advantageous (i.e. the most expeditious)and recommends that route to us. Although it is only a forecast, it is extremely useful. Nonetheless, Kawasaki-san, if we follow the weather routing service's instructions and consequently encounter a storm and fall behind schedule or, God forbid, have an accident, who do you think has to bear the responsibility?" |
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"You
mean the captain is responsible?" |
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"That's right. The
captain is responsible for selecting the ship's route and navigating the ship.
That's why I have to watch the weather so closely and select our route carefully." |
| The Captain then pulled out a drawing with several lines drawn over the Pacific Ocean together with a line graph. |
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"These lines (Chart
2)are typical routes from Long Beach to Tokyo. This graph (Graph 1)shows each
route's sailing time by month. As you can see, the shortest course, the
Great Circle, is not necessarily the fastest route." |
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"During
January and February, it's faster to take a more northerly route than the Great
Circle. " |
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"That's right, mainly
due to the influence of the path of low pressure. The arrows on this chart(Chart
3)show the pattern of low pressure systems' movement in the north Pacific during the
winter. As you can see, at that time of year the low pressure path is close to
the Great Circle route between Long Beach and Tokyo. Consequently, it's more advantageous, particularly for westbound ships, to head farther north than the Great Circle and travel to the north of the low pressure pathway." |
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"In what
way is it advantageous?" |
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"In northern hemisphere
low pressure systems, winds blow counterclockwise, which means that easterly
winds blow to the north of the low pressure. These winds are tail winds for
westbound ships." |
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"So you
can count on getting an extra push from the wind" |
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"That's right. In
addition, the sea is comparatively calmer to the north of the low pressure.
That's why when returning from America in the winter we often go through the Bering Sea, north
of the Aleutian Islands. To head to Japan after sailing west through the Bering Sea, we veer south offshore eastern Kamchatka and sail down alongside the Kurile Islands. In this area, we almost always run into a low pressure system that has passed over Japan. As you well know, atmospheric pressure alternates between peaks and troughs. Consequently, when we are suffering from an encounter with a low pressure system, the Pacific coast of Japan's main island is often enjoying dry, clear winter weather due to the winter atmospheric pressure pattern in Japan high in the west, low in the east." |
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"I see. Thank
you, Captain" |
| Kawasaki san, who was wearing short sleeves, went back to his cabin to get a sweater. |
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