Navigation NAU 102 Lesson 29

Weather has a significant impact on every voyage. Weather Instruments Weather has a significant impact on every voyage. The safety of crew, passengers, cargo and the ship itself is dependent on making good weather decisions.

Weather Instruments We are our own weather station at sea. We must develop an accurate forecast using data received from shore and that we collect ourselves. You’ll learn how to make a forecast in Meteorology.

Duties of the Mate At all times, be knowledgeable of the current weather. Wind direction and speed Wave direction and height Visibility Trend of the barometer

Duties of the Mate At all times, be knowledgeable of the weather forecast. Obtain 24 hour forecast from weather fax, messages, etc. Interpret the forecast for your area. When necessary, make your own forecast.

Meteorological data is collected by: Weather Instruments Meteorological data is collected by: Buoys Satellites Ships (that’s you)

Duties of the Mate Conduct marine surface weather observations. Make observations Log observations Transmit observations to National Weather Service

Conduct marine surface weather observations. Duties of the Mate Conduct marine surface weather observations. Voluntary program 7000+ ships worldwide (600 US) 4 times each day 0000, 0600, 1200, 1800 GMT

Weather Observations Data is collected and formatted in accordance with the Marine Observer’s Handbook. http://www.nws.noaa.gov/om/marine/handbk1.pdf

Weather Instruments Aneroid Barometer Measures atmospheric pressure. Air pressure exerts a crushing force on a thin metal element. Linkages move to a pointer to indicate the pressure on a scale.

Aneroid Barometer

Aneroid Barometer Must be properly adjusted. (Usually done by a professional, often for free.) Reading must be adjusted for elevation to give pressure at sea level. (Note: Mercurial Barometers must also be corrected for latitude and temperature.)

Barometers

Weather Instruments Barograph A recording barometer.

Weather Instruments Thermometer Measures air temperature. You must be able to convert between Celsius and Fahrenheit scales.

Temperature Conversions

Temperature Conversions

Measures Relative Humidity and Dew Point. Weather Instruments Hygrometer Measures Relative Humidity and Dew Point.

Dew Point – the temperature at which water vapor will condense. Weather Instruments Relative Humidity – ratio of amount of water vapor in the air to the amount it could hold at the current temperature. Dew Point – the temperature at which water vapor will condense.

Most common type: sling psychrometer. Weather Instruments Hygrometer Most common type: sling psychrometer.

Two Thermometers: 1 wet, 1 dry Temperature difference is noted. Sling Psychrometer Two Thermometers: 1 wet, 1 dry Temperature difference is noted.

Sling Psychrometer

Psychrometer

Measures apparent wind direction. Measures apparent wind speed. Weather Instruments Wind Vane Measures apparent wind direction. Anemometer Measures apparent wind speed.

Weather Instruments Wind Vane Anemometer

Wind Definitions True wind. The speed and direction from which the wind blows with reference to a fixed point. Apparent wind. The speed and direction (true or relative) from which the wind appears to blow with reference to a moving point.

Wind Examples True wind: East wind, 10 knots. Ship: course 090° T, speed 5 knots. Apparent wind: From 090° T (000° R), 15 knots.

Wind Examples True wind: West wind, 10 knots. Ship: course 090° T, speed 5 knots. Apparent wind: 270° T (180° R), 5 knots.

Wind Anemometers and wind vanes give apparent wind (unless the vessel is stopped). We must solve for True wind for our weather observations. Best done graphically on maneuvering board, plotting sheet or compass rose.

Wind Example 1: A ship is proceeding on course 240° at a speed of 18 knots. The apparent wind is from 040° relative at 30 knots. What is the true wind?

Step 1: Plot ship’s course & speed. Wind Step 1: Plot ship’s course & speed.

Wind Example 1: A ship is proceeding on course 240° at a speed of 18 knots. The apparent wind is from 040° relative at 30 knots. Step 2: Convert relative direction of apparent wind to true direction, if necessary. R + S = T 040° R + 240° = 280° T

(Remember, draw the direction the wind is blowing towards!) Step 3: Draw the apparent wind from the tip of the ship’s course & speed vector. (Remember, draw the direction the wind is blowing towards!) Wind

Step 4: Complete the triangle. Result = True wind. Direction = 315 °T Speed = 20 knots

Wind Measurement Beaufort Scale If no instruments are available, speed may be estimated by observing the effect on the water. Scale was devised in 1806 by English Admiral Sir Francis Beaufort (1774-1857).

Force 0

Force 1

Force 2

Force 3

Force 4

Force 5

Force 6

Force 7

Force 8

Force 9

Force 10

Force 11

Force 12

Introduction to Navigation Questions?