Forecasting

Question 1

Which instrument is used to measure the relative humidity? How does it work?

Answer 1

Stevenson Screen with Hygrometer. A dry and wet bulb thermometers, the wet one has a damp muslin wick causing a depression in the wet bulb. When the air is dry it’ll cause a greater difference, and when it’s humid it’ll cause a smaller difference.

Question 2

How many Stevenson Screens are usually on a ship? Which one should you use?

Answer 2

Two, one on each side of the bridge. You should use the windward one.

Question 3

What instrument is used to determine the wind speed and direction?

Answer 3

An anemometer. It gives us the relative wind speed and direction.

Question 4

What instrument is used to measure the pressure? What corrections must be made to the reading?

Answer 4

Aneroid Barometer. We make sure the door is open while taking the reading.

• Height of Eye
• Temperature
• Index Error

Question 5

What is a barograph?

Answer 5

An instrument used to produce a graphical record of the change in atmospheric pressure over time.

Its paper usually lasts one week and needs to be replaced.

Question 6

What are long-term sources of weather information?

Answer 6

• The Mariner’s Handbook (NP100)
• Sailing Directions
• Ocean Passages of the World (NP136)
• Routing Charts

Question 7

Where can you get short-term weather sources?

Answer 7

• VHF Radio Broadcasts Ch16 ALRS Vol.3
• NAVTEX Using ALRS Volume 5
• SafetyNET
• Weather Routing Services
• Private & Official Websites

Question 8

What information can you get from a synoptic chart?

Answer 8

• Areas of high and low pressure
• Isobars
• Fronts Information
• Calculate the Geostrophic Wind Speed

Question 9

How would you measure the real wind speed on a synoptic chart?

Answer 9

• Measure distance between two isobars
• Use the geostrophic wind scale
• We place our measurement on the latitude (Y-Axis) and read off the wind speed (X-Axis)
• Reduce the figure by half if over land or third if over water

Question 10

What is the change in pressure from one isobar to the adjacent one on a synoptic chart?

Answer 10

4 mb

Question 11

What is frontolysis?

Answer 11

Front is dissipating because the temperature gradient is decreasing.

Question 12

What instrument is used to determine the density of the water?

Answer 12

Hydrometer.