Meteorology

Question 1

What is a Hygrometer?

Answer 1

Measures relative humidity

The difference between the wet and the dy bulb is an indication of the humidity. If they are exactly the same, it is 100% humidity. The bigger the difference between the bulbs, the drier the air.

Can find dew point using table in NP-100 Mariners Handbook.

Question 2

Fog types

Answer 2

Advection (sea) fog: Warm moist air rolls over cool sea, causing air mass to cool, condense and reach its dew point. Once the dew point temperature reaches the sea temp, fog will form.
Radation fog: Warm air rises on clear nights, cooling air mass close to ground, forming fog
Frontal fog: Warm and cool air mixing on either side of the front

Question 3

Sources of weather information

Answer 3

• Own ship
• VHF
• Navtex
• Sat-C
• Internet
• Weather fax
• Local forecasts posted in marinas

Question 4

Publications containing weather information

Answer 4

• Mariners Handbook - NP-100
• ALRS Vol 3: Maritime safety information (what channel & when)
• ALRS Vol 4: Meteorological observations
• Routing charts
• Sailing directions
• Ocean passages of the world

Question 5

Actions in dangerous/navigable semi-circle

Answer 5

In Northern hemisphere

Dangerous semi-circle: Put the wind on the starboard bow and alter course to starboard as the wind veers

Navigable semi-circle: Put the wind on the starboard quarter and alter course to port as the wind backs

Question 6

How would you predict fog?

Answer 6

• Hygrometer
• Calculate depression (difference between dry and wet bulb)
• Mariners handook - dew point tables
• Compare depression against dry bulb temp to find dew point
• Plot dew point against sea temperature

Question 7

How does a TRS form?

Answer 7

Conditions:
- Water temperatures >27 degrees C
- Far enough from equator for sufficient coroilis effect - between 5 and 30 degrees latitude

Formation:
- Warm moist air expands and rises in areas of low pressure
- The cool air from high pressure areas rush in to fill the space left by rising warm air. This air also warms and rises, starting a cycle
- The warm moist air cools and condenses, forming clouds
- Coriolis effect deflects the air to the right, however is also sucked into the centre of the low, causing a spinning effect
- Moisture from the sea continues to feed the TRS, until it reaches landfall and will dissipate due to the lack of moisture