Met and Synoptic Charts and TRS

Question 1

Types of weather instruments on the Bridge and their uses?

Answer 1

Hygrometer

• Measures the humidity of the air
• Two thermometers (one wet, one dry) stored in a Stevenson screen on the bridge wing
• Difference between the wet and dry bulb can be used to work out dewpoint using the dewpoint table in the Mariners handbook
• When the dewpoint is within 5° of the sea surface temperature then fog can be expected

Barometer

• Measures atmospheric pressure
• Periodical readings can inform you of the trend
• High pressure = calm, clear, sunny conditions but also possible trapped cloud or fog
• Low pressure = Clouds and precipitation can be expected with areas of deep lows potentially leading to storms

Anonemeter
- Measures the wind speed and direction

Sea Surface Temperature

• Sensor in the hull
• Water intake temp sensor

Question 2

What are the key relationships when describing weather on a synoptic chart?

Answer 2

1st Relationship
- Clouds = Rain = Reduced vis

2nd Relationship
- Pressure drop = closer isobars = wind = sea

These relationships can be used to describe the weather where two air masses meet (a weather front)

Question 3

Describe the weather as you pass front a warm front through to a cold front?

Answer 3

Warm Front

• Warm air rising over cold air
• Warm air condenses forming an increasing amount of cloud, rain and reducing vis
• Pressure drops, the wind picks up and sea state worsens
• So, as you pass under a warm front the weather will deteriorate

Warm Sector

• Pressure steady = steady wind
• light rain or drizzle
• Potential for fog and mist - if clouds have formed low enough
• Overall, the weather will remain the same

Cold Front
- Cold air moving under a warm air
- Rising warm air = clouds = heavy rain (potential hail and thunder) = poor vis
- Pressure drop = squally wind = worsening sea state
Behind the Cold Front
- Just cold air = fewer clouds (to blue skies) = no rain = better vis
- No rising air = reducing wind strength = calmer seas

Question 4

What is a TRS?

What actions will you take?

Answer 4

TRS = Tropical revolving storm

• They are a very low depression and warm in the water around the equator
• A TRS can be sectioned into two halves a dangerous and a navigable
• The dangerous half will always be in the direction that the TRS recurves

Apply Buy’s Ballots Law
- Put your back to the wind and stick your left arm out to the side and that is the centre of the storm

N Hemp

• Dangerous half = Keep the wind on the starboard bow
• Navigable half = Keep the wind on the starboard quarter

Question 5

What the signs of an approaching TRS?

Answer 5

• Possible TRS if the barometer is 3mb below mean for that area- found in sailing directions
• Definite TRS if the barometer is 5mb below mean
• Significant change in wind and strength and direction
• Long low swell from the direction of the TRS
• Squally heavy rainfall in the vicinity of TRS

Question 6

What weather can be expected at an occluded front?

Answer 6

• This is when the cold front has caught up with the warm front meaning the warm sector has gone. Both warm front weather and cold front weather would be experienced here
• So, low heavy cloud continuous rain, moderate wind and sea, poorer vis. Immediately after the warm front passes you will get heavy squally showers associated with the cold front, wind backs then veers considerably, confused seas, very poor vis
• Once this passes the weather will then start to improve