What is the ITCZ?
Where is it found (does it move)?
What weather do you get there?
- Intertropical Convergence Zone is a belt of low pressure where the trades winds of the North and South Hemispheres meet.
- It is found on or near the equator. It moves seasonally, following the sun.
- Light winds alternating with squalls and thundery showers.
What is the STAC?
Weather at STAC?
- The Sub Tropical Anti Cyclone (STAC) is an area of high pressure.
- Calm winds, sunny skies, little precipitation.
When is the NE Monsoon and typical weather?
- In N hemisphere winter, anticyclone develops over COLD Asian continent (high pressure).
- November to March
- Winds mod to fresh, weather cool, fair, broken clouds, (their dry season).
When is the SW Monsoon and typical weather?
- In N hemisphere summer, land heats up, creates an area of low pressure
- Persistent SW Monsoon winds from May to September
- Winds fresh to strong, larger seas, significant amount of clouds and rain, there wet season.
When is the inter-monsoon period and typical weather?
- April to May and October to November
- Usually light variable winds and warm weather. However, this is also the time that TRSs can occur.
How is fog formed?
- When air is cooled to its dew point it becomes saturated with water vapour and the condensation of this water vapour is what causes fog.
What are the different types of fog?
- Advection (Sea) Fog
- Radiation Fog
- Sea Smoke
When does Advection fog occur?
- When warm moist air flows over a cold sea.
- Typical in areas where a warm and cold current meet (e.g. Labrador and Gulf Stream off New Found land)
When does sea smoke occur?
- Occurs in high lats when very cold air blow over a relatively warm sea.
- The water vapour evaporating from the sea cannot be held by the very cold air so it condenses and forms fog.
When does radiation fog occur?
- Occurs over low lying land, on clear nights, typically.
- The heat trapped in the earth cools and forms fog.
- Can blow out onto the sea but never more then 10-15 miles
How can you forecast sea fog?
- 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
What is a TRS?
Where do they form?
Typical movement and speed?
TRS = Tropical revolving storm (Typhoon, Cyclone, Hurricane)
- A very low depression.
- Form in warm water (≧26°C) of the tropics.
- Start off around 10-15 knots in a WNW direction (N HEMP), WSW (S HEMP).
- They may recurve around the STAC but movement can be erratic.
- If a TRS does recurve then it usually slows down, possibly even stop, and then speed up again to 20-25 knots.
Describe the signs of an approaching TRS
- Possible TRS if barometer is 3mb below mean for that area (mean found in sailing directions)
- Definite TRS if the barometer is 5mb below mean
- Significant change in wind strength and direction
- Long low swell from the direction of the TRS
- Squally heavy rainfall in the vicinity of TRS
Imagine you are on a ship whose course will pass through the eye of an approaching TRS. Describe the weather you would expect?
Edge of Storm (200 to 250 miles from eye)
- Pressure 5mb below seasonal mean.
- Winds F 6/7.
- Some rain from outer bands may be encountered.
125 miles from centre
- Gale force winds.
- Cloud and rain becoming more continuous.
80 miles from centre
- Pressure 10mb below mean, falling rapidly
- Hurricane force winds
Eye Wall
- Pressure 20mb below mean
- Driving wind and spray reducing visibility to zero
Eye
- Winds light or mod
- Clear sky
- Vis still near zero due to high, confused seas
What is the dangerous semi-circle and actions to take?
- The dangerous half will always be in the direction that the TRS recurves
- Put the wind on a bow and alter course to the same side as the wind backs/veers
Three reasons why the the advance quadrant of the dangerous semi-circle is the most dangerous?
- Strongest winds.
- Wind is pushing you towards centre of the storm.
- The would recurve towards the vessel.
What is the navigable semi-circle and actions to take?
- Navigable semi-circle is opposite to the dangerous side. Storm would recurve away from the vessel
- Put the wind on a quarter and alter course to the opposite side as wind backs
How can determine the centre of the storm?
Apply Buy’s Ballots Law
- Face the wind, storm centre will lie 12 points to right in N.hemp and 12 points to the left in S.hemp.
How can I determine if I am behind or in front of the trough line?
- If pressure is decreasing you are in front
- If pressure is increasing you are behind
If I am in a TRS but the wind does not appear to be backing or veering, where am I and what action should I take?
- You are in the path of the storm.
- Take action as you would in a navigable semi-circle.
Which way does the wind rotate in N.hemp and S.hemp?
N.hemp
- Anti-cyclone - clockwise
- Cyclone - anti-clockwise
S.hemp
- Anti-cyclone - anti-clockwise
- Cyclone - clockwise
What information should be reported by the master when encountering a TRS?
- Time, date, position of ship at time of observation
- Pressure
- Pressure trend
- True wind direction
- Wind force
- Sea
- Swell
- True course and speed
After the Masters first report how often should further reports be made?
- Ideally every hour, not exceeding 3 hours
Define Relative Humidity?
- A ratio expressed in percent, of the amount of atmospheric moisture present relative to the amount that would be present if the air were saturated.
