Weather Flashcards
Weather
**Sources of Met information:
ALRS 3 & 4 - Maritime Weather services and Met observation stations
1. Window
2. Naxtex - B, E 518 kHz - 490kHz
4. Inmarsat C - Safety Net
5. Sailing directions for local and climatic data
5. Weather Routing (WRI)
6. Weather Fax
7. Internet (check source data - ECMWF)
8. Coast Radio Stations
9. Onboard instruments
Weather
What is radiation fog?
Radiation = Land Fog (imagine someone hugging a radiator) = Landlubber
- Warm moist air over cooling land mass
- Very clear sky at night time
- Low wind
- Can drift out over the sea
- Worse before dawn / sunrise
- Air is a poor conductor of heat - land a good conductor (whilst the warm moist air is sitting on the cold ground and it condenses out into fog)
- 3 - 5 mph of wind the fog is lifted off the ground
- >5 mph of wind it become a low cloud
How would you predict the likely onset of fog?
- Stevenson Screen protects the hygrometer from sun, wind etc.
- Measures Relative humidity,
- Allows you to calculate dew point
- Wet (Muslin with wick into distilled water) and Dry bulb
- Dry bulb temp vs Wet bulb temp gives you the depression (Difference between the two thermometers)
- Dew point table is found in the Mariner Handbook NP100
- Need to plot the the dew point on a graph to see if the dew point and the dry bulb temp lines converge, if they converge there will be dew.
- If you plot dew point against seawater temp and where the two temps come together there will be advection fog
- Hang it to windward as we want to see the temp of the air
- The bigger the difference between the two thermometers = the dryer the air is. (More evaporation, cooling the “wet thermometer”. If they were the same / close means more humidity.
- Plot a course for warmer water if fog likely
When dewpoint is within 5 degrees f sea surface temperature for is likely
Weather
Ballots Law
As per NP100 Northern Hemisphere - face the wind, centre of the low pressure will be 90 to 135 degrees in your right side. And left side in South Hemisphere
Veering is clockwise
Backing is anti clockwise
Weather
Synoptic Chart - Be able to point out:
- Surface pressure chat.
- High Pressure - Low Pressure - Warm Front - Cold Front - Occluded
- Front - Wind speed - measured by distance between isobars
(Off Geostrophic wind scale on synoptic chart will give wind speed at 600m - 70% of that will be at sea level) - How to read a synoptic chart
- Warm sector between a warm and cold front, cloudy variable weather.
- Black lines mark troughs - rain as a result of warm air trying to get above cold air. Areas of turbulent air
- High pressures air moves around clockwise
- Low pressures air moves around anti clockwise
Weather
Met Instruments
What is a Hygrometer?
What is a Barometer?
Masons Hygrometer / Whirling Hygrometer: Measures relative humidity
Barometer: Measures atmospheric pressure
Must be corrected for altitude and Diurnal variation considered via sailing directions
Barograph: Measures trend of atmospheric pressure over 7 days on a paper scroll
Anemometer: Measures wind speed and pressure
Weather
Met Instruments:
What is a Barograph?
What is a Anemometer?
Barograph / Barometer / Aneroidbarometer:
- Measures a pressure trend on a mechanized chart drum over a seven day period.
- Enables you to try and predict weather.
- 1030 mb is a high pressure, fine clear weather? Need a trend one reading is not sufficiently. Need a tendancy. The steeper the change the windier it will have been.
- Diurnal variation of the barometer: Atmostpheric pressure wave that goes around the world recorded daily, must compare day by day.
- Must be corrected for height, latitude, temperature and diurnal variation
Anemometer:
- Measure wind speed (apparent / relative)
- If GPS input available you will get a true wind speed also
Weather - passing of a low pressure system
Low pressure systems:
-
As the warm front approaches
- The pressure would fall
- The wind direction steady/backing
- Wind force increasing
- Temperature steady
- Cloud would be high cirrus and cumulus, then cirro stratus and finally nimbo stratus
- Light rain increasing to moderate
- Visibility deteriorating
-
In the warm sector:
- The pressure would stop falling
- Wind freshens
- Temperature sharply increases
- Cloud would be nimbo stratus, then stratus (Thick layering)
- Rain would stop or change to drizzle
- Visibility could be poor
- Approaching Cold Front:
- Increase in low-level cloud
- Wind freshening
- Barometer falling
-
As the cold front begins to pass:
- The pressure starts rising
- The wind veers
- The wind force increases/squalls
- Temperature falls (Due to wind veering blowing from north)
- Clould - cumulonimbus
- Heavy Rain, possibly hail and thunderstorms
- Visibility Poor
-
In the rear of the cold front:
- Wind direction steady
- Wind force decreasing
- temperature steady
- Cloud cumulus
- Possible showers, clearing
- Visibility clearing
Weather
What are your actions in the different sectors of a TRS?
-
Wind Veering = Dangerous semicircle
- Wind on STBD bow and steam away at full speed,
- Whilst altering course to STBD keeping the wind on the STBD bow (Outwards and away from the center of the storm)
-
Wind Backing = Navigable Semicircle
- wind on STBD quarter If the wind is a steady direction then you are in the path of the storm and would do the same as if you were in the navigable semicircle
Northern Hemisphere
- Veering = Dangerous - STBD Bow -
- Backing = Navigable - STBD Quarter
Southern Hemisphere:
- Veering = Navigable - PT Quarter
- Backing = Dangerous - PT BOW
Weather
Winds
Katabatic
Anabatic
Land Breeze
Sea Breeze
- Land Breeze sets in late in the evening dies down by sunrise
- Sea Breeze sets in by full noon (Land at its hottest, creating a low pressure, filled in by the sea)
- Katabatic, intense radiation (clear night) causes cooling over sloping ground - cold air falling down steep land mass creates a very strong land breeze, can set on rapidly. In the valley warm air is rising and is subsequently replaced by the cold falling wind.
Norway, N Adriatic, E Black Sea and Antarctica - Anabatic: Rising warm air rises up the sides of mountain slopes until it gets to a point it is cooled below its due point and forms cumulus clouds. As warm air rises a low pressure is created in the valled and begins a cycle.
Weather
What are the requirements for a TRS to form and what are the signs of an approaching TRS?
- Latitude of 5 - 15 degrees N or S of the equator
- Sea temp above 26 degrees
- Tropical disturbance
- A wind sheer causing an uprising of the wind
- Drop in pressure of more than 3mb for seasonal average corrected for diurnal variation (Average can be obtained from routing charts or sailing directions) raises suspicion
- 3mb or more = suspicion
- 5mb or more means there is little doubt that a TRS is in the vicinity
- Must take avoiding action
- Long lowly swell coming from a different direction to the wind
- Appreciable change in wind direction and strength
- Lack of wildlife
- Strange and colourful cloud formations (usually cirrus followed by broken cumulus)
Weather
What is frontal fog?
Frontal Fog = Warm Front and Cold Front Meeting or Occlusion
- Fog is due to the mixing of the warm and cold air of the two mixing fronts.
- High wind speed with an occluded front (30 - 35 kts)
- Artic Sea Smoke - Occurs in high latitudes when extremely cold air blows over a relatively warmer sea surface.
Weather
What is advection fog?
Advection = Sea fog
- Warm Moist air flows over a cold sea
- Need between 5-25 kts of wind
- > 25 kts it become low cloud
- Local Spring and early summer when sea temp is at its lowest
- Polar regions in summer / Grand Banks in NFL / NW Pacific Ocean / British Isles in spring/early summer.
Publications for weather
-NP100 Mariners Handbook
-ALRS Vol 3
-Admiralty Routing Charts
-Ocean Passages of the World- 2 parts
-Sailing Directions
What information is given on a Admiralty Routing Chart?
12 charts - 1 per month per Ocean
-Contains wind roses giving statistical analysis of likely wind direction/strength (amount of observations, % of variables, % of calms)
-Frequency of wind - force 7 and above
-Ocean currents
-Frequency of fog as percentage (less than 1000m)
-Sea currents
-Ice limits
-Shipping routes
-Past TRS tracks
-Load Lone divisions
