METEOROLOGY Flashcards
What is the typical weather in the UK? Why does UK get bad weather?
Area of low pressure with associated frontal activity
UK is situated in temperate climates between 50 and 60 degrees north. The polar front is the boundary in between cold polar air coming from the north and warm wet tropical air from the south.
Where these to air masses meet causes a pressure difference causing a Jetstream. Blows from West to East. Causes development of low pressure systems
Warm wet air is forced to rise quickly leading low pressure.
Surface air winds will blow in towards the centre of the low pressure.
The Coriolis force in the northern hemisphere causes the winds to be deflected to the right leading to an anticlockwise circulation of air around the Low pressure system.
The anticlockwise circulation of air draws a wedge of warm air into the cold air to the north, forming 2 distinctive fronts (warm front and cold front)
Movement of the Low pressure system itself is driven by the prevailing South westerly winds as well as the Jetstream which blows west to east.
Low pressure system generated on the eastern seabed of north America.
Describe the passage of a low pressure system in the UK
Warm fronts are slow moving, the warn air climbs above the cold air ahead.
Cold front are more active and vivacious. Moves faster as the cold air undercuts the warm air ahead. The cold front forces the air to rise.
As Low-pressure approaches
- Drop in barometric pressure (3 Hpa in 3 hours)
- Strengthening winds. (Higher pressure gradients mean stronger wind speeds)
- Initially cirrus clouds, then stratus clouds. Cloud level dropping.
- Moderate and persistent precipitation as the warm front approaches
As warm front passes
- Distinct veer in the wind direction (SE to SW)
- Temperature rises slightly
- Humidity increases significantly
- Barometer falls more slowly
- Rain ceases
- Skies clear for now
Warm sector - Wind strength and direction steady - Barometer falls slowly until we reach the closest point of the As Cold front passes - Wind veers (From Sw to NW) - Temperature falls dramatically - Humidity falls - High towering cumulous clouds form - Heavy precipitation and potentially thunderstorms - Barometer rises fast In cold sector - Clear skies - Fair weather - Altocumulus clouds
What weather in an occlusion?
- Stratocumulus cloud. Covers most of the sky and high vertical extent
- High precipitation
- Tend to occlude at the centre of the low pressure
- High wind speeds
What is a trough?
An area of organised precipitation
What is a TRS?
An area of intense low pressure that forms over warm seas.
What are the conditions required for a TRS to form?
- Minimum sea temperature of 26 degrees Celsius
- A pre-existing low pressure system
- Between latitudes 5 and 25 North or south of the equator.
- Certain time of year
- Minimum air turbulence
Draw a diagram of a TRS
Draw
How does a TRS form?
There required conditions for the formation of the TRS must be met
A pre-existing low pressure would find its way over the warm sea temp.
The sea temp will cause the air in the centre of the low pressure to start to rise rapidly. Rising warm wet air causes the pressure of the centre of the low pressure to deepen intensely.
Causing the surface winds to blow strongly towards the centre of the depression. The Coriolis force bends the wind to the right. Causes a very tight circulation of air around a column of rising air
A tight circulation of air causes the storm itself to rotate. The resultant vortex in the centre of the storm causes the air to rise even faster, which causes the pressure in the centre to deepen further.
You find that there is a TRS. What action do you take?
- Revise voyage plan consider contingency plans
- Consider ports of refuge
- Brief the crew
- Stow for sea
- Close WTD
- Ensure optimum stability
- Monitor weather forecasts and navigational warnings
- Stay in the navigable sector where possible
What are indications of a TRS?
- Navigational warnings
- Barometric fall. (Including corrections). If the barometric reading is 3 mb below the seasonal average, there is a possibility of a TRS. If it is 5 mb below the seasonal average, there is a TRS.
- Change of wind direction than normal trade winds.
- Long low swell from the direction of the storm
- Dramatic clouds. (Cirrus clouds in a v point in the direction of the storm)
- Vivid sunrises and sunsets
- Lack of marine wildlife
What is diurnal variation?
Daily fluctuation of atmospheric pressure caused by the temperature
What is the dangerous quadrant of a TRS? What is the navigable semicircle? How to evade a TRS?
Dangerous quadrant. The reason it is called the dangerous quadrant is due to the
• increased wind speed due to the advance of the storm
• Due to the potential of being in the path of the storm
Signs that we are in the dangerous quadrant is that Wind veering. To evade the dangerous quadrant, put the wind on the SB bow
Navigable semicircle. Signs that we are in the navigable semicircle. Wind backing. Put the wind on the SB Quarter
If steady wind direction and barometer dropping, vessel is in the path of the storm.
1,2,3 rule.
What is the track of a TRS? What causes re-curvature of the TRS?
The predicted track of a hurricane is usually West – North West. Driven by prevailing winds.
Re-curvature could be caused by:
• Change in conditions in the leading edge of the storm
• Seeking warm water
• Coriolis force
What are the names and times of year of TRS?
N Atlantic Hurricane May – December NE Pacific Hurricane May – Nov NW pacific Typhoon Apr - Dec N Indian Ocean cyclone Mar – Dec S Indian Ocean cyclone Nov - May Australasia Hurricane Nov – May
What is a barometer? What is a barograph? What is a precision aneroid barometer?
Metal chamber, partially exhausted of air and hermetically sealed. It is susceptible to the slightest changes of pressure, which causes it to expand or contract. These movements are mechanically amplified and displayed on a dial.
Barograph The mechanical linkage movements are recorded on a chart attached to a revolving drum.
Precision aneroid barometer. The mechanical linkage between the cell and the display is replaced by an electronic link with increased accuracy
