GSK - Meteorology Flashcards
Items of Meteorological Information found on Routing charts
-ocean current - set/ rate/ drift
-wind direction and force of weather rose
-mean air temperature
-mean air pressure
-hurricane forecasting
-fog percentage
-low visibility percentage
-TRS winds
-Shipping Routes
-Limits of ice
Synoptic Chart
The synoptic chart shows pressure systems (High and Low), isobars and weather fronts (Warm & Cold) at a given level over a large area at a given date and time
Its used for weather forecasting and the weather to be expected
Warm Front
- cirrus, altostratus, Cumulus, Stratus Clouds
- light to moderate prolonged rain ahed of front, patchy behind
- moderate or poor visibility in Fog
- Wind increases and backs
- pressure drops
- temperature rises
Occluded Front
- happens when either warm or cold front is suspended due to one catching up with the other and so no longer reaches sea level
- the depression is filling and so no more pressure starts to rise
-persistant rain and showers with moderate visibility - light winds as the system is decaying and slowly advancing
- increased amount of mid-level clouds conditions like those in warm sector
Cold Front
-brief Period of clear skys ahead of front
- Cumulonimbus clouds
- heavy rain and thunder at the front,
- very poor visibility at the front, excellent after
- wind decreases and veers
- conditions rapidly improving after
- pressure rapidly increases after
- temperature falls
Factors determining strength and direction of wind
- geographical location
- proximity of High and Low pressure systems
- pressure gradient between High and Low pressure systems
- proximity of land
Meteorological indications of approaching TRS (Tropical revolving storm)
-Falling Pressure 3mb below normal
-long low swell from direction of TRS
-appreciable change in wind strength and direction
-extensive cirrus clouds followed by altostratus
-good visibility
Karabatic Winds
Dangers…
- Occurs when there is intense radiation on a clear night
- this causes cooling over sloping ground
- cold dense air ´falls´down the mountain slope when the air below is less dense under the influence of
gravity - producing the Katabatic wind.
Dangers: strong offshore wind arriving without warning
Mistral wind is example
Barometer
barometer
records pressure at a given time
by using fixed pointer from a previous reading the change over a period of time can be observed.
Barograph
barograph
gives a continual readout on graduated paper by use of a pen and revolving drum thus provides for the yachtsman the ‘trend/tendency’.
Land Fog
Land (Radiation)
- forms over low-lying land on clear nights, when conditions are ideal for maximum radiative cooling, especially during the winter months
- Radiation Fog forming over land is cool and so the air is denser and will flow downhill.
- Occasionally it drifts out to
sea, but it is found no farther than fifteen miles offshore, as the relatively
- high sea surface temperature causes the fog to disperse
Sea Fog
When warm moist air flows over a cold sea surface and cools it to its dew point (vapor condenses)
Sea (advection) fog is formed.
- main type of fog experienced at sea
- may form and persist with moderate or even strong winds
Most common in: - Polar regions
- Grand Banks of Newfoundtland (Canada)
- near cold ocean currents, California, Chile Peru
- SW approaches to English channel
- north west Pacific Ocean
Effect of Land masses on the general direction of winds
land masses disturbs the ideal distribution
- due to local heating and cooling, pressure systems move
Weather in ITCZ
- variable light winds
- squalls and thunder storms clouds
- very hot and humid
Weather in Variables/ Subtropical Anticyclone (STACs)
- fairly nice weather
- small amounts rain and clouds
Buys Ballots Law
in the northern Hemisphere when standing with your back to the wind, Low Pressure will. be on your left
Land Breeze
- land cools at night faster than at sea
- Cool air is denser and falls creating a H Pressure
- Sea cools more slowly creating a relatively Low Pressure
- Wind blows offshore from H to L
Name 6 Air Masses UK experiences
- Arctic Maritime (very cold, moist)
- Polar Continental (cold and dry)
- Tropical Continental (hot and dry)
- Tropical Maritime (hot and moist)
- Returning Polar Maritime (cold, very moist)
- Polar maritime (cold, moist)
ITCZ
Inter Tropical Convergence Zone)
How do clouds form
form as result of condensation of water vapor in the atmosphere
- Ascent in convention currents
- Uplifting at a front
- air being forced over higher grounds (mountains)
- radiation cooling
Trade Winds Weather
- persistent strong winds
- fair and small cumulus clouds
- generally nice weather
Warm Sector Weather
- Stratus clouds
- light to moderate showers
- moderate to poor visibility
- steady wind
- steady low pressure
- steady warm Temperature
Sea Breeze
- Land heats faster than sea during the day
- Sea warms more slowly creating a relatively Low Pressure
- warm air rises creating Low pressure at surface and H pressure is formed aloft
-pressure aloft is higher then same level at sea and so High Pressure moves towards the Low Pressure above the sea to equal out - cooler air falls, creating High Pressure at sea surface
- sea breeze results blowing onshore
Hygrometer
- Obtain a range of readings of the dry and wet bulb Maisons Hygrometer.
- Take the difference between the two (depression) and enter Mariner’s Handbook Dewpoint Table
- Plot graph with time against temperature
- At the same time obtain regular readings of the sea surface temperature.
- where 2 lines cross, Fog is present
- Start of Advection fog is likely when the Dew Point of the air is within 5° C of the sea surface temperature
Where can we source Weather Forecast ?
- local Forecast from VHF
- Navtex
- Sat C
- Weatherfax
- Weather Routing charts
- Television
- Internet
Dew Point
temperature below which air becomes saturated and water droplets begin to condense and dew forms
Relative Humidity
amount of water vapour present in air expressed as a % of the amount needed for saturation at the same temperature
Published Sources of Weather information
- Mariners Hanbomok
- Routing Charts
- Admiralty Sailing Directions
- Ocean Passages of the world
- A.R.L.S Volumer 3
If an unsaturated parcel of air is cooled beyond its dew point, what will happen?
It will become saturated and some of the water vapour will condense out, forming clouds.
What is the difference between absolute and relative humidity?
Absolute humidity is the quantity of water in the air, expressed in grams per cubic metre. Relative humidity is absolute humidity in relation to the quantity required for the air to be saturated, expressed as a percentage.
What is the troposphere?
The troposphere is the lowest layer of the atmosphere. It is the layer which contains almost all of the water and most of what we think of as
‘weather’
Complete the sentence:
When air rises, it…
cools, due to expansion.
What type of atmospheric stability results in clear skies?
Absolute stability, where the parcel of air is always cooler than the surrounding air and therefore wants to sink back down to its original altitude.
What does SALR stand for?
Saturated Adiabatic Lapse
Rate.
What is it called when air is forced to rise because of two or more air flows meeting?
Convergence.
What is it called when air is forced to rise due to land?
Orographic uplift.
What is a fluffy cloud at low altitude called?
Cumulus.
What does a nimbostratus cloud look like?
nimbostratus cloud is a flat layer of cloud at low altitude which brings rain.
How might clouds change when a depression is approaching?
As a depression approaches, high cloud will appear and it will thicken and lower as the depression gets closer.
What are squalls?
A squall is a sudden and dramatic increase in the wind speed, which lasts longer and is more significant than a regular gust. It can also cause a significant shift in the wind direction.
Which air mass forms over
Northern Canada and
Greenland, and moves directly across the North Atlantic to the UK?
Polar Maritime.
How is an air mass which originates over land termed?
Continental.
What are the characteristics of maritime air?
Maritime air masses are more humid (contain more water) than continental air masses.
What are the expected weather conditions when the
UK is under the influence of a tropical maritime air mass?
This air mass is warm and moist. As it moves towards the UK and cools a little, it will become stable and saturated.
When it reaches the UK, it will cause low cloud and drizzle, and perhaps some fog.
What are the expected weather conditions when the
UK is under the influence of a tropical continental air mass?
tropical continental air mass brings warm, dry weather and clear skies because it
originates in a warm place and doesn’t carry much moisture with which to form clouds or rain.
Which type of breeze occurs because land absorbs heat more quickly than water?
Sea Breeze
What causes a land/offshore breeze?
At night, the land cools more quickly than the sea. The air over the land is cooled and sinks, creating a high pressure, in contrast to the lower pressure over the sea. This creates a pressure gradient which results in the land/offshore breeze.
What drives the north-east monsoon?
The intense winter cold across the Tibetan Plateau and
Central Asia.
Why is the summer monsoon south-westerly when the pressure gradient is north-south?
Because the Coriolis effect bends/deflects the air flow to the right in the northern hemisphere, resulting in a south-westerly wind.
What is the pressure over central Australia in the wet season and what is the effect of that pressure?
The wet season is the result of the intense heat in central Australia causing air to rise and thus creating a low pressure. This draws warm, humid air from the Indian Ocean over Northern Australia, which causes the wet season.
Why are the seas in the Arabian Sea rougher during the SW monsoon than during the NE monsoon?
Two reasons. One, wind speeds are higher during the more powerful SW monsoon.
Two, SW’ly winds have a much greater fetch (distance travelled) and are thus larger.
What are the weather conditions along the India’s
SW coast during the SW monsoon?
The SW monsoon draws warm, humid air from the Indian Ocean so the weather is very unstable with high levels of precipitation and squally conditions.
Which of the jet streams drives the weather experienced in Northern
Europe?
The Polar Jet.
What causes El Niño?
Weaker than normal trade winds allow the warm water that is normally pushed over to the western Pacific Ocean spread more evenly across the ocean. This reduces the normal temperature gradient.
What does ITCZ stand for?
Inter-Tropical Convergence
Zone.
Which cell is driven by the intense heat of the sun causing air to rise at the equator?
Hadley Cell.
Why are there often deserts at approximately 30° to 40°
latitude?
Because that is the latitude at which the air descends between the Hadley and
Ferrel Cells. This air is typically cool, dry and stable, so there is very little rainfall.
What are created along the Polar Front and blown towards Northern Europe by the Polar Jet?
Depressions (low pressure systems).
What is the average global atmospheric pressure?
1013 mb
What is ‘pressure gradient’?
The change in air pressure per unit of horizontal distance.
Which unit of pressure is equivalent to millibars (mb)?
Hectopascals (hPa).
What does an isobar show?
Isobars link areas of equal pressure on a pressure chart, in much the same way that contours link areas of equal depth or height on charts and maps.
What is the difference between geostrophic wind and gradient wind?
Geostrophic wind blows parallel to the isobars, at altitude. Gradient wind (also referred to as ‘real wind’) is the result of friction slowing the geostrophic wind and causing it to cross the isobars by the angle of indraft.
Which way does a low pressure system rotate in the southern hemisphere?
Clockwise
What is a depression?
Depression is a term often used to describe mid-latitude low pressure systems.
Which way does a low pressure system rotate in the northern hemisphere?
Anti-clockwise.
How does front symbology indicate the motion of the front?
The triangles/semi-circles point in the direction of motion of the front.
What happens when the cold front catches up with the warm front?
It lifts the warm air mass off of the ground and causes an occlusion.
Where do depressions form?
They typically form along the Polar Front, which is the boundary between the Ferrel and Polar Cells.
Does an occluded front occur when a cold front catches up with a warm front or when a warm front catches up with a cold front?
Cold front catches up with a warm front.
How will cloud cover and precipitation change as a warm front approaches?
As a warm front approaches, cloud cover will increase and lower. Precipitation will initially be light and intermittent but it will become moderate or heavy in the vicinity of the front.
Which front is steeper?
cold front.
What happens to the wind as the cold front passes in the northern hemisphere?
As the cold front passes, the wind will veer dramatically and sudden squalls may occur.
What is the warm sector?
The warmer air mass wedged between colder air ahead and behind.
Why are conditions in an antic@clone typically stable with clear skies?
Because the descending air is generally cool and dry.
Why does the temperature in an anticyclone differ with the seasons?
It’s warmer in summer, because clear skies mean that there’s nothing to block the intense infrared radiation from the sun. In winter, it’s colder because there’s not a lot of infrared radiation but there’s also no cloud cover to prevent what warmth there is from escaping into the atmosphere.
Which way does an anticyclone rotate in the northern hemisphere?
Clockwise
Is air rising or falling in an anticyclone?
Falling
Why do tropical storms not normally form on the equator?
They need the Coriolis effect to begin rotating and there is insuffcient Coriolis effect below approximately 5 degrees of latitude.
What are tropical storms called in the Indian Ocean?
Cyclones.
What is the minimum sea temperature for tropical storms to form?
The sea temperature needs to be more than 26°C for tropical storm to form.
What is a tropical storm?
A tropical storm is an intense and rapidly rotating low pressure system which develops within tropical areas of the world
Why can’t tropical storms form over land or at higher latitudes?
They can only form in an area where there is a lot of warm, moist and rising air. There isn’t enough moisture over land and it’s not warm enough (more than 26°C) at higher latitudes.
What is the approximate Hurricane season in the
Caribbean Sea?
Hurricane season in the Caribbean Sea can be summarised as JASON or
JJASON, i.e (June), July, August, September, October and November.
How can El Niño and La Niña affect tropical storm formation and strength?
Multi-annual fluctuations in
Pacific Ocean sea surface temperatures caused by El Niño and La Niña, can lead to weaker and stronger tropical storms in different parts of the world.
What magnitude of a fall in atmospheric pressure is confirmation that a tropical storm is in the vicinity?
5mb
What change in the wind might signal that a tropical storm is inbound?
If the wind strengthen to a degree and/or was coming from direction which is not normal for the region (i.e.
trade winds).
At what wind speed (in knots and on the Beaufort Scale) is a hurricane declared?
Wind speeds of over 64 knots
(Force 12) are ‘hurricane-force’
Which organisation is the primary source of hurricane forecasting in the Caribbean?
The National Hurricane
Center of the United State’s
National Oceanic and
Atmospheric Administration.
How can swell indicate the presence of a tropical storm?
long low swell, travelling outwards from the centre of the storm, may be the first indication of a tropical storm’s approach. Particular attention should be paid when the swell direction and height differs from the norm in the region.
In the northern hemisphere, if the wind is veering and the pressure is falling, where should the relative wind be positioned in order to take avoiding action?
Starboard bow
What is the average speed of a tropical storm prior to it recurving?
Tropical storms tend to move at approximately 10 knots in their early stages. As they gain latitude, this may increase but rarely exceeds 15 knots before they have recurved.
What is Buys Ballot’s Law and how can it be used in the southern hemisphere?
Buvs Ballot’s Law can be used to determine the centre of an area of low pressure from the perspective of the observer. In the southern hemisphere, if the observer faces the wind the centre of the low will be 100° to 125° to their left or 10° to 35° over their left shoulder.
If the pressure is falling and the wind is backing, where are you in relation to the tropical storm?
Advance Left.
Where is the Dangerous Quadrant in the Northern hemisphere?
Advance Right.
What is the correct action to take if you find yourself in the Dangerous Quadrant in the Southern hemisphere?
Place the wind on the port bow and make best speed.
Maintain the relative wind direction as the wind backs.
Describe, in general terms, the movement of a ‘standard’ tropical storm in the North Atlantic.
品
Having formed in low latitudes in the mid- or eastern-Atlantic, tropical storms tend to move roughly W’ly to WNW’ly across the Atlantic before recurving away from the equator in the vicinity of 25°N latitude and eventually settling onto a north-easterly track.
Why is the Dangerous
Quadrant so called?
The Dangerous Quadrant is so named because the storm is more likely to recurve that way, the wind and waves push any nearby vessels into the path of the storm and the wind
speed is higher.
What is the Beaufort Scale used to quantify?
Wind speed, based on observed sea conditions.
*Which meteorological instrument is used to measure humidity?
hygrometer.
How do you take a pressure reading with a precision aneroid barometer?
The micrometer dial on the right hand side is rotated whilst the black button is compressed until the indicator light jumps from one setting to another to indicate that the value indicated matches the ambient pressure. The pressure is then read from the read-out.
What is the difference between a barograph and a barometer?
A barometer measures pressure at a point in time, whereas a barograph records pressure change over time.
What must the OOW be cautious about when measuring the wind speed?
They must understand
whether the wind speed being fed into the Bridge equipment is relative or true and, when more than one anemometer is fitted, they must ensure that the windward one is being used.
What causes depression of the wet bulb?
The tip of the wet bulb thermometer is wrapped in clean muslin which is dipped into a reservoir of distilled water. The evaporation from the muslin absorbs heat from the thermometer, lowering the temperature recorded. The drier the air, the greater the evaporation and the greater the depression.
Which series of publications should be consulted during
NAVTEX set-up?
Admiralty List of Radio Signals
How many routing charts are there?
Each ocean has twelve routing charts, one for each month of the year.
What is weather routing?
Weather routing is the use of weather information to determine a safer, quicker and/or more economical route between two ports.
What are Viking, Fair Isle and Forties?
They are sea areas in the Met Office’s Shipping Forecast.
Why does geostrophic wind not correctly predict the wind speed at ground level?
Geostrophic wind speed is reduced at the surface due to friction between the moving air and the water or land below.
What is frontolysis?
Frontolysis means that the front is dissipating because the temperature gradient is decreasing at the surface.
What is the change in pressure from one isobar to the adjacent one?
4 of 5
4mb
What is the symbology and direction of travel for a cold front?
cold front is indicated by a blue line with blue triangles pointing in the direction of travel of the front.
Which type of fog occurs when warm, moist air passes over cold water?
Advection fog.
Which type of fog occurs at the boundary between warm and cold air masses?
Frontal fog
What causes advection fog?
Advection fog occurs when warm, moist air passes over a colder surface. The colder surface cools the warm, moist air to below its dew point which causes condensation.
What is the definition of fog?
A cloud at ground level, which reduces the visibility to below
1000m.
What causes radiation fog?
As the land cools during the night, it cools the air close to the surface. This air is cooled to below its dew point, causing condensation and forming fog. Although it has formed over land, it can flow out over the sea, although not normally further than 10-15m and it is normally burnt off fairly quickly once the sun rises.
What is the ‘depression of the wet bulb’?
The difference between the dry and wet bulb temperature is known as the ‘depression of the wet bulb’.
Where can be Dew Point Table be found?
In the Mariner’s Handbook
(NP 100).
