Meteorology Flashcards
What is a barograph?
An aneroid barometer provided with a lever and a pen, which records variations of pressure on a chart attached to a revolving drum. The drum is driven by clockwork and revolves every 7 days.
- can detect errors
- continuous record for reference
What is a an Aneroid Barometer?
Used to measure atmospheric pressure. A partially exhausted/vacuumed compressible box which is connected to a spring. Air pressure distorts the box and and moves a needle which is preset.
What is a hygrometer and how is it used?
It is used to measure realtime humidity. Two Mercury filled thermometers. One has muslin cloth wrapped around the bulb and is dampened by water, the other is exposed. Evaporation causes the wet bulb to be cooler than the exposed bulb. The depression between the two and the air temp compared in psychometric tables.
What are the different types of fog?
- Radiation fog
- Advection (sea) fog
- Frontal fog
What is fog?
Caused by the cooling of air to its dew point at which it becomes saturated by the water vapour that is present within it. Condensation of this water vapour into tiny water droplets produces fog.
What is radiation fog?
On calm cloudless nights land looses heat and cools down. The cold land now cools the air and fog is formed. Does not occur at sea but may be blown on to the sea surface by land breeze.
What is advection (sea) fog?
Formed when warm moist air moves over cold sea. It may persist in wind
- usually in spring and early summer
- Canada, north east coast of US
What is frontal fog?
Air masses involved in a frontal system are forced to mix.
What is a land breeze?
- Start in the evening and fade shortly after sunrise
- Land cools more quickly than the sea
- The warmer air over the ocean is buoyant and is rising
- The denser cool air over the land flows offshore to replenish the buoyant warm air
What is a sea breeze?
- Usually starts in the morning and is gone by sunset
- Land warms, quickly heating the air above it, which becomes less dense (low pressure) and rises
- Cooler air from over the sea is
drawn in to replace it
What are katabatic winds?
These form as intense radiation, on clear nights, causes cooling over sloping ground. Cold dense air flows downhill due to gravity
What sources of weather information are there?
- MSI (navtex, Inmarsat)
- Own ship (look out the window)
- VHF, MF, HF radio listing to coast stations for weather broadcasts. ALRS vol 3 (To obtain weather report broadcast times from coast stations)
- Weather routing services
- Weather fax
- Internet - Government Met offices (NOAA, UK MET Office)
- Local radio
- Harbour master
What is a TRS?
Tropical Revolving Storm
Form over oceanic regions, they require sea surface temperatures of at least 26°C, they tend to form in the Eastern part of an ocean.
What are the signs of an approaching TRS?
- Barometer drops 3mb below the mean reading
- Significant change in the direction or strength of the wind
- Long swell observed from the centre of the storm
- Heavy rain, hot and damp atmosphere
- High cirrus cloud, altostratus, followed by broken cumulus
What is a synoptic weather chart?
Shows pressure systems, isobars and weather fronts at a given level over a large area at a given date and time.
What is a warm front?
A warm front is the boundary between a mass of warm air and a retreating mass of cold air. At constant atmospheric pressure, warm air is less dense than cold air, and so it tends to override, rather than displace, the cold air.
What is a cold front?
A cold front is defined as the transition zone where a cold air mass is replacing a warmer air mass. Cold fronts generally move from northwest to southeast. The air behind a cold front is noticeably colder and drier than the air ahead of it.
What is an occluded front?
The cold front generally moves faster than the warm front and eventually overtakes it, thereby closing or occluding the warm sector of the depression.
- Either the warm or the cold front is suspended and no longer reaches sea level.
- The depression is filling and so the pressure starts to rise.
- Persistent rain and showers with moderate visibility.
- Light winds as the system is decaying and only slowly advancing.
- Increased amount of mid-level clouds, conditions like those in the warm sector.
What is a stationary front?
Forms when a cold front or warm front stops moving. This happens when two masses of air are pushing against each other, but neither is powerful enough to move the other.
There are 4 stages of a passage of a low (depression), what are they?
- Approaching warm front
- Warm sector (behind warm front)
- Approaching cold front
- At cold front
What conditions would you expect in the approaching warm front section of a passage of a low?
Wind N Hemisphere: Backs SW to S and freshens
Cloud: Thickens and cloud base lowers. Overcast skies obscure the sun. (Cirrus and altostratus)
Weather: Intermittent light rain increases to moderate to heavy just ahead of the front
Change in barometric pressure: Increase rate of fall
Change in temp: Very slow increase
Visibility: Steady reduction in increasing rain
What conditions would you expect in the warm sector of a passage of a low?
Wind N Hemisphere: Veers and may freshen
Cloud: Medium and high level cloud breaks up. Typically, lower level cloud remains (Nimbostratus)
Weather: Rain eases and may turn to drizzle. Fog banks may develop if sea surface temp is high compared to dew point
Change in barometric pressure: Rate of fall slows and becomes steady
Change in temp: Sharp increase
Visibility: Moderate or poor
What conditions would you expect in the approaching cold front section of a passage of a low?
Wind N Hemisphere: Backs slightly and freshens
Cloud: Increase in medium and high level cloud. (sometimes large cumulonimbus)
Weather: Rain recommences
Change in barometric pressure: Fall
Change in temp: Steady
Visibility: Moderate
What conditions would you expect at the cold front section of a passage of a low?
Wind N Hemisphere: Veers markedly to W or NW. May be accompanied by sudden squalls
Cloud: Bank of cloud breaks up as front clears to East
Weather: Moderate or heavy rain followed by clear conditions as front clears to East
Change in barometric pressure: Sharp rise
Change in temp: Sharp decrease
Visibility: Sharp increase
What is a mistral?
- Strong NW
- France
- Winter and early spring
- High Pressure over bay of Biscay
- Low pressure in Gulf of Genoa
What is the principle of buys ballots law?
Back to the wind in the northern hemisphere low on the left
What publications are available onboard to help the OOW assess weather information?
- Admiralty list of radio signals
- Mariners handbook
- Pilot books/sailing directions
- Routing charts
- Marine observation handbook
- Yachtsman’s handbooks/almanacs
State the months that hurricanes are generally expected in the Caribbean Sea
May - November
