Meteorology Flashcards
Explain a sea breeze
As land is heated during daytime the air over it is heated by conduction. This heating causes a reduction in density and thus a pressure decrease over the land.
The sea temperature remains more or less constant and the pressure over it is relatively high compared with the land and a pressure gradient is formed.
The difference in pressure creates an airflow from high to low - from the sea to the land
Explain katabatic winds
Occurs when there is intense radiation on a clear night. This causes cooling over sloping ground. The colder and denser air adjacent to the slope will now flow down the slope under the influence of gravity producing the katabatic wind
State at least 5 items of meteorological info to be found on a admiralty routeing chart
Ocean current - set and rate
Mean sea temperature chartlet
Wind direction and force - weather rose
Dew point chartlet
Mean air temperature
Fog chartlet
Mean air pressure
Low visibility chartlet
hurricane forecasting and track
Define the term synoptic chart and its use for weather forecasting
The synoptic chart shows pressure systems, isobars and weather fronts,
At a given level over a large area at a given date and time.
To indicate the weather to be expected
Describe and compare the information obtained from a barometer and barograph and explain their use in weather forecasting
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 - gives a continual readout on graduated paper boy use of pen and revolving drum thus provides for the yachtsman the trend/tendency
Both used to obtain the atmospheric pressure and trend and to predict future weather. From the pressure trend, a mariner can determine the likely weather to be experienced, in simple terms ie- rising pressure indicated improving weather and the opposite with decreasing pressure.
Define the term prognostic chart
Is a weather chart forecasting the state of the atmosphere over a large area at a given level and at a future time
List four factors which determine the strength and direction of winds
Geographical location
Proximity of areas of high and low pressure systems
Pressure gradient between areas of high and low pressure systems
Proximity of land
Describe the weather conditions expected whilst navigating in the ‘variables’
Winds are light and variable and the weather is generally fair with small amounts of cloud and rain
Describe trade winds, their location and how they migrate with seasons
Are air streams originating in the sub tropical oceanic high pressure areas of the N. & S. Hemisphere blowing on the E. And equatorial sides of the areas of high pressure towards the equator.
The general direction is NE in the N. HemiS
And SE in the S. HemiS
They are encountered and blow with remarkable persistence over all major oceans
EXCEPT the N. Indian Ocean and China seas
The zones migrate seasonally and in each hemisphere extend to about 30* N and S latitude in the respective summers
And 25* N and S in winter
State five meteorological publications consulted when planning a passage
The mariners handbook
Admiralty list of radio signals VOL3
Admiralty routing charts
Admiralty sailing directions (pilot books)
Weather forecast source
Commercial sailing directions / Pilot Books e.g : Adlard Coles
Commercial routing charts e.g. Imray
US Hydrographic office monthly pilot charts
Explain the terms ‘synoptic’ and ‘prognostic’ chart used in weather forecasting
The synoptic chart shows pressure systems (highs and lows), isobars and weather front (warm and cold), at a given level over a large area at a given date and time
Describe a weather routing service AND explain the advantages of using this service
A weather service is a commercial service provided by a meteorological organization, details of whom can be foi8nd in ALRS VOL3, specifying and updating an individual recommended route for a specific voyage by means of the following:
Makes use of actual weather (as opposed to expected climatic conditions)
Provided by highly trained an experience teams for forecasters and mariners
Route constantly reviewed and modified if required according to the actual and predicted weather conditions
Route “Optimized” to achieve a specified aim such as least time, minimum storm damage, etc..
List SIX publications that may be available onboard to help the OOW assess weather information
Admiralty list of radio signals
Mariners handbook
AdmiraSailing directions/ Pilot books
Routing charts
Marine observation handbook
Yachtsman’s handbooks/almanacs
State the months that hurricanes are generally expected in the Caribbean Sea
1st of June to 30th of November
Describe the advantages of using a weather routing service
Makes use of actual weather/sea conditions (as opposed to expected climatic conditions)
It’s provided by highly trained and experienced teams of forecasters and mariners.
Route constantly reviewed and modified according to actual & predicted conditions
Route optimized to achieve specified aim:
Least time, minimum storm damage, safest passage, reduce fuel costs, ETA,
Potential reduction on wear and tear
Critical information received in time to take avoiding action around danger areas
Insurance companies may insist on their use
Reduced insurance costs overall
Describe a whirling psychrometer and how and what it is used for
Consists of a wet and dry bulb thermometer, is similar to a football rattle and is rotated as per instructions by hand. The values of wet and dry temperatures are the read, the difference (depression) is then used to enter the appropriate tables along with the dry temperature to obtain.
Practical use: to obtain either the dew point temperature or the relative humidity of air (depending on tables used)
This can then aid the forecast of sea fog in conjunction with sea temp
Describe how katabatic winds can occur
On clear nights the slope of mountains radiate out their heat and the temperature falls.
The air in contact with the ground becomes heavier and rolls down the slope.
Normally as it descends it would warm at the adiabatic rate lapse rate (variation of air temperature with height), but because it is still in contact with the cold ground this effect is reduced and the air increases its flow down the slope.
These winds can reach high speeds and are often funneled down water-courses
Describe the formation of land and sea breezes
The land is heated by the sun during the day and loses heat at night relatively quickly and therefore the adjacent surface air changes temperature accordingly.
Hot air rises and therefore the pressure drops, similarly cold air falls and the pressure increases.
The sea temperature however changes very little and will therefore throughout a 24-hour period will be relatively hotter or colder than the adjacent land.
The change of pressure over the land causes on - and off shore winds to blow.
Sea breezes may reach force 5 in places, but land breezes are normally not as strong
Define the ITCZ
Intertropical convergence zone
High temperatures at the solar equator create the low surface pressure where the trade winds converge.
In this equatorial trough (or tropical low) lies a zone of calm and weak winds of no prevailing direction. Here the air, which is very moist and heated by the sun, tends to expand and rise, maintaining the low pressure of the area.
These winds, which are roughly between 5N and 5S, are generally known as the doldrums.
This area is called the ITCZ or the equatorial belt of variable winds and calms. Because of the converging moist air and high potential for rainfall in the doldrums this region coincides with the worlds latitudinal belt of heaviest precipitation and most persistent cloud cover
State the months when TRS would expected in the Caribbean
Tropical Revolving Storms
1st June - 30th November
List 4 broadcast and 5 published sources of weather information
Broadcasts:
Radio weather forecasts for shipping
Radio navigation warnings
Navtex
Internet
Published sources:
Admiralty sailing directions Routing charts Ocean passages of the world Mariners handbook Admiralty list of radio signals - vol 3/4
Describe buy ballots law for both hemispheres
N Hemisphere - with ones back to the the TRUE wind, the low pressure will be on the left hand side
S Hemisphere - with ones back to the true wind, the low pressure will be on the right hand side.
Define dew point
Is the temperature at which water vapor in the air condenses into liquid water, such as in the form of dew, fog, or possibly rain
Define relative humidity
Relative humidity is the ratio between the current amount of water vapor in the air at a given temperature to the maximum amount of water vapor possible in the air at that temperature.
Measured as a percentage %
Use of a marine hygrometer will give you this.
