Meteorology Flashcards
What is a Barometer?
A Scientific MET instrument used to measure atmospheric pressure. Pressure tendency recordings can be used to forecast the weather, predicting surface troughs, pressure systems and frontal boundaries
What is a Masons Hygrometer?
Two thermometers, a wet and dry bulb, placed inside a Stevensons Screen.
Wet bulb is wrapped in muslin and soaked in distilled water.
Difference between the two is called the ‘wet bulb depression ‘
This is used to calculate the dew point, and relative humidity, via the respective tables in NP100
Use of a Masons Hygrometer
- Obtain the Dry bulb reading and the wet bulb depression from the hygrometer
- With these readings, pull the Dew point from the Dew point table in NP100
- Create a graph, with the x axis labeled ‘temperature’, and the y axis labeled ‘time’
- Plot dew point readings, continue to plot and draw a LOBF
- Meanwhile, take recordings of the Sea level temp, and plot. Also Draw a LOBF
- Where the two lines intersect is the Dew point (fog onset)
- The point at which the two lines are 5 degrees apart is considered the ‘warning of approaching dew point’
Define Relative Humidity
The Ratio of the partial pressure of water vapour in an air parcel, to the saturated pressure of water vapour at a prescribed temperature
Define Dew Point
The temperature at which an air parcel reaches 100% humidity and cannot hold any more water vapour
When attempting to weather forecast using a barograph, what should the mariner be aware of?
Diurnal Variation
This results from atmospheric pressure waves, with a period of nearly 12 hrs, that sweep from E-W around the Earth.
Mariner must take this into account when weather forecasting
What is a Precision Aneroid Barometer?
How do we use it?
A barometer, as it says on the Tin, that is more precise.
Press the back switch button, turn the knob until the light breaks. take the reading when the light breaks.
I.e. 1018.7mb
This reading must then be corrected to mean sea level using the correction card provided.
What is advection fog?
Advection fog forms when warm air blows over a relatively warm body of water
This generally occurs during spring and early summer. It is generally quite low lying.
As the sun warms the air further, the fog will increase until the heat is enough to burn it off entirely
Places Advection fog is likely to occur:
British Isles
Polar regions in Summer
North Sea
NW africa
NW Atlantic (Newfoundland)
NW pacific (Kamchatka current)
What is Radiation fog:
Occurs over low-lying land on clear nights
Especially during winter months
Thickest during the later parts of the night, early morning
Sometimes extends out to sea (10-15nm max)
What is Frontal Fog:
Forms ahead of a warm front
Warm moist air from the warm sector combines with cold air from ahead of the front
Rain at the front can increase the severity of frontal fog
Usually confined to a narrow belt at the front
Can form into advection fog
Sources of MET information
Internet
Own Ship
VHF/MF/HF
NAVTEX
Weather Facsimile
Inmarsat C EGC Safety Net
Weather Routeing service
MET instruments on board
MET publications used in passage planning
Admiralty Sailing directions (local winds)
ALRS vol 3
Admiralty Routeing charts (weather patterns for each month in chosen areas of navigation)
Ocean passages for the world (Each ocean passage and the typical weather associated)
Ice limit charts
NP100 (weather associated in areas, info on TRS’s, dew point, relative humidity tables)
Anabatic vs Katabic winds
Anabatic winds travel uphill due to rising in convection currents
Katabatic winds, travel downhill. Cold air builds upon a plateau, and is forced downhill, creating dangerous conditions below for navigators
Mistral Wind
Katabatic wind from above the SOF, into the Gold Of Lyon
Geostrophic wind deflection over land vs sea:
Speed and direction
Speed: 40% less over land, 25% less over sea
Direction: 30 degrees over land, 15 degrees over sea
Define Gradient wind
The simple wind direction from a H to L pressure system
Define Geostrophic wind
Gradient wind effected by the Coriolis effect
Define Surface wind
Geostrophic wind after friction / deflection from land and sea masses,
How to tell of an approaching TRS:
Potential if a drop of more than 3mb, accounting for Diurnal Variation
Definite if a drop of more than 5mb
Winds increasing to ≤ force 6
Buys Ballots Law relative to the Hemisphere
Long, Low, Lazy Swell
Significant change in expected wind direction and strength
Heavy Rain
Huge dark cumulus clouds
Extremely low atmospheric pressure, lack of wildlife
WEATHER REPORT OF TRS
In NH, how to tell if you are in the navigable semi circle, and un navigable semi circle of a TRS:
Buys ballots law
Back to the wind, wind on your left side
Navigable:
Wind backing
Un-navigable:
Wind Veering
Actions if found in un-navigable or navigable semi circle of a TRS
Un navigable, wind on STBD bow until clear
Navigable, wind on STBD quarter until clear
Define a Synoptic Chart
A weather chart giving geostrophic weather conditions and lines of pressure the present time
Define a prognostic chart
A Synoptic chart predicting the future conditions
Weather at an occluded front
Lots of rain,
Warmer temperature
Lower pressure
Unstable air and winds
Clouds similar to unstable air pocket, cumulonimbus etc
Poor visibility
SIMILAR to a trough
4 stages of the passage of a low
Stationary front (2 air masses meet)
Developing front (Coriolis effect takes hold)
Mature front (fully developed)
Occluded front
The Passage of a Low: before the warm front
Clouds: Cirrus, Alto Stratos, Cumulus, Stratos
Precipitation: Light to moderate showers
Visibility: Moderate, poor if fog is present
Wind: Increasing and Backing
Temperature: Rising
Pressure: Dropping
The Passage of a Low: Warm Sector
Clouds: Low lying Stratos clouds
Precipitation: light-moderate showers
Visibility: Moderate to poor visibility
Wind: steady wind direction and speed
Temperature: Steady, warm
Pressure: Steady, low
The Passage of a Low: After the cold front
Clouds: Cumulonimbus
Precipitation: Heavy rain, lightning, thunder
Visibility: Pouring rain, followed by excellent visibility
Wind: Decreasing in speed and veering
Temperature: Sharp decrease in temperature
Pressure: Increasing Pressure
What percentage of the reading is the surface wind, when using the geostrophic wind scale on a surface analysis chart?
70% of the reading on the scale
What is a Typhoon?
A typhoon is a TRS that hovers over the Pacific (usually East Asia)
Develops between 100°E and 180° in longitude
