Meteorology Flashcards
Define Barometric pressure
The amount of air pressure exerted by a column of air above the measuring point
Surface pressure varies with temparature
What is the mean atmospheric pressure?
1013 hPa (millibars)
Define Stable air
An air parcel that is cooler than the surrounding atmosphere will sink as the cool air is less dense than the warm air
This creates a stable air parcle as it does not continue to rise and create precipitation etc
Define Un stable air
An air parcel that is warmer than the surrounding atmosphere will continue to rise as cold air is denser than warm air
This produces clouds and rain
Explain the Coriolis effect
The speed of the Earths rotation is seen to be faster at the equator than at higher latitudes, due to the Earths circumference.
As the Earth rotates in an E’ly direction, Air masses moving towards the poles are seen to be deflected to the Right in the Northern Hemisphere, and the Left in the Southern Hemisphere, seemingly overtaking the Earths surface
Backing
When winds are deflected in an anti clockwise direction they are described as backing
Veering
When winds are deflected in a clockwise direction they are described to be Veering
Northern hemisphere direction of wind rotation in pressure systems
Low pressure systems rotate in an anti clockwise direction
High pressure in a clockwise direction
Southern Hemisphere direction of wind rotation in pressure systems
Low pressure systems rotate in a clockwise direction
High pressure in an anti clock wise direction
What are the four factors that help to determine local wind strength and direction
- Geographical Location
- Proximity of areas of high and low pressure systems
- Pressure Force Gradient between areas of high and low pressure systems
- The proximity of land masses
Clouds form due to these four factors
- Ascent in convection currents
- Uplifting at a front
- Air being forced over high ground (ornographic Cloud formation)
- Radiation cooling
- Turbulence Mixing
Define Dew Point
The atmospheric temperature (varying with pressure and Humidity) below which the air becomes saturated, water droplets begin to condense and Water droplets that we can see (clouds), and dew begins to form
Define Relative Humidity
The amount of water vapour present in the air expressed as a % of the amount needed for saturation at the same temperature
(the amount of water vapour an air parcel can hold)
Relative Humidity Formula
Actual vapour density / Saturation vapour density x 100
How is Advection fog Formed
Warm moist air being transported over cold Sea surface or ocean currents
Most commonly in spring and early summer when the ocean temperatures are at their lowest
Where is Advection fog most commonly found
In areas where the prevailing winds transport warm moist air over areas of cold water or over the major cold ocean currents
Polar regions in summer
Grand banks of Newfoundland (Labrador current)
NW Pacific Ocean (Kamchatka current)
Cold ocean currents off the Western seaboards of continents lying within the trade wind belts (California, Chile, Peru, SW Africa, Morocco)
British isles
What does Buys ballot Law state (N hemisphere)
If you stand with your back to the surface wind in the N hemisphere:
The centre of the LP system will be to your left and slightly forward
The centre of the HP system will be to your right and slightly backwards
What does Buys ballot Law state (S Hemisphere)
If you stand with your back to the surface wind in the S hemisphere:
The centre of the LP system will be to your right and slightly forward
The centre of the HP will be to your left and slightly back
How does Frontal Fog Form?
Forms due to the mixing of cold and warm air on two sides of a passing (warm) frontal system.
Rain ahead of the front may help to raise the relative humidity and aid the formation of fog
Usually confined to a narrow belt in front of the fog, however Advection fog can form in the warm moist air behind the front.
How does Radiation fog form
Due to radiation cooling of the Earth
Forms over low-lying land, valleys, water and damp vegetation
Ground cools, damp moist air flows upwards, creating low lying fog
Usually late at night and early hours of the morning
Published sources of Weather Information
Mariners Handbook (NP100)
Admiralty Sailing directions (pilot books)
Commercial nautical almanacs
Admiralty Routeing charts
Ocean passages for the World
ALRS Vol 3 (Maritime Weather Services. Safety Information broadcasts. Worldwide NAVTEX and SafetyNET information)
Function of a barograph
Allows the reader to follow trends in a barometer
Records changes in Barometric pressure via a lever and pen, recording variations of pressure on a chart attached to a revolving drum. This coincides with a Barometer
Drum makes one revolution in 7 days
Should be secured in a position where least likely to be affected by vessel movement
Hygrometer
Two thermometers, one with a dry bulb, one with a wet bulb covered in muslin, kept damn with fresh water
Stored inside a wooden box painted white with louvered sides (Stevensons screen)
The wet muslin allows the air to be fully saturated around the bulb
The difference in temperatures between the two Thermometers (wet bulb depression) is the measurement used to measure the relative humidity and dew point of the ambient air
This is done via the Dew point tables in the Mariners handbook
Areas where advection fog may occur
North Sea
North West Atlantic (Newfoundland)
North West Africa
Polar regions in Summer
North West Pacific Ocean (Kamchatka Current)
British Isles
Some items found on an Admiralty Routeing chart
General weather conditions for the year, based on previous observations
Ice limits
Shipping routes
Ocean currents
Wind Roses
Mean Sea temp
Mean Dew point temp
Load Line Limits
Define the Inter Tropical Convergence Zone (ITCZ)
And its weather patterns
The equatorial Trough
Considered to be the Earths thermal equator.
Moves North and South with the declination of the sun and change of seasons.
Weather will usually be fair, with variable or light winds, alternating with squalls and thundery showers.
Conditions are generally worst when the trade winds are strongest
Mistral Wind
Cold Northerly Catabatic wind on the Southern French Coast
Sirocco Wind
Warm Dry continental wind from Africa over Europe
Levante
Strong wind of a convergence zone running between the semi permanent LP of the Med, and the semi permanent HP of western Europe
Causes strong easterlies down through the Gibraltar straights
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,
Define Saturation Point
The state of the atmosphere in which an air parcel is at the maximum amount of water vapour it can hold, at a specific temperature
What is a precision aneroid barometer
What it says on the tin. Reads, very precisely, the atmospheric pressure at a specific height
Usually found on MET reporting ships
Must be corrected for temp and height above sea level according to the included calibration card
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 measurement, 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
- The point at which the two lines are 5 degrees apart is considered the ‘warning of approaching dew point’
What are the 6 Air masses surrounding the UK
Arctic Maritime
Polar Continental
Tropical Continental
Tropical Maritime
Returning Polar Maritime
Polar Maritime
What is a ‘mid latitude depression’
Essentially a LP system and frontal system, with a warm and cold front.
Moves NE towards the UK
How to interpret a wind rose on an Admiralty Routeing chart
Define an Isobar
A line of Equal Pressure
