Meteorology Flashcards
List 5 published sources of weather information
- Admiralty Sailing Directions
- Admiralty list of Radio Signals
- Ocean Passages of the World
- Routing charts
- Mariners Handbook
Explain Anabatic and Katabatic Winds
On clear nights the slope of a mountain radiates out their heat and the temperature falls. The air in contact with the slope becomes heavier and rolls down the slope. These Katabatic winds can reach high speeds and are often funnelled down water-courses.
Anabatic winds are the relatively mild uphill reverse of Katabatic winds. As the sun heats up a side of a slope the air in contact with the slope heats up and rises up the slope.
Describe the air movement and associated weather for a stationary front
Two air masses meet. Neither advances to the others territory- Extensive cloudiness & precipitation
What are the four main factors that drive the Earths weather?
- Hot air rises, cool air falls. Hot air being light, and cool air being heavy. This causes low pressures and high pressures respectively.
- Pressures try to equalise.
- Latent heat contained in evaporation and condensation.
- Coriolis effect
How is wind caused?
Wind is caused by air flowing from a high pressure area to a low pressure area.
Explain pressure gradient
Is the force driving the air towards the lower pressure, and acts perpendicular to the isobars.
Explain the Coriolis Effect
Due to the rotation of the Earth, the Coriolis force deflects the direction of the wind to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This causes wind-flow around high and low pressure systems to circulate in opposing directions in each hemisphere. Coriolis force is one of the main factors affecting wind direction.
What is an isobar
Isobar is a line on a meteorological chart joining places of equal pressure. These are plotted at 4 millibar/hPa intervals.
What terms will be used to describe changes in wind direction?
If wind direction changes clockwise it is said to “veer”. If it changes anti-clockwise it is said to “back”.
What are the forces influencing the Gradient wind?
- Pressure Gradient
- Coriolis Force
- Centrifugal Force
What effect does the friction of the Earth’s surface have on wind?
It modifies the direction and strength of the wind and will back and veer according to what angle it strikes a land mass.
What relationship does the distance between isobars have with wind speed?
Wind speed is inversely proportional to the distance between the isobars
Explain Buys Ballots’ Law
In the Northern Hemisphere with one’s back to the true wind, the low pressure will be on the left-hand side.
In the Southern Hemisphere with ones back to the true wind, the low pressure will be on the right-hand side.
Summarise how planetary wind belts are formed
The suns radiated energy is greatest at the equator, where the sun’s rays are most direct. This causes the earth’s surface to be very hot at the equator, causing the air in that region to heat up through conduction, causing a low pressure area, due to air rising to great heights, eventually this air cools and falls back to earth at roughly 30°. Cooler air on the earths surface will fill in to replace the air at the low pressure area’s. At 30° latitude there will be a relatively high pressure, from the sinking cooler air.
The air at the poles will be cold and heavy resulting in the creation of high pressure at both poles. Between the high of 30˚and the polar high, a region partially heated by the sun at about 60˚, an area of relatively low pressure will be created. Warm rising air, upper level winds, falling cool air and surface level winds to replace the air at low pressure areas cause convection cells between these significant latitudes.
What is another name for the Inter-Tropical Convergence Zone, and what wind conditions can be expected here?
The Doldrums- Light and variable winds
What direction do the trade winds blow in each hemisphere?
NE in Northern Hemisphere, SE in Southern Hemisphere
What wind conditions can be expected in the Horse latitudes?
Light, calm and variable
Explain land and sea breezes
The land is heated during the day and cools at night. Sea temperature changes very little and therefore in a 24 hour period will be relatively hotter or colder than the adjacent land. When the land is heated during the day the hot air rises causing a relative low pressure on the land compared to the sea, causing on-shore winds to blow. This is a sea breeze
At night the air over the land cools and falls, and air pressure increases. This causes a relative high pressure on the land compared to the sea, causing off-shore winds to blow. This is a land breeze
How does advection fog occur?
Warm moist air passes over a cool surface by advection (wind) and is then cooled. It is most common at sea when moist air passes encounters cooler waters, including areas of cold water upwelling.
How does radiation fog occur?
Radiation fog occurs at night in clear conditions, where heat radiates from the land and escapes into the atmosphere, which cools the air close to the surface to the dew point, causing fog.
How does frontal fog occur?
Different air masses in a frontal system are forced to mix, the temperature of the warm moisture laden air may drop to below dew point, causing fog.
What is the difference between mist and fog?
Mist has a visibility of more than 1000 metres, fog has a visibility reduced to less than 1000 metres.
What is lapse rate?
Reduction of temperature of the air as it rises, averages 1°C per 100 metres.
What is Adiabatic lapse rate?
The rate at which atmospheric temperature decreases with increasing altitude in conditions of thermal equilibrium.
What is the difference between the atmosphere being stable and unstable?
If a small parcel of unsaturated air moves upwards its temperature will fall at its Adiabatic lapse rate. As it rises its temperature will get closer and closer to the surrounding air until it becomes the same, when this occurs the movement will cease and the atmosphere is then said to be stable. Until this occurs the rising parcel of air is said to be unstable.
How are clouds formed?
When air parcels are cooled to a temperature where it becomes saturated and this cooling continues to dew point, water vapour will condense and a cloud will form.
List and describe 5 ways in which air parcels are cooled?
- Radiation: Water vapour radiates heat and cools the layer without upward movement. This forms layers of cloud of the Stratus type.
- Thermal Convection: A parcel of air is heated by contact with the earth’s surface
and rises as a convection current. It will cool and reach its dew point, producing
clouds of the Cumulus or Cumulonimbus type. - Frontal: Where the air is forced to rise in association with a frontal depression.
- Orographic: When a hill or mountain obstructs the path of the air and forces it to
rise. There will be different types of cloud dependant on the strength of uplift. A
strong uplift could cause Cumulus cloud while a moderate uplift could result in
Stratus. - Turbulence: Moist air being carried up by the turbulence caused by the earths
surface creating a cloud layer. - Convergence: Air is forced to rise due to horizontal converging winds, causing the air to cool at its adiabatic lapse rate.
List 10 types of clouds from highest in the atmosphere to lowest
- Cirrus: Detached delicate fibrous appearance.
Cirrocumulus Small white flakes in groups, lines or ripples. - Cirrostratus: Thin whitish veil.
- Altocumulus: Cotton wool-like puffs.
- Altostratus: Layer or patch of flattened globular mass.
- Nimbostratus: Dark layered rain clouds.
- Stratus: Uniform layer, resembling fog but not on the ground.
- Stratocumulus: Large globular mass, soft, grey with dark patches in groups, lines or
waves. - Cumulus: Thick with vertical development and a horizontal base.
- Cumulonimbus: Heavy masses with great vertical development (Rain).
How does precipitation occur?
Water droplets at great heights will probably be so cold they are in the form of ice crystals. When the water droplets become too large to stay suspended in the air, they will begin to fall. If the air currents are not strong, they will continue to fall, warming as they descend and reach the surface as rain.
If strong uprising air currents force the ice crystals or water droplets up again, the droplets will continue to grow until they are large enough to fall as heavy rain.
If this process continues and the ice crystal continues to grow, it can form a large ball of ice, which
may reach the surface before it melts, as hail.
If the ground temperature is low and the cooling
has been rapid enough, water vapour may skip the liquid phase of condensation and freeze
immediately to fall as snow.
Define relative humidity
The amount of water vapour present in air expressed as a percentage of the amount needed for saturation at the same temperature.
What is used to measure atmospheric pressure?
Aneroid Barometer
Define dew point
The dew point is the temperature the air needs to be cooled to in order to produce a relative humidity of 100%. This temperature depends on the pressure and water content of the air.
What would it mean if the wet-bulb read higher than the dry-bulb on a Wet and Dry Bulb Hygrometer?
The dry-bulb ventilation could be inadequate,
or the wet-bulb could be too dry.
How could the dew point reading warn against fog?
In areas where a rapid fall in sea surface temperature may be encountered a reliable warning of
fog will be given when the dew point is within 5°C of the sea surface temperature.
What is a Whirling Psychrometer?
A whirling psychrometer is a rotating
board on which are mounted the wet and dry
bulb thermometers. The rotation of the
board will provide the correct level of
ventilation and a desired 4-knot wind speed.
List 5 broadcast sources of weather information
- Admiralty Sailing Directions
- Admiralty List of Radio Signals
- Ocean Passages of the World
- Routing Charts
- Mariners Handbook
What are routing charts and what information do they display?
Charts are published for the main ocean areas of the world and are produced for each month of
the year. They contain a wealth of information on winds and currents etc such as:
- Wind roses showing wind speed and direction
- Frequency of Calms
- The frequency and intensity of storms and low pressures
- Ocean currents
- Sea temperatures
- Air temperatures
- Air pressure
- Ice limits based on data averages
- Percentage frequency of low visibility of less than 5 miles and percentage frequency
of fog, where visibility is less than 0.5 mile
List 6 broadcast sources of weather information
- Radio Weather Forecasts for Shipping
- Radio Navigational Warnings
- NAVAREA Warnings
- Coastal Warnings
- Local Warnings
- NAVTEX
What are the NAVTEX subject indicator letters used to show the category of the messages?
- A Navigational Warnings (See also L)
- B Gale Warnings
- C Ice Reports
- D Initial Distress Information
- E Weather Forecasts
- F Pilot Service Messages
- G Decca Messages
- H Loran C Messages
- I Omega Messages
- J Satnav Messages
- L Navarea Warnings (Trial Service). Coastal and Navarea warnings relating to the positions of mobile drilling rigs.
- Z No messages to hand.
What is a Synoptic Chart and what can be found on them?
Synoptic Chart is a chart showing the weather conditions over a large sea area at a given
instant in time.
Pressure systems, cold and warm fronts, depressions, relevant land masses, isobaric patterns.
What is a Surface Analysis Chart and what can be found on them?
Is a chart showing a summary of the general synoptic situation in the area concerned and often an indication of the movement of the pressure systems.
HP& LP systems, temperature, cloud conditions, fronts, air pressure, troughs, squall lines, Beaufort scale, visibility
What is a prognostic chart?
Is a chart showing the forecast positions of the pressure centres, isobars and
fronts 24 hours ahead of the most recent main synoptic hour chart.
What is a surface wave chart?
Chart showing wave height isopleths
and direction of movement. Normally wave heights of less than 2 metres are ignored. The wave
conditions are intimately related to the weather systems existing or forecasted, whether they are
sea waves or swell waves that are considered.
What considerations should be made when passage planning using weather routing?
- Use routing, weather and wave charts
- Obtain latest weather forecasts
- Routing to avoid the worst of the waves which are associated with wind strength
- Prevailing currents
- Access to safe havens if bad weather cannot be avoided
- Avoid possibility of fog
Describe the air movement and associated weather for a cold front
Forces warm air up along a steep front- Clouds, showers thunderstorms
Describe the air movement and associated weather for a warm front
Warm air displaces cold air, warm air encounters less friction with the ground- Extensive cloudiness & precipitation
Describe the air movement and associated weather for a occluded front
Moves rapidly that it overtakes a warm front- Precipitation
Describe the air movement and associated weather for a stationary front
Two air masses meet. Neither advances into the others territory- Extensive cloudiness & precipitation
