MET Flashcards
(105 cards)
1
Q
If an unsaturated parcel of air is cooled beyond its dew point, what will happen?
A
It will become saturated and some of the water vapour will condense out, forming clouds.
2
Q
What is ‘dew point’
A
The temperature at which a parcel of air is saturated and ahs a relative humidity of 100%.
3
Q
What is the difference between absolute and relative humidity?
A
- Absolute humidity is the quantity of water in the air, expressed in grams per cubic metre.
- Relative humidity is absolute humidity in relation to the quantity required for the air to be saturated, expressed as a percentage
4
Q
What is the troposphere?
A
The lowest layer of the atmosphere. It is the layer which contains almost all of the water and most of what we think of as ‘weather’.
5
Q
Complete the sentence:
When air rises, it
A
cools due to expansion.
6
Q
At what rate does unsaturated air cool with altitude?
A
10 degrees C/km.
7
Q
What type of atmospheric stability results in clear skies?
A
Absolute stability, where the parcel of air is always cooler than the surrounding air and therefore wants to sink back down to its original altitude.
8
Q
What does SALR stand for?
A
Saturated Adiabatic Lapse Rate.
9
Q
What is it called when air is forced to rise because of two or more air flows meeting?
A
Convergence.
10
Q
What is it called when air is forced to rise due to land?
A
Orographic uplift.
11
Q
What is a fluffy cloud at low altitude called.
A
Cumulus.
12
Q
What does a nimbostratus cloud look like?
A
A nimbostratus cloud is a flat layer of cloud at low altitude which brings rain.
13
Q
How might clouds change when a depression is approaching?
A
As a depression approaches, high clouds will appear and it will thicken and lower as the depression gets closer.
14
Q
What are squalls?
A
A sudden and dramatic increase in wind speed, which lasts longer and is more significant than a regular gust. It can also cause a significant shift in the wind direction.
15
Q
Which air mass forms over Northern Canada and Greenland, and moves directly across the North Atlantic to the UK?
A
POLAR maritime.
16
Q
How is an air mass which originates over land termed?
A
Continental.
17
Q
What are the characteristics of maritime air?
A
Maritime air masses are more humid (contain more water) than continental air masses.
18
Q
What are the expected weather conditions in the UK when under influence of a tropical continental air mass?
A
- Warm.
- Dry.
- Clear skies.
19
Q
Which type of breeze occurs because land absorbs heat more quickly than water?
A
Sea/onshore breeze.
20
Q
What causes a land/offshore breeze?
A
- The land cools quicker than the sea at night.
- Air over the land cools and sinks generating a high pressure in contrast to the lower pressure over the sea.
- This creates a pressure gradient which results in the land/offshore breeze.
21
Q
What drives the north-east monsoon?
A
The intense winter cold across the Tibetan Plateau and Central Asia.
22
Q
Why is the summer monsoon south-westerly when the pressure gradient is north-south?
A
Because the Coriolis effect bends/deflects the air flow to the right in the northern hemisphere, resulting in a south-westerly wind.
23
Q
What is the pressure over central Australia in the wet season and what is the effect of that pressure?
A
The wet season is the result of the intense heat in central Australia causing air to rise and thus creating a low pressure. This draws warm, humid air from the Indian Ocean over Northern Australia, which causes the wet season.
24
Q
Why are the seas in the Arabian Sea rougher during the SW monsoon than during the NE monsoon?
A
- Wind, speeds are higher during the more powerful SW monsoon.
- South Westerly winds have a much greater fetch (distance travelled) and are thus larger.
25
What are the weather conditions along the India's SW coast during the SW monsoon?
The SW monsoon draws warm, humid air from the Indian Ocean so the weather is very unstable with high levels of precipitation and squally conditions.
26
Which of the jet streams drives the weather experienced in Northern Europe?
The Polar Jet.
27
What causes El Nino?
Weaker than normal trade winds allow the warm water that is normally pushed over to the western Pacific Ocean spread more evenly across the ocean. This reduces the normal temperature gradient.
28
What does ITCZ stand for?
Inter-Tropical Convergence Zone.
29
Which cell is driven by the intense heat of the sun causing air to rise at the equator?
The Hadley cell.
30
What are the three cells that make up the globes circulation?
- Hadley.
- Ferrel.
- Polar.
31
Why are there often deserts approx 30-40 degrees Lat?
Because that is the latitude at which the air descends between the Hadley and Ferrel Cells. This air is typically cool, dry and stable, so there is very little rainfall.
32
What are created along the Polar Front and blown towards Northern Europe by the Polar Jet?
Depressions (low pressure systems).
33
What is the average global atmospheric pressure?
1013 mb.
34
What is 'pressure gradient'?
The change in air pressure per unit of horizontal distance.
35
Which unit of pressure is equivalent to millibars (mb)?
Hectopascals (hPa).
36
What doe an isobar show?
Isobars link areas of equal pressure on a pressure chart, in much the same way that contours link areas of equal depth or height on charts and maps.
37
What is the difference between geostrophic wind and gradient wind?
Geostrophic wind blows parallel to the isobars, at altitude. Gradient wind (real wind) is the result of friction slowing the geostrophic wind and causing it to cross the isobars by the angle of indraft.
38
Which way does a low pressure system rotate in the southern hemisphere?
Clockwise.
39
What is a depression?
Depression is a term often used to describe mid latitude low pressure systems.
40
Which way does a low pressure system rotate in the northern hemisphere?
Anti-clockwise.
41
Which way does a depression rotate in the southern hemisphere?
Clockwise.
42
How does front symbology indicate the motion of the front?
The triangles/semi-circles point in the direction of motion of the front.
43
What happens when the cold front catches up with the warm front?
It lifts the warm air mass off of the ground and causes an occlusion.
44
Where do depressions from?
They typically from along the Polar Front, which is the boundary between the Ferrel and Polar Cells.
45
What causes depression to dissipate?
- Once most of the warm air in the warm sector has been forced upwards, the temperature gradient between the different sectors will reduce because the warm air cools as it rises.
- At this point the air stabilises and the system dissipates.
46
Does an occluded font occur when a cold front catches up with a warm front or when a warm front catches up with a cold front?
Cold front catches up with a warm front.
47
How will cloud cover and precipitation change as a warm front approaches?
As a warm front approaches cloud cover will increase and lower. Precipitation will initially be light and intermittent but it will become moderate or heavy in the vicinity of the front.
48
Which front is steeper.
Cold front.
49
What happens to the wind as the cold front passes in the northern hemisphere?
As the cold front passes, the wind will veer dramatically and sudden squalls may occur.
50
What is the warm sector?
The warmer air mass wedged between colder air ahead and behind.
51
Why are conditions in an anticyclone typically stable with clear skies?
Because the descending air is generally cool and dry.
52
Why does the temperature in an anticyclone differ with the seasons?
- It's warmer in the summer, because clear skies mean that there's nothing to block the intense infrared radiation from the sun.
- In winter, it's colder because there's not a lot of infrared radiation but there's also no cloud cover to prevent what warmth there is from escaping into the atmosphere.
53
Which way does an anticyclone rotate in the northern hemisphere?
Clockwise.
54
Is air rising or falling in an anticyclone?
Falling.
55
Why do tropical storms not normally form on the equator?
They need the Coriolis effect to begin rotating and there is insufficient Coriolis effect below aprrox 5 degrees of latitude.
56
What are tropical storms called in the Indian Ocean?
Cyclones.
57
What is the minimum sea temperature for tropical storms to form?
The sea temperature needs to be more than 26 degrees C for tropical storm to form.
58
What is a tropical storm?
A tropical storm is an intense and rapidly rotating low pressure system which develops within tropical areas of the world.
59
Why can't tropical storms form over land or at higher latitudes?
They can only form in an area where there is a lot of warm, moist and rising air. There isn't enough moisture over land and it's not warm enough (more than 26 degrees C) at higher latitudes.
60
What is the approximate Hurricane season in the Caribbean Sea?
Hurricane season in the Caribbean Sea can be summarised as JASON or JJASON, i.e (June), July, August, September, October and November.
61
How can El Nino and La Nina affect tropical storm formation and strength?
Multi-annual fluctuations in Pacific Ocean sea surface temperatures caused by EL Nino and La Nina, can lead to weaker and stronger tropical storms in different parts of the world.
62
What magnitude of a fall in atmospheric pressure is confirmation that a tropical storm is in the vicinity?
5mb.
63
What change in the wind might signal that a tropical storm is inbound?
If the wind strengthen to a degree and/or was coming from direction which is not normal for the region (i.e trade winds.)
64
At what wind speed (in knots and on the Beaufort Scale) is a hurricane declared?
- Over 64 knots.
- Force 12, (hurricane force).
65
Which organisation is the primary source of hurricane forecasting in the Caribbean?
The National Hurricane Center of the United State's National Oceanic and Atmospheric Administration.
66
How can swell indicate the presence of a tropical storm?
A long low swell, travelling outwards from the centre of the storm, may be the first indication of a tropical storm's approach. Particular attention should be paid when the swell direction and height differs from the norm in the region.
67
In the northern hemisphere, if the wind is veering and the pressure is falling, where should the relative wind positioned in order to take avoiding action?
Starboard bow.
68
What is the average speed of a tropical storm prior to it recurving?
Tropical storms tend to move at approximately 10 knots in their early stages. This may increase but rarely exceed 15 knots before they have recurved.
69
What is Buys Ballot's Law and how can it be used in the southern hemisphere?
Buys Ballot's Law can be used to determine the centre of an area of low pressure from the perspective of the observer. In the southern hemisphere, if the observer faces the wind the centre of the low will be 100-125 degrees to their left or 10-35 degree over their left shoulder.
70
If the pressure is falling and the wind is backing, where are you in relation to the tropical storm?
Advance Left.
71
Where is the Dangerous Quadrant in the Northern hemisphere?
Advance Right.
72
What is the correct action to take if you find yourself in the Dangerous Quadrant in the Southern hemisphere?
Place the wind on the port bow and make best speed. Maintain the relative wind direction as the wind backs.
73
Describe, in general terms the movement of a 'standard' tropical storm in the North Atlantic.
Having formed in low latitudes in the mid- or eastern-Atlantic, tropical storms tend to move roughly Westerly to West North Westerly across the Atlantic before recurving away from the equator in the vicinity of 25 N degrees latitude and eventually settling onto a north-easterly track.
74
Why is the Dangerous Quadrant called what it is?
The Dangerous Quadrant is so named because the storm is more likely to recurve that way, the wind and waves push any nearby vessels into the path of the sto0rm and the wind speed is higher.
75
Which sea state is worse: rough or high?
High.
76
What is the Beaufort Scale used to quantify?
Wind speed, based on observed sea conditions.
77
Which meteorological instrument is used to measure humidity?
A hygrometer.
78
Which hygrometer should be used when taking temperature readings?
The one on the windward side of the vessel.
79
What is the difference between a barograph and a barometer?
A barometer measures pressure at a point in time, whereas a barograph records pressure change over time.
80
How do you take a pressure reading with a precision aneroid barometer?
The micrometre dial on the right hand side is rotated whilst the black button is compressed until the indicator light jumps from one setting to another to indicate that the value indicated matches the ambient pressure. The pressure is then read from the read - out.
81
What must the OOW be cautions about when measuring the wind speed?
They must understand whether the wind speed being fed into the Bridge equipment is relative or true and, when more than one anemometer is fitted, they must ensure that the windward one is being used.
82
What causes 'depression of the wet bulb'?
The tip of the wet bulb thermometer is wrapped in clean muslin which is dipped into a reservoir of distilled water. The evaporation from the muslin absorbs heat from the thermometer, lowering the temperature recorded. The drier the air, the greater the evaporation and the greater the depression.
83
Which series publications should be consulted during NAVTEX set-up?
ALRS
84
how many routing charts are there?
Each ocean has twelve routing charts, one for each month of the year.
85
What is weather routing?
Weather routing is the use of weather information to determine a safer, quicker and/or more economical route between two ports.
86
What are Viking, Fair Isle and Fortier's?
Sea areas in the Met Office's Shipping Forecast.
87
Why does geographic wind not correctly predict the wind not correctly predict the wind speed at ground level?
Geostropic wind speed is reduced at the surface due to friction between the moving air and the water or land below.
88
By how much is geostrophic wind reduced to get the real/gradient wind at the surface?
By half if over land or a third if over water, to determine the real wind speed at the surface.
89
What is frontolysis?
Frontolysis means that the front is dissipating because the temperature gradient is decreasing at the surface.
90
Wat is the change in pressure from one isobar to the adjacent one?
4 mb.
91
What is the symbology and direction of travel for a cold front?
Acold front is indicated by a blue line with blue triangles pointing in the direction of travel of the front.
92
Which type of fog occurs when warm, moist air passes over cold water?
Advection fog.
93
Which type of fog occurs at the boundary between warm and cold air masses?
Frontal fog.
94
What causes advection fog?
It occurs when warm, moist air passes over a colder surface. The colder surface cools the warm, moist air to below its dew point which causes condensation.
95
What is the definition of fog?
A cloud at ground level, which reduces the visibility to below 1000m.
96
What causes radiation fog?
At the land cools during the night, it cools air close to the surface. This air is cooled to below its dew point, causing condensation and forming fog. Although it has formed over land, it can flow out over the sea, although not normally further than 10-15nm and it is normally burnt off fairly quickly once the sun rises.
97
What is the 'depression of the wet bulb'?
the difference between the dry and wet bulb temperature is known as the 'depression of the wet bulb'.
98
Where can the Dew Point Table be found?
NP100
99
What is plotted against the dew point to determine the likelihood of fog forming in the future?
The sea surface temperature.
100
What are the two types of ice accretion?
Salt water and fresh water.
101
Below what air temperature does salt water ice accretion occur?
-2 degree C.
102
Which factors must be present for salt water ice accretion to occur?
- Air temperature of less than -2 degree C.
- Adverse weather conditions.
- Vessel' motion causing significant amounts of spray and/or water on deck.
103
Which type of ice accretion creates equipment and safe access issues but is unlikely to place the vessel in danger?
Fresh water ice accretion.
104
How can ice accretion affect GNSS and radio antennas?
GNSS antennas covered in a layer of insulating ice may not receive the relatively weak signals from orbiting satellites, so position accuracy may suffer. Radio antennas may suffer from reduced effective range and whip aerials could snap if the ice becomes too heavy.
105
Which factors must be present for fresh water ice accretion to occur?
- Air temperature of less than 0 degree C.
- Precipitation.
