meteorology- pressure

Question 1

Atmospheric pressure is measure by a….

Answer 1

barometer

Question 2

How does an aneroid barometer work

Answer 2

A sealed capsule of thin metal expands and contracts in response to changes in external atmospheric
pressure.
This movement is magnified mechanically to rotate a pointer.
A graduated circular scale indicates the corresponding pressure.
An adjustment is provided to set the correct pressure.

Question 3

What is a problem with common aneroid barometers

Answer 3

do not maintain accuracy particularly well.

Question 4

How can changes in pressure be measure on a barometer

Answer 4

by noting the pressure at regular intervals.

Question 5

What makes a precision aneroid barometer better

Answer 5

it is consistently more accurate

Question 6

In an precision aneroid barometer the sensing element consists of _____ pressure capsules.

Answer 6

three

Question 7

What is pressure read to

Answer 7

0.1hPa
0.05 is rounded to the odd tenth

Question 8

procedure for precison aneriod barometer

Answer 8

Press the button switch.
Turn the knurled knob in accordance with the instructions until the Magic Eye is just continuous then
discontinuous with small adjustments of the knob.
Continuous - increase reading,
Discontinuous - decrease reading.

Question 9

A Damping Cap, which has a small orifice and restricts air flow, is fitted to

Answer 9

minimise the effects of
rapidly fluctuating pressure.

Question 10

Altitude correction

Answer 10

Pressure for reporting purposes is standardised as pressure at sea level.
The instrument is sited above sea level and reads a lower pressure.
The temperature of the air affects its density and therefore the pressure difference.
The reading is corrected for altitude above sea level by means of a table.

Question 11

Who is the barometer checked by

Answer 11

y Port Meteorological Officers against a standard instrument.
The battery is replaced when exhausted.

Question 12

What does a barograph do

Answer 12

records changes in atmospheric pressure.

Question 13

Recording atmospheric pressure, the operation:

Answer 13

The aneroid mechanism is used to operate a pen drawing a trace on graduated paper on a chart on a
drum turned by clockwork.
The changes in atmospheric pressure can be readily seen and used for forecasting purposes.
The barograph supplied to Selected Ships has a cylinder filled with oil around the pressure capsule to
reduce the effects of vibration and rapid pressure fluctuations.

Question 14

Care and maintenance of barograph

Answer 14

The barograph should be sited in a shaded position not subject to vibration or concussion.
It should be aligned athwartships so that the arm does not lift off the paper when the ship rolls.
The chart is changed weekly.
The date is recorded on the chart.
The corrected atmospheric pressure is recorded on the chart when it is changed.
The chart time is aligned with UT.
The chart is time marked at 12:00 UT each day.
The clockwork is wound when the chart is changed.
The pen arm is adjusted to minimise the pressure, consistent with a continuous trace.
The pen is refilled or changed when exhausted.
The indicated pressure is adjusted if significantly in error. This adjustment is noted on the chart.

Question 15

Ships record the amount of cloud cover using the Beaufort notation.

b=
bc =
c=
o =

Answer 15

no cloud
partly clouded- less than half sky covered
cloudy- more than half sky covered
overcast- completely covered

Question 16

Selected ships report in more detail.
By doing what…

Answer 16

• The amount of cloud in eighths of the sky covered, oktas.
• The types of cloud at different levels.

Question 17

Wind direction is found by

Answer 17

observation of the sea waves generated.

Question 18

Wind direction is

Answer 18

is the direction from which the wind is blowing.
It is perpendicular to the crests of the waves.

Question 19

Wind direction is recorded

Answer 19

in points of the compass by merchant ships.

Question 20

Wind speed is estimated from

Answer 20

the state of the sea waves generated

Question 21

The Beaufort scale describes

Answer 21

the various sea states associated with different wind speeds, assuming that
the observation is made in deep water away from land after the wind has been blowing for long enough
to raise the appropriate sea, approximately 24 hours.

Question 22

Various factors affect the relationship between wind speed and sea state:

Answer 22

• Swell present must be discounted; only sea waves are relevant to wind estimation.
• Fetch. A high wind blowing offshore will not generate waves of the expected height close to the
  shore.
• Duration. Time is required for the wave height to develop to the appropriate level.
• Current and/or tidal stream. Water movement contrary to wind direction shortens and steepens
  the waves, and conversely.
• Precipitation has a smoothing effect on the sea surface.
• Shallow water steepens and shortens sea waves.
• Wind waves may be higher for a given wind speed if the air is significantly colder than the sea.

Question 23

Wind speed is classified in terms of

Answer 23

Beaufort force

Question 24

Ships record wind direction in

Answer 24

points of the compass

Question 25

Anemometers and wind vanes may be used to

Answer 25

measure relative wind speed and direction

Question 26

Anemometers and wind vanes are subject to inaccuracy due to

Answer 26

turbulent flow around the ship’s structure.

Question 27

The movement of the vessel must be allowed for by vector analysis to find the true wind speed and direction.
How would you go about doing this

Answer 27

• Plot the vessel’s movement vector WO using a suitable scale.
• Plot the relative wind vector OA.
• Measure the true wind vector WA.

Question 28

Wave direction is

Answer 28

the direction from which waves are approaching.

Question 29

Wave height is

Answer 29

is the height from trough to crest.

Question 30

Wave period is

Answer 30

the time in seconds taken for successive waves to pass a fixed point.

Question 31

Wind waves are

Answer 31

the waves being generated by the wind that in blowing at the time of the observation.
The height of the waves is recorded as a sea state.

Question 32

Description of sea state Wave height m
Calm _
Rippled -._
Smooth .-.
Slight .-.__
Moderate .__-.
Rough .-_
Very rough -
High -
Very high _-__
Phenomenal >__

Answer 32

0
0-0.1
0.1-0.5
0.5-1.25
1.25-2.25
2.5-4
4-6
6-9
9-14
>14

Question 33

Swell waves are

Answer 33

waves that have been generated by a wind in another place.

Question 34

When objects are in sight at known distances these can be used to

Answer 34

estimate visibility

Question 35

The quality of the horizon may be used

Answer 35

in the absence of other indicators.

Question 36

A clear horizon indicates ____ visibility; an indistinct horizon indicates ____ visibility.

Answer 36

good
poor

Question 37

Meteorological reports are encoded to

Answer 37

minimise the volume of traffic.

Question 38

The Ships’ Code and Decode Book gives full details.
Latitude and Longitude are reported in

Answer 38

Degrees and tenths, rounded down, not to the nearest tenth.
Wind direction is coded in tens of degrees.