meteorology- pressure Flashcards
Atmospheric pressure is measure by a….
barometer
How does an aneroid barometer work
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.
What is a problem with common aneroid barometers
do not maintain accuracy particularly well.
How can changes in pressure be measure on a barometer
by noting the pressure at regular intervals.
What makes a precision aneroid barometer better
it is consistently more accurate
In an precision aneroid barometer the sensing element consists of _____ pressure capsules.
three
What is pressure read to
0.1hPa
0.05 is rounded to the odd tenth
procedure for precison aneriod barometer
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.
A Damping Cap, which has a small orifice and restricts air flow, is fitted to
minimise the effects of
rapidly fluctuating pressure.
Altitude correction
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.
Who is the barometer checked by
y Port Meteorological Officers against a standard instrument.
The battery is replaced when exhausted.
What does a barograph do
records changes in atmospheric pressure.
Recording atmospheric pressure, the operation:
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.
Care and maintenance of barograph
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.
Ships record the amount of cloud cover using the Beaufort notation.
b=
bc =
c=
o =
no cloud
partly clouded- less than half sky covered
cloudy- more than half sky covered
overcast- completely covered
Selected ships report in more detail.
By doing what…
- The amount of cloud in eighths of the sky covered, oktas.
- The types of cloud at different levels.
Wind direction is found by
observation of the sea waves generated.
Wind direction is
is the direction from which the wind is blowing.
It is perpendicular to the crests of the waves.
Wind direction is recorded
in points of the compass by merchant ships.
Wind speed is estimated from
the state of the sea waves generated
The Beaufort scale describes
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.
Various factors affect the relationship between wind speed and sea state:
- 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.
Wind speed is classified in terms of
Beaufort force
Ships record wind direction in
points of the compass
Anemometers and wind vanes may be used to
measure relative wind speed and direction
Anemometers and wind vanes are subject to inaccuracy due to
turbulent flow around the ship’s structure.
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
- Plot the vessel’s movement vector WO using a suitable scale.
- Plot the relative wind vector OA.
- Measure the true wind vector WA.
Wave direction is
the direction from which waves are approaching.
Wave height is
is the height from trough to crest.
Wave period is
the time in seconds taken for successive waves to pass a fixed point.
Wind waves are
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.
Description of sea state Wave height m
Calm _
Rippled -._
Smooth .-.
Slight .-.__
Moderate .__-.
Rough .-_
Very rough -
High -
Very high _-__
Phenomenal >__
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
Swell waves are
waves that have been generated by a wind in another place.
When objects are in sight at known distances these can be used to
estimate visibility
The quality of the horizon may be used
in the absence of other indicators.
A clear horizon indicates ____ visibility; an indistinct horizon indicates ____ visibility.
good
poor
Meteorological reports are encoded to
minimise the volume of traffic.
The Ships’ Code and Decode Book gives full details.
Latitude and Longitude are reported in
Degrees and tenths, rounded down, not to the nearest tenth.
Wind direction is coded in tens of degrees.
