meteorology- temps Flashcards
What are meteorological observations used for
forecasting, and planning operations such as cargo ventilation.
To measure air temperature accurately the thermometer must be
shielded from direct radiation from the sun or other heat sources.
The thermometer is placed in a Marine Screen, also known as a Stevenson Screen. This has a solid top and bottom, and louvred sides
to permit free flow of air while sheltering the thermometer from radiant heat, rain and spray.
Why is the stevenson screen painted white
to reflect heat
Why should the stevenson screen be hung on the windward side
To measure air temperature free of any influences from the ship
How is humidity measured
Masons hyrdrometer
What does a masons hydrometer consist of
This consists of a Marine Screen with two thermometers inside it.
One thermometer measures the air temperature. This is the Dry Bulb thermometer.
The other thermometer has a wick over the bulb leading to a reservoir of pure water.
The water migrates up the wick and keeps it damp.
Water evaporates from the wick drawing Latent Heat from it and reducing the temperature shown by the wet Bulb thermometer.
The depression of the wet bulb is
the amount by which the Wet Bulb thermometer reads lower than the
Dry Bulb thermometer
The rate of evaporation is
directly proportional to the temperature and inversely proportional to the
humidity of the air
When temperature is high and / or humidity is low….
evaporation is rapid, drawing large amounts of latent
heat, and the depression of the wet bulb is large.
When temperature is low and / or humidity is high…..
evaporation is slow, drawing small amounts of latent
heat, and the depression of the wet bulb is small.
When the air is saturated
no evaporation occurs, no latent heat is drawn, and the depression of the wet bulb is zero.
Why are temps read at 0.1°C
for humidity purposes
In the MH how often must the wick be renewed
Weekly
And after rough weather when spray may contaminate the water
In the MH how often must the reservoir be replenished
Weekly with pure water, or more frequently in conditions of low
humidity.
Why is it important the the reservoir remains pure
Impure water evaporates at different rates, leading to inaccurate measurements.
Precautions when observing temperatures:
The screen must be to windward
Why?
to minimise the effect of the ship.
Precautions when observing temperatures:
Read the dry bulb then the wet bulb
Why?
as radiation is affecting them as soon as the door is
opened.
Precautions when observing temperatures:
The dry bulb must be what?
Clean and dry
Precautions when observing temperatures:
The wet bulb wick must be
Must be damp, not dripping, if temperature is above freezing.
Precautions when observing temperatures:
The wet bulb wick must be what if temps are below freezing?
Must be coated with a thin layer of ice if temperature is below freezing. Water may have to be carefully applied, and the temperature allowed to stabilise.
How is sea water temp measured
bucket method
hull sensor
engine intake
What is the bucket method
Ships making observations for the Meteorological Office are supplied with a specially constructed sea
temperature bucket.
This has a double skin and is constructed to
ensure flow through the bucket when it is towed through the
water.
The bucket is attached to a line, which is attached to the ship.
- It is lowered into the sea clear of ___________ of water from machinery or cargo spaces.
- It is towed through the water to allow the bucket to acquire the ______ _____________.
- It is then hauled aboard and taken to a __________ location.
- The thermometer is inserted in the bucket and allowed to acquire the temperature of the water.
- The temperature is read to an accuracy of .° C without removing the bulb of the thermometer
from the water.
discharges
water temperature
sheltered
0.1
Hull sensor
Certain ships may be fitted with a _______ attached to the inside of the ship’s ____ just below the water
line which gives a remote measurement of sea temperature by conduction through the hull plating.
This is only suitable for ships whose ________ does not change significantly, or sensors may be fitted for light and loaded conditions.
The cabling required makes this an ___________ method.
sensor
hull
draught
expensive
Engine intake
Other ships use the sea temperature measured at an intake of sea water to the engine room.
This is the least accurate method because:
- the point of intake is significantly below the surface
- the water entering the intake has been affected by the ship’s hull.
