MANCLIM Manual of Climatological Observations: temperature

• 3.1 General
• 3.2 Equipment
• 3.3 Reading of thermometers
• 3.4 Resetting of thermometers
• 3.5 Defects in thermometers
• 3.6 Maintenance of thermometers
• 3.7 Storage of thermometers
• 3.8 Reporting temperature observations

3.1 General --

Temperature may be defined as the degree of warmth or cold as measured on some definite temperature scale. The temperature scale most commonly used in meteorology is the Celsius scale.

The Celsius scale is devised so that there are 100 divisions, or degrees, between the freezing point and boiling point of water (see Figure 3-1). Note that water freezes at 0°C and boils at 100°C. Other references on the Celsius scale are body temperature (37°C), room temperature (20°C), and the freezing point of mercury (-39°C).

Figure 3-1: Celsius temperature scale

3.2 Equipment

The equipment normally supplied for the measurement of air temperature consists of:

• A Stevenson Screen complete with stand;
• A maximum thermometer; and
• A minimum thermometer.

If spare thermometers are available, they may be provided for back-up purposes. Spare thermometers must be stored according to the MSC guidelines (see 3.7).

3.2.1 The Stevenson Screen

The Stevenson Screen was designed by Thomas Stevenson (1818 - 1887), a British civil engineer and father of the author Robert Louis Stevenson. The Stevenson Screen is a double-louvered wooden box (see Figure 3-2), painted white and specially designed to shield the thermometers from the direct rays of the sun, from precipitation, and from heat radiation from the ground. The louvered construction allows the free flow of air over the thermometer bulbs. So that meaningful comparisons can be made with other stations, the screen is always mounted on a stand at such a height that the thermometer bulbs are between 1.25 m and 2 m above ground level. The door faces north and opens downward.

Figure 3-2: Stevenson Screen (mounted on stand)

The site for the Stevenson Screen is usually chosen by the MSC Maintainer and should not be changed without this person's approval. The Stevenson Screen should be positioned as follows:

• Ideally on a level plot of ground at least 7 m x 7 m in size and covered with grass that is kept short;
• Removed from any tree or building, etc., by a distance at least twice the height of the tree or building, etc.; for example, the screen should be at least 20 m away from a tree that is 10m high;
• Screen must be level;
• Screen door must face north so that the sun will not shine on the thermometers when the door is open; and
• The stand on which the screen is mounted comes prefabricated and, if installed as shown in Figure 3-2, the floor of the screen will be approximately 115 cm above ground level. One side of the stand does not have a cross-brace. This will be the north side.

The inside of the screen should be kept clean and free from debris. Care should be taken not to block airflow by storing large objects in the screen. The screen will be replaced or painted by an MSC Maintainer as required.

Figure 3-3: Stand for Stevenson Screen

3.2.2 Thermometers

The liquid-in-glass thermometer is the standard used by official climatological observers. Liquid-in-glass thermometers operate on the principle that the volume of the liquid increases as the temperature rises, thus causing the liquid in the tube to expand. As the temperature falls, the liquid in the bulb decreases in volume and the liquid column becomes shorter in most thermometers, the maximum thermometer being the exception.

Each thermometer is inserted into a protective sheath to which two rings are attached. The sheath containing a thermometer hangs from brass hooks in the screen. The bulb end of each thermometer should be to the left so the figures on the stem of the thermometer will be upright.

The minimum thermometer, in its protective aluminum sheath, is hung on the appropriate hooks in the thermometer shelter in a horizontal position (see Figure 3-2).

The maximum thermometer, in its protective aluminum sheath, should be hung in the screen nearly horizontally but with the bulb very slightly lower than the stem (see Figure 3-2). It should hang below the minimum thermometer, because the upper part of the screen is more likely to be warmed by the sun near the time of maximum temperatures, thus giving an exaggerated high reading.

3.2.3 The maximum thermometer

The maximum thermometer is a mercury-filled thermometer. A maximum thermometer records the highest temperature which has occurred since the maximum thermometer was "reset."

In the maximum thermometer, the tube through which the mercury rises has a constriction just above the bulb (see figures 3-4 and 3-5). This constriction in the maximum thermometer permits the expanding mercury to pass through the narrow opening, but as the temperature decreases the mercury is trapped above the constriction and remains there until the observer reads the maximum temperature. The thermometer must be reset by forcing the mercury to return through the constriction into the bulb. When this thermometer has been properly reset, it will indicate the air temperature at the time of resetting.

Figure 3-4 Maximum thermometer in protective sheath

Figure 3-5: Maximum thermometer

The maximum thermometer is graduated in half degrees to indicate temperatures within the range 50°C to -39°C. There are no graduations below -39°C because the mercury freezes at this temperature. To avoid damage which would result from the freezing of the mercury, the maximum thermometer must be taken indoors when the air temperature drops to -37°C. It then becomes necessary to estimate the maximum temperature from the minimum thermometer (see 3.3.1).

3.2.4 The minimum thermometer

The minimum thermometer is alcohol-filled. The identifying feature of the minimum thermometer is the small dark dumb­bell-shaped rod called an index, which is immersed in the alcohol and moves freely through the column (Figure 3-7). As the temperature drops, the index remains at the end of the retreating alcohol column. When the temperature begins to rise, the end of the index farthest from the bulb will indicate the lowest temperature that was reached.

The minimum thermometer is graduated in half degrees to indicate temperatures within the range 45°C to -70°C.

The freezing point of the alcohol thermometer is much lower than the mercury thermometer; therefore, there is no requirement to bring it indoors at any time.

Figure 3-6: Minimum thermometer in protective sheath

Figure 3-7: Minimum thermometer showing the index

3.3 Reading of thermometers

Do not stand any closer to the thermometer than is necessary to get an accurate reading. The observer is cautioned to avoid breathing on the thermometers or using matches to light the interior of the screen after dark. These actions could increase the temperature appreciably, especially during cold weather. When additional light is needed in the screen, a flashlight should be used.

To read a maximum mercury thermometer accurately, the eye should be level with and at right angles to the end of the column. In mercury thermometers, the meniscus is convex (see Figure 3-8)- that is, the end of the mercury column bulges out, or away from the bulb end. This bulging may not be noticeable, but in any event, it is the extreme end of the column that should be observed.

When reading the minimum alcohol thermometer, it should be at right angles to the end of the index farthest from the bulb. The reading should be taken from the right-hand end of the index.

Figure 3-8: Reading a maximum thermometer

Temperatures are more reliable when they are observed according to a routine plan. To achieve this reliability, the following steps are recommended:

It is important that the resetting procedure always be carried out in the above-described order. If the mercury in the bore is not in contact with the constriction at the beginning of the downward stroke, it may bang against it on the downswing and break or shatter the constriction. This undoubtedly leads to the failure of many maximum thermometers, especially in cold weather.

Note: Maximum thermometers manufactured by the JUMO Company may appear to have a break in the mercury column in the area of the constriction. No attempt to reunite the column in this area should be made after the thermometer has been reset to the current air temperature. This does not indicate that the thermometer is broken.

Figure 3-12: Procedure for resetting maximum thermometer

When handling the thermometer, never hold it at the bulb end because the heat from your hand will heat the mercury in the bulb and it will not be possible to correctly reset it to the current air temperature until the thermometer cools down again.

When the sun is shining, try to keep the thermometer out of the direct rays of the sun (by shading it with your body) because the mercury reacts very quickly to this heat source and warms up, thus making it impossible to reset the thermometer to the current temperature.

Some maximum thermometers prove very hard to reset in cold weather and it may sometimes be entirely impossible to do so. In this case, replace the thermometer with a spare. In all cases when a thermometer is replaced by a spare, report it to your supervising office and a replacement will be sent.

The minimum thermometer should be reset without removing it from the screen. Simply unhook the right eyelet of the protective sheath and lower the right end of the thermometer until the index slides to the end of the alcohol column. Re-hook the right eyelet of the protective sheath to return the thermometer to its horizontal position. Check again to make sure that the index is resting against the end of the alcohol column.

3.5 Defects in thermometers

In a maximum thermometer, if the constriction is damaged by improper resetting, it may act as an ordinary thermometer, because the mercury thread fails to break off at the constriction. Such a thermometer is known as a "retreater." If the thread does not break at all, the defect is soon discovered, since the readings of the maximum thermometer will always coincide with the air temperature at the time of the observation. Occasionally, a thermometer is found in which the column will "retreat" for a degree or two and then break off. This is much more serious, because it is more likely to remain undiscovered, especially at climatological stations where readings are not necessarily taken near the normal time of the maximum temperature. This fault is also more likely to occur in cold weather. A thermometer showing this defect should be taken out of service and replaced by a spare maximum. If there is doubt about whether or not the maximum thermometer is defective, and a spare is available, the observer may run a test for a few days with both thermometers installed in the screen.

In a minimum thermometer, the alcohol column can quite often develop breaks or separations (see Figure 3-13). This defect could be caused by rough handling during shipment or it could occur in the screen due to vibrations caused by the wind, banging of the screen door, etc.

Figure 3-13: Broken alcohol column

Alcohol separations are caused by distillation. The alcohol that separates from the main column will usually collect in the extreme end of the thermometer farthest from the bulb. The observer must carefully inspect the condition of the alcohol column from end to end.  If the alcohol column is found to be broken into sections, the thermometer should be taken out of service and replaced by a spare.

The defective minimum thermometer may be repaired by the observer. The broken alcohol column can be reunited by holding the thermometer with the bulb end down and tapping it gently on the palm of the hand, the inner pages of a book or a piece of soft rubber (see Figure 3-14).

Figure 3-14: Reuniting a broken column

After all the bubbles or breaks in the column have been removed, the thermometer should be kept indoors for at least two hours in a vertical position, bulb downward, to allow any liquid that may have collected on the walls of the bore to drain down into the main column. This thermometer can be retained as a spare.

When the above-mentioned methods fail to unite the column, or if for any other reason the thermometer is defective, the nature of the defect should be reported to your supervising office with your request for a replacement thermometer.

3.6 Maintenance of thermometers

Thermometers supplied to climatological stations have low maintenance requirements. Thermometers should be cleaned occasionally to remove dust and dirt that can accumulate on the outer surface and make readings difficult. This is done simply with a damp cloth.

During periods of heavy blowing snow, it may be necessary to remove any accumulation that has entered the screen through the louvers.

Care should be taken  never to let thermometers be unduly heated. Close proximity to a strong source of heat can cause the thermometer to burst.

3.7 Storage of thermometers

The amount of mercury in a maximum thermometer is not large but a spill indoors can be hazardous to your health. Mercury thermometers should never be stored indoors. Mercury is a hazardous substance. Observers should store spare maximum thermometers outside whenever possible. Maintenance on maximum thermometers should also be performed outdoors. Spare thermometers must be stored as recommended in the MSC guidelines. Care should be taken never to store the thermometers in close proximity to a strong source of heat as the thermometer can burst.

The MSC Maintainer for your site must be informed immediately if breakage occurs and mercury is spilled.

3.8 Reporting temperature observations

For thermometers at climatological stations that are equipped with Meteorological Service of Canada instruments only for measuring temperature and precipitation or precipitation, the official station record is the entry in COOLTAP/IVR system. Refer to the COOLTAP or IVR manual for instructions on how to use these applications.

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