This chapter contains instructions for the operation of meteorological instruments, related equipment, care and use, and instrument evaluating procedures of various equipment.

Prepare sky cover height tables for ceiling lights as follows:

a.  Compute tables for each ceiling light baseline on the basis of:

h = b x tan Q; where "b" is the baseline, "Q" is the angular reading, and "h" is the sky cover height or vertical visibility.

b.  Add the difference between the height of the observation site and the field elevation to each tabular value. (Use ground elevation for stations not located at airports.)

c.  Round each of these sums to the reportable height increment.

(1)  5,000 feet or less - to the nearest 100 feet.

(2)  5,001 to 10,000 feet - to the nearest 500 feet.

(3)  Above 10,000 feet - to the nearest 1,000 feet.

17-3.   CEILING LIGHT

If a ceilometer is not operational or available, night-time sky cover heights and vertical visibility values may be determined with a clinometer and a ceiling light projector by using the following procedures.

a.  Turn on the ceiling light projector, and allowing the pendant to swing freely, sight through the clinometer and center the intersection of the cross-hairs on:

(1)  the brightest portion of the light beam spot when the sky cover is aloft, or

(2)  the upper limit of the light beam penetration when the sky is completely obscured by a surface-based layer.

b.  When the pendant has come to rest, lock it in position without moving the clinometer. Read the indicated angle to the nearest whole degree.

c.  Repeat this procedure two more times; average the three angular readings and refer to the table for the baseline (i.e., distance from clinometer to projector) to obtain the equivalent height value of this averaged angular reading.

17-4.   MAINTAINING CEILING LIGHT

a.  Daily Checks. At least once each day turn on the lamp, if shut off by a timer, to determine if it is operational. During daylight hours, the operation of the lamp can be observed from the office if a metal reflector is placed at the top edge of the cover. Replace the lamp if it is determined to be defective.

b.  Weekly Checks. Clean the outside of the cover glass with water and detergent once each week or more often if the local conditions require it.

c.  Monthly Checks. Clean the mirror and the inside of the cover glass using water and detergent. Wipe both surfaces completely dry using a soft lint-free cloth. CAUTION: The concentrated rays of the sun can burn the skin or injure the eyes of personnel and may damage equipment. Therefore, if direct rays of the sun reach the mirror, take precautions to shade the projector. If the projector has drainage holes in the mirror or housing, clean the holes to ensure adequate drainage and ventilation. With the light turned off, inspect its condition and if the envelope has darkened appreciably, replace it.

d.  Painting. As frequently as necessary to protect the instrument, paint the stand using a suitable primer and a finishing coat of aluminum paint. All rust should be removed by sanding before painting.

Once each month check the clinometer as follows:

a.  Examine the scale graduation and clean the scale if necessary.

b.  Test the action of the clutch and indicator for normal operation. When unlocked, the indicator should react freely to elevation angle changes of 1 or 2 degrees. If it does not, a drop of SAE 10 oil on the bearings may improve the action. When the clutch is locked, ordinary vibrations or light shocks should not disturb the indicator.

c.  Examine the cross wires for proper alignment.

a.  Storage. Ceiling balloons should be stored in a dry location at room temperature, away from large electric motors and generators, and in such a manner that oldest balloons will be used first.

b.  Conditioning. When balloons from a particular package have a tendency to break during inflation or flight, the remaining balloons from that package should be conditioned within 24 hours before inflation. Conditioning consists of uniformly heating the balloon for 30 minutes to 8 hours depending on the temperature to which the balloon is heated; that is, 30 minutes at air temperatures near the boiling point of water (about 100°C), or 8 hours at temperatures near 49°C. Temperatures below 49°C will not condition the balloon and much above 100°C will likely scorch or harden them.

(1)  Balloons may be conditioned by suspending them in a stream of hot air (above 49°C) from a heating system, or by placing them on a hot water or steam radiator, or on the reflector of a small incandescent lamp (not over 100 watts). When balloons are heated through contact with a solid object, they should be turned occasionally to ensure uniform conditioning. This precaution is particularly important with high temperatures. The surface to be used can be checked quickly with a few drops of water. If the water boils or evaporates rapidly, the surface is too hot and may damage the balloon.

(2)  Balloons may also be conditioned in boiling water. Plug the neck of the balloon to keep water out and immerse all of the balloon, except the neck in the water for about 5 minutes. Shake all water from the balloon and have it relatively dry before inflation.

c.  Inflation. The balloon should be dry inside and relatively dry outside, especially during freezing weather. Remove any trapped air by folding and squeezing the balloon in the hand; then stretch the neck sufficiently to insert the inflation nozzle. Place the nozzle so that the tubing connecting the nozzle to the regulator rests on a support and is not suspended from the nozzle. Inflate the balloon slowly and turn off the gas as soon as the balloon begins to lift the nozzle. Add or remove gas from the balloon until the nozzle is suspended just above the support when any motion of the balloon has stopped. Listen for leaks in the balloon. Tie the balloon neck securely just above the nozzle and remove the balloon from the nozzle. Double the end of the neck over the tied portion and either tie it or secure it with a rubber band. Figure 17-1, 10-Gram Balloon Ascension Rates, presents 10-gram balloon ascension rates. Figure 17-2, 30-Gram Balloon Ascension Rates, presents 30-gram balloon ascension rates.

Figure 17-1. 10-Gram Balloon Ascension Rates