A ceiling balloon also called a pilot balloon or pibal, is used by

meteorologists A meteorologist is a scientist who studies and works in the field of meteorology aiming to understand or predict Earth's atmospheric phenomena including the weather. Those who study meteorological phenomena are meteorologists in research, while t ...

to determine the height of the base of

clouds In meteorology, a cloud is an aerosol consisting of a visible mass of miniature liquid droplets, frozen crystals, or other particles suspended in the atmosphere of a planetary body or similar space. Water or various other chemicals may com ...

above ground level In aviation, atmospheric sciences and broadcasting, a height above ground level (AGL or HAGL) is a height measured with respect to the underlying ground surface. This is as opposed to height above mean sea level (AMSL or HAMSL), height above elli ...

during daylight hours. In the past, and sometimes today, a

theodolite A theodolite () is a precision optical instrument for measuring angles between designated visible points in the horizontal and vertical planes. The traditional use has been for land surveying, but it is also used extensively for building and i ...

was used to track the balloon in order to determine the speed and direction of winds aloft. The principle behind the ceiling balloon is that timing of a balloon with a known ascent rate (how fast it climbs) from its release until it disappears into the clouds can be used to calculate the height of the bottom of the clouds.

How it is used

A ceiling

balloon A balloon is a flexible bag that can be inflated with a gas, such as helium, hydrogen, nitrous oxide, oxygen, and air. For special tasks, balloons can be filled with smoke, liquid water, granular media (e.g. sand, flour or rice), or light so ...

is a small, usually red, (fluted)

rubber Rubber, also called India rubber, latex, Amazonian rubber, ''caucho'', or ''caoutchouc'', as initially produced, consists of polymers of the organic compound isoprene, with minor impurities of other organic compounds. Thailand, Malaysia, and ...

balloon commonly measuring 76 mm (3 in) across prior to inflation, inflated to ~40 cm (~15.75 in) diameter. After inflation the balloon is taken outside and released. By timing the balloon from release until it enters the cloud a ceiling height can be obtained. When correctly inflated the balloon will rise at rate of 140 m/min (460 ft/min). The bases of clouds are very rarely flat and solid, so the ceiling height is not when the balloon disappears but when the colour begins to fade. The balloon can also be used to measure the vertical visibility into a layer of

fog Fog is a visible aerosol consisting of tiny water droplets or ice crystals suspended in the air at or near the Earth's surface. Reprint from Fog can be considered a type of low-lying cloud usually resembling stratus, and is heavily influ ...

blowing snow Blowing snow is snow lifted from the surface by the wind, at eye level () or more, that will reduce visibility. Blowing snow can come from falling snow or snow that already accumulated on the ground but is picked up and blown about by strong winds. ...

. In this case the balloon will begin to fade as soon as it is released, so the vertical visibility is when the balloon disappears. If the balloon is visible for a considerable distance into the cloud layer the observer should make note of it as it is of importance to aircraft. The ceiling balloon is a reliable, safe and simple way to get an indication of the height of clouds. However, it does suffer from some disadvantages that the observer must be aware of.

Rain Rain is water droplets that have condensed from atmospheric water vapor and then fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides water f ...

and wet

snow Snow comprises individual ice crystals that grow while suspended in the atmosphere—usually within clouds—and then fall, accumulating on the ground where they undergo further changes. It consists of frozen crystalline water throughout ...

may slow the ascent of the balloon, giving a falsely high ceiling and high winds and poor visibility may cause the balloon to appear to enter the cloud before it actually does. As the balloon rises at a rate of 140 m/min (460 ft/min) it will take over five minutes for the balloon to reach 700 m (2300 ft). Beyond this height the ability to follow the balloon, even with binoculars, is poor, as even the slightest movement of the eye off the balloon will almost certainly ensure that it vanishes. At night when it is not practical to use a balloon the

ceiling projector The ceiling projector or cloud searchlight is used to measure the height of the base of clouds (called the ceiling) above the ground. It is used in conjunction with an alidade, usually positioned 1000 ft (300 m) away and wherever pos ...

is used. However, during twilight it may be impossible to use the ceiling projector and then a pibal (pilot balloon) light may be used. This is a simple flashlight bulb attached to a battery. To charge the battery it is immersed in water for three minutes and then tied to the balloon prior to inflation. These are rarely used today.

Technical details

The balloons and associated equipment are usually stored in a cabinet mounted on a wall close to the gas cylinders. The cabinet has three doors one of which opens down and to it the filler stand is attached. At the top of the filler stand is a "L" shaped pipe with two rings, a small one on the bottom and a larger one on the top called the

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nozzle. The rings stop the tube from dropping through the stand or rising too far when the balloon is inflated. The top ring has several grooves cut into it, to help grip the balloon which is fitted to it. At the bottom of the pipe is a weight that, when the precise amount of gas has been added, will lift to indicate the balloon is full. A rubber hose is attached to this pipe and passes through the filler stand twice. The first hole is larger than the tube to permit movement, while the second is used to hold the tubing in place. From there the tube runs to a

needle valve A needle valve is a type of valve with a small port and a threaded, needle-shaped plunger. It allows precise regulation of flow, although it is generally only capable of relatively low flow rates. Construction and operation An instrument ne ...

that controls the amount of gas flowing to the balloon. A second tube will then run from the valve to a regulator valve that is attached to the gas cylinder. This valve has two pressure gauges attached. One showing the total pressure remaining in the gas cylinder and the second showing the amount of gas flowing through the tubing. Typically the cylinder, which is made of steel and weighs about 140 lb (65 kg). It contains the equivalent of about 200 ft³ (5.7 m³) of gas at standard pressure, stored at a pressure of 2000 psi (14

megapascal The pascal (symbol: Pa) is the unit of pressure in the International System of Units (SI), and is also used to quantify internal pressure, stress, Young's modulus, and ultimate tensile strength. The unit, named after Blaise Pascal, is define ...

s) and will inflate approximately 120 balloons (according to a nominal diameter of 45 cm). On the opposite side of the cabinet is space to store balloons, string and pibal lights. The gas used to fill the balloon is

helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...

. Because of its low cost ceiling balloons are often filled with hydrogen gas, but sometimes helium is used. The balloon is attached to the inflation nozzle and a piece of string is wound around the neck. After donning safety glasses and hearing protection a check is made to ensure the needle valve is fully closed. The main valve on the cylinder is then opened, followed by the regulator valve. Next, the needle valve is opened and the balloon begins to inflate. As the balloon reaches the correct size the inflation nozzle will begin to lift. At this point the needle valve is closed along with the regulator valve and cylinder valve. The string is then used to tie off the balloon neck to ensure that no gas can escape. Caution must be used during inflation because the occasional faulty balloon can burst. If the person inflating the balloon is not wearing goggles or hearing protectors then eye or ear damage can result.

References

*Environment Canada - Atmospheric Environment Services, Technical Manual (TM07-01-01) Ceiling Balloon Equipment 76 mm (3 in). {{Meteorological equipment Balloons (aeronautics) Meteorological instrumentation and equipment

How it is used

Technical details

See also

References