Met Service of Jamaica Online

Accuracy Barometer

The Transfer Standard Accuracy Barometer is an electronic instrument that measures atmospheric pressure. The display gives continuous digital recordings of instantaneous pressure and trends.

A change in pressure results in vibrations within the cylinder walls and sets up a corresponding electromagnetic field. Electromagnetic coils sense the frequency of vibration and condition and amplify the signals. State-of-the-art electronic circuits within the instruments then utilize a special (curve-fit) equation to produce pressure calculation algorithms and the result is output in the form of a digital display.

The pressure is applied to the interior of a thin walled sensing cylinder made from a magnetic alloy.

Stevenson Screen

The purpose of the Stevenson Screen is to create, as near as possible, uniform-temperature enclosure at the same temperature as the air outside. .

The screen is designed to shield thermometers from precipitation and direct radiation from the sun, while allowing air to circulate freely round them. It houses maximum and minimum thermometers and a wet-and-dry-bulb hygrometer. It is made from wood with louvered sides to allow the free and even flow of air. The front panel

is hinged at the bottom to form a door, which allows for maintenance and the reading of the thermometers. The whole screen is painted white to reflect radiation

Thermometer

The Thermometer is used to measure (air) temperature. The instruments (usually four: maximum, minimum, wet bulb and dry bulb) are usually housed in a Stevenson Screen, which provides a uniform temperature environment for the thermometers.

With the exception of the minimum thermometer, thermometers for meteorological use are mercury-in-glass instruments. The maximum thermometer has a constriction in the bore between the bulb (which contains a reservoir of mercury) and the beginning of the scale. When temperature rises, expansion forces the mercury column past the constriction.

When temperature falls, the constriction prevents the mercury from being drawn back into the reservoir and the upper part of the column remains at the extreme position reached until it cools down (or is shaken down in the case of the minimum thermometer) giving the user a chance to get a reading.

The minimum thermometer uses clear alcohol instead of mercury. A small index made of black glass is placed within the tube and rests on the meniscus of the alcohol column. As the alcohol expands and contracts, readings are obtained using the position of the index.