Manual Instruments

What are Meteorological Instruments?
Meteorological instruments are the equipment used to sample the state of the atmosphere at a given time.

A meteorological station is a facility either on land or sea, with instrumemnts and equipment for measuring atmospheric conditions to provide information for weather forecasts and to study the weather and climate.

Tempertature

Stevenson Screen

This wooden screen is used to shield the thermometers from direct sunlight, rainfall and other environmental factors. It allows air to circulate freely creating a uniform temperature in the screen. It is positioned at about 1.25–2 meters above ground on a metal stand and is white in color to reflect direct solar radiation.

Dry Bulb Thermometer

The dry bulb thermometer is used to measure temperature of freely exposed air, but shielded from radiation and moisture.

Wet Bulb Thermometer

The WBT measure the temperature read by a thermometer covered in water-soaked cloth (muslin) over which air is passed.

The Psychrometer is a hygrometer consisting of one DBT and one WBT. The difference between the readings of the thermometers gives an indication of the atmospheric humidity.

Maximum Thermometer

This is mercury in glass thermometer with a constriction in the bore between the bulb and the beginning of the scale. This constriction prevents the mercury column from receding with falling temperature. This thermometer is reset by forcing the mercury back through the constriction. This is done with a quick rotation of the wrist.

Minimum Thermometer

This is an alcohol in glass thermometer with a glass index immersed in the alcohol. When the temperature falls, the surface tension of the meniscus of the alcohol pushes the index down to the bore of the thermometer. When the temperature raises again the index remain at its lowest position. The end of the index furthest from the bulb indicates the lowest temperature. Raising the bulb end so that the index slides down the bore of the thermometer resets the thermometer.

Precipitation

Standard Rain Gauge

This rain gauge consists of a funnel leading into a glass bottle inside a brass can. The base is embedded about 6–8 inches in the ground for stability. An extra bucket is added to prevent loss of rainfall due to overflow of bottle. Rainfall is collected in the bottle and is measured using a graduated measuring cylinder.

Recording Rain Gauge

Tilting Siphon Recorder (Float)
Contains a float inside a drum. When it rains water is collected inside the drum. The float which is connected to a pen then rises and records the amount of rainfall on a chart.

Tipping Bucket Rain Gauge
Rainfall from the funnel falls into the first bucket. When this is filled it tips and empties its water. The second bucket now begins to collect water, when this is filled it empties the water. With each tip it sends a signal to a recorder where the amount of rainfall is recorded. The recorder can either be an electrical one or a chart on a drum, which is connected to a clock, and rotates once a week.

Soil Temperature

Soil Thermometers

These are mercury in glass thermometer mounted in a glass or plastic tubes with their bulb embedded in wax. The wax enables the thermometer to be removed from the tubes and read before the temperature has time to change appreciably. The thermometer is placed in a steel tube, which is sunk into the earth to a depth of either 1 foot it 4 feet. The thermometer is held by a chain that is connected to a metal cap which covers the steel tube.

Grass Minimum Thermometer

The grass minimum temperature recorded in open air ground on short turf with the bulb of the thermometer just in contact with the tops of the blades of grass. The alcohol column overnight causes a small indicator within the stem of the thermometer to descend and record the lowest temperature.

Evaporation

Evaporation Pond

Evaporation of water is measured using a Class-A Evaporation Pan.

During the day the sun heats the water in the pan and causes it to evaporate. Since the water is evaporated the level of the water in the pan falls. The water level in the pan is measured using a hook-gauge which consists of a movable scale fitted hook. When properly set the point of the hook breaks the ripples that may be present in the pan and also acts as a support for the hook-gauge. The pan is reset whenever more than 25mm of water is evaporated or when it is overflowed.

Wind

The Anemometer measures the Wind Speed and the Wind Vane measures the Wind Direction.

Anemometer

The Anemometer consists of three cups positioned at 120° apart. The cups are connected to an electrical generator. When the wind blows, the cups turn and thus the generator motor turns. Current is the produced in the generator and is displayed on a dial. The current produced is proportional to the wind speed.

Wind-vane

The Wind-vane consists of a fin type tail and a round knob position in the direction the wind is blowing from. When the direction of the wind changes from a specific direction, this same action is repeated and displayed on a dial where it is read.

Cup-counter Anemometer

This instrument measures the average wind speed for the day. A counter is fitted to an anemometer, which turns according to the wind speed. By observing the reading at the beginning and end of a period, the average wind speed for that period can be calculated.

Sunshine

Campbell-Stokes Sunshine Recorder

The Campbell-Stokes sunshine recorder is used to measure sunshine hours. It consists of a glass sphere which focuses the sun’s rays on heat sensitive cards causing it to burn a trace.

There are three types of cards which correspond to the position of the sun at different times of the year.

1. Long curved cards are used from April 12 to September 2;
2. Straight cards are used from September 3 to September 14;
3. Short curved cards are used from October 15 to February 28 or 29.
4. Straight cards are then used from March 1 to April 11.

Solarimeter

This instrument measures both sunshine hours and radiation. The Solarimeter consists of two sets of heat sensitive diodes inside a glass dome. When there is a temperature change between the two sets of diodes caused by the heat of the sun the voltage changes and this change is transmitted to the integrator. This change in voltage is proportional to the energy input in watts per square meters.

Pressure

Kew Pattern is used to check other barometers for accuracy. The barograph gives graphical displays of the pressure changes over time.

Digital Barometer

The application ranges of the series DB-X are the measurements of pressure in meteorological stations, aviation use and in laboratories. The barometric pressure sensor is designed for measurements that require excellent accuracy, fast dynamic response and long-term stability and reliability.

Radiosonde

The radiosonde is a balloon borne, battery operated instrument used together with a ground receiving equipment to create a vertical profile of the atmosphere. It sends a signal of all measured variables back to the ground receiving stations where a computer calculates the measured variables and displays them on a screen. This instrument measures pressure, temperature, relative humidity, wind direction and wind speed. It uses these measurements to determine the stability of the atmosphere.