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Weather Instruments

Some surface weather instruments used at a weather station are:


An anemometer measures wind speed by a rotating fan (which consist of 3 or 4 cups). Data is collected when the wind propels the fan and is measured by recording revolutions of the fan versus a peiod of time. Speed may be recorded in miles/hour, meters/second, or nautical miles/hour (knots).  


To make a simple anemometer:
You would need:
5 paper cups
2 straws
Paper punch
A sharp pencil with an eraser attached


(1) Punch a hole in the side of each of the 4 paper cups and connect them to the fifth cup using the straw (as shown in the diagram on the left). Note: the open ends of each cup must face the same direction.

(2) Jon the intersection of the two straws onto the end of the eraser of the pencil with the pin.

(3) The wind velocity of the wind can be found by counting the number of rounds per minute and by measuring the circumference of the circle (in cm) made by the rotating cups. The units for the wind velocity would then be in cm per min.

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A barometer monitors changes in air pressure. There are two types of barometers - Mercury Barometer and Aneroid Barometer. The latter may also be linked to a pen that records a clock-driven drum chart. Such an instrument is called a Barograph and it provides a continuouss record of air pressure with time.

Principle: The Mercury Barometer consist of a glass tube inserted into a dish of mercury. The surrounding air presses down on the mercury in the dish and forces some of thee mercury up into the tube. The height to which the mercury rises in the tube is directly proportional to the atmospheric pressure.


Mercury Barometer



To make a simple barometer:
You would need:
1 glass tube 36 inches long, closed at one end
Mercury or coloured water
Ring stand with a clamp
Cardboard strip
A meter ruler
(1) Fill the glass tube with mercury (or coloured water).

(2) Place a finger over the open end of the tube and invert the tube, lowering it into the beaker containing the remainder of the mercury. Clamp the tube to the stand.

(3) Mark a scale of centimeters on the cardboard, labelling it from 60cm to 100cm.

(4) With the meter ruler, measure the actual height of the mercury column and attach the scale to the proper spot on the tube.

(5) Record 1 or 2 pressure readings per day. Increasing air pressure often means that a high pressure area is approaching, bringing clearing or fair weather. Decreasing air pressure often indicates the apprach of a low pressure area, which often brings clouds and precipitation.


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A hygrometer measure humidity or the amount of moisture in the air. This is done by measuring the change in length of an organic fibre (e.g. human hair) brought about by the absorption of moisture. A hair hydrometer may be designed to activate a pen on a clock -driven drum. Such an instrument, called a Hydrograph gives a continuous record of fluctuations in humidity over a given time period.
Principle: The human hair cells will indicate the level of moisture in the air by expanding (when air is moist) and contracting (when air is dry).



Thermohydro Electronic Thermo-Hygro Recorder

To make a simple hydrometer:
You would need:
A piece of wood
A flat piece of thin plastic
Some glue
2 small nails
3 long strands of human hair (about 8 inches long)
(1) Cut the plastic into a triangle to make a pointer

(2) Fix a nail loosely through the pointer and glue the hair onto the nail

(3) Pull the hair taut and glue the other end of the hair to the another nail

(4) Attach the nail to the top of the wood making sure the pointer is perpendicular to the hair

When the air is moist, the hair will expand and lengthen. This will make the pointer point down. When the air is dry, the hair will contract and the pointer points up.


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Nephoscopes, used for measuring the speed and direction of cloud motion, may be divided into two types: Grid Nephoscope and Mirror Nephoscope.

Grid Nephoscope

Grid Nephoscope

Principle: The grids in the Grid Nephoscope are used as referenc in finding the speed and direction of the cloud. The vertical pointer in the Mirror Nephoscope is placesd such that the image of a cloud appears in line with it and the center of the disk. The image is then watched, keeping in line with the pointer. The direction fo the cloud motion can be read from the graduation of the edge of the disk.

Mirror NephoscopeMirror Nephoscope

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A Psychrometer measure the humidity of the air. To make measurements the wick is dipped in distilled water. One such psychrometer is a Sling Psychrometer (shown on the right) where measurements are taken by whirling the psychrometer with the handle.

Principle: Evaporation fo water from the wick on the wet bulb thermometer leads to cooling. Using a relative humidity table, the temperature difference between the two thermometers gives the relative humidity of the air (in percent)




To make a simple psychrometer:
You would need:
2 Thermometers
Masking tape
Wet Gauge and elastic bands
A piece of cardboard and a fan
Relative Humidity Table
(1) Tape the thermometers onto the cardboard

(2) Using elastic bands, tie the wet gause around the bulb of one of the thermometers

(3) Turn the fan onto the thermometers until the temperatures stop falling. Record the temperatures. Subtract the temperatures on the wet thermometer from that of the dry bulb thermometer.

(4) Look up the Relative Humidity Table. The value where the row and column readings intersect gives the relative humidity of the air.


Relative Humidity Table

Relative Humidity Table

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A Rain Gauge measures rainfall over a period of time. It consists of a funnel at the top that directs rainwater into a long and narrow cylinder which is seated inside a larger outer cylinder.  

Rain Gauge

A Tilting-siphon Rain Recorder

Rain Gauge Rain Gauge

To make a simple rain gauge:
You would need:
A high cylinder
A funnel
A ruler inserted into a measuring tube

(1) Place the measuring tube into the funnel and place then into the cylinder as shown

(2) Leave the rain gauge in an open area. When it rains, rain water will enter the measuring tube through the funnel. The amount of rain water collected over a period of time can then be read off from the scale of the ruler.

Rain Gauge

Rain Recorder Rain Recorder

Weather radars, Lightning detectors, Wind profilers, Satellites and Radiosondes are also used as tools in forecasting the weather

Lightning Detection System

Lightning Detection System (Provides location of lightning activities. Blue dots on map (below) shows the location of the Lightning Detectors.)


Wind Profiler System (Provides the direction and speed of the winds from just above ground level to over 5 km)

Wind Profiler

Radar Tower

Weather Radar (Provides locations of the thunderstorm clouds, height of cloud top, wind shear, velocity of wind and amount of precipitation in clouds)


Radiosonde (A hydrogen filled balloon carrying a radiosonde is released twice daily at the Upper Air Observatory. Data such as wind direction and velocity, temperature and relative humidity of the atmosphere up to 100,000 ft are recorded and transmitted to the ground station for further processing)



Satellites (Both Polar Orbiting and Geostationary satellites give us valuable weather information. Geostationary satelite images are obatined hourly while Polar Orbiting Satellite images are obtained only when the satellite passes ver our region.)


Other weather information are obtained from ship reports, aircraft reports, reports from oil rigs, weather buoys and other unmanned weather stations. These reports provide velocity of winds and weather conditions in the oceans and in the sky.

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©2007 National Environment Agency