Annual Weather Review 2011

  1. The National Environment Agency
  2. Corporate Functions
  3. Resources
  4. Publications
  5. Annual Reports
  6. Annual Weather Review 2011
  7. Weather Instruments

Weather Instruments

Anemometer

An anemometer measures wind speed by a rotating fan (which consists 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 period of time. Speed may be recorded in miles/hour, metres/second or nautical miles/hour (knots).

Anemometer

Barometer

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 continuous record of air pressure with time.

The mercury barometer consists 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 the mercury up into the tube. The height to which the mercury rises in the tube is directly proportional to the atmospheric pressure.

Barometer

Hygrometer

A hygrometer measures 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 hygrometer 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).

Hygrometer

Psychrometer

A psychrometer measures 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.

Evaporation of 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)

Psychrometer

Rain Gauge

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. A chart in the tilting-siphon rain recorder records the amount of rainfall over the period.

Tilting-siphon Rain Recorder

Tilting-siphon Rain Recorder  

Rain Gauge

Rain Gauge

Rain Recorder

Rain recorder 

Radiosonde

A helium 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.

Weather balloon is inflated with helium gas.

The weather balloon is inflated with helium gas.

Release of weather balloon

The balloon is released with a radiosonde attached to its base.