• A thermometer is a major weather instrument.thermometer image by timur1970 from Fotolia.com
  
  Meteorologists use a variety of instruments to measure and forecast the weather. Some standard weather instruments have become more sophisticated over centuries, while others have changed only a little in their basic design.
  
  

Temperature

• A thermometer measures temperature. The National Weather Service keeps each of its official thermometers 6 feet above ground in a white box to keep it shaded. You should also place any outdoor thermometer out of direct sunlight for an accurate reading of temperature. There are electronic and traditional mercury thermometers available.
  
  

Pressure

• A barometer measures atmospheric pressure. The relative difference in pressure between geographic areas drives weather events. For example, the centers of storms are areas of low air pressure. High air pressure normally brings mild weather. Forecasters use atmospheric pressure changes to track changes in weather patterns and make forecasts.
  
  

Precipitation

• A rain gauge measures precipitation. There are two main types of rain gauges. One is a tipping bucket type and the other is a weighing rain gauge. The tipping bucket rain gauge can be pole-mounted or bolted to a level plate. The weighing rain gauge has an attachment that fits around the gauge that creates a vacuum. The vacuum helps reduce the effect of wind and allows the gauge to catch more precipitation. Generally, 10 inches of snow equates to 1 inch of rain.
  
  

Winds

• An anemometer measures wind speed. Some anemometers are hand-held devices, while others are mounted to a stationary object such as a pole, magnetic mount or window. A wind vane measures wind direction.
  
  

Humidity

• A hygrometer measures the relative humidity of air. There are many physical effects that occur as a result of the amount of water vapor in the air. These have been used over the centuries to construct early versions of hygrometers, called psychrometers, which use evaporative cooling of a thermometer bulb. Once the temperature of this bulb and a similar dry bulb were known, the relative humidity could be calculated by using tables. Today, digital and analog hygrometers are available.