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.

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).

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)

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.

  Rain Gauge