The transparency (or clarity) of water is measured using a Secchi disc. A Secchi disc is a circular disc, typically 20 cm in diameter, divided into alternating black and white quadrants. It is lowered into the water on a calibrated rope or chain until it is no longer visible from the surface. The depth at which the disc disappears from view is called the Secchi depth, and it provides a simple, standardised measure of water transparency.
Water transparency is an important ecological parameter because it indicates how far light can penetrate, which directly affects photosynthesis by aquatic plants and phytoplankton. Low transparency (shallow Secchi depth) may indicate high levels of suspended sediment, algal blooms, or dissolved organic matter.
The other instruments measure entirely different environmental factors:
A hygrometer measures the humidity (moisture content) of the atmosphere.
A barometer measures atmospheric pressure.
An anemometer measures wind speed.
Exam tip: Ecological measurement instruments are commonly tested. Remember that the Secchi disc measures water clarity, while a hydrometer (not hygrometer) measures the relative density of liquids.
