Glossary of terms

cloud -a visible collection of tiny water droplets (or ice particles) in the atmosphere above the earth’s surface; the droplets by convention have diameters less than about 40 um, a range of sizes where they have negligible settling velocities.

collector -a device that intercepts water droplets present in the air; droplets impact on the vertical surface of the collector as a result of being given a horizontal velocity component by the wind.

condensation -the physical process by which water molecules in the vapor form are changed into a liquid; this results in the formation of dew, fog and cloud.

dew -water condensed onto grass and other objects near the ground.

drizzle – small water drops that have an appreciable settling velocity but can be suspended by moderately strong rising currents of air; the drops are larger than fog droplets and smaller than raindrops; in meteorological observations the diameters are less than 0.5 mm and in cloud physics generally less than 0.3 mm; the low end of drizzle drop sizes is poorly defined but in cloud physics usage would extend down to cloud droplet sizes of 40 um.

drop -a small spherical particle of water; the term is usually used for particles in drizzle and rain; smaller particles are called droplets.

droplet -a very small spherical particle of water; the term is used for particles in clouds or fogs; larger particles are called drops.

fog – a visible collection of water droplets (or ice particles) suspended in the air and touching the earth’s surface; by international definition it reduces visibility to less than 1 km; the droplet diameter range is the same as for cloud, that is 1 to 40 um, however, because fogs are often not very deep, the droplets are normally at the low end of the size range.

haze – fine dust or salt particles dispersed through a portion of the atmosphere; the particles are very small with diameters of the order of 0.1 um; at humidities considerably less than 100 %, some haze particles begin to acquire a coating of water molecules
mist -this term causes much confusion as there are two
different definitions in common use; by international definition it is an aggregate of microscopic water droplets intermediate between haze and fog; in practice this is an exceedingly difficult distinction to make as haze itself is imprecisely defined.

in North American usage mist is considered the same as drizzle, that is intermediate between fog and rain; the term should be avoided in scientific discussions.

occult precipitation -a term that has been around since the last century and really refers to an observed effect rather than a process; when the ground was wetted, particularly under trees, and no rain had been measured, the water was attributed to ‘occult’ precipitation; later it was realized that fog may have been the source of the water but the name persisted even though fog is not a form of precipitation.

precipitation -any water or ice particle that falls from the atmosphere-and reaches the ground.

rain -precipitation in the form of liquid drops which have diameters greater than 0.5 mm, or, if widely scattered, the drops may be smaller; in cloud physics, drops with diameters greater than 0.3 mm are usually considered rain.

vapour -a substance existing in the gaseous state at a temperature lower than that of its critical point; all water substance found in the atmosphere in the gaseous state is called water vapour.