Memnon, King of Ethiopia, was son of Aurora, goddess of the Dawn. Memnon, rather foolishly, went to help the Trojans at the Siege of Troy, and came to grief in a brief encounter with Achilles.
Aurora, to this very day, is unconsoled, and her tears flow frequently in mourning for her only son. They may be seen at early morning in the form of glistening dew-drops twinkling on the grass.
Like cloud and fog, dew occurs because air at a particular time contains more moisture than it can comfortably hold. The first and rather obvious requirement, therefore, for any of these phenomenon, is that the relative humidity be high.
It may be that the air mass has a high humidity anyway - or sometimes the abundance of moisture may be purely local; a recent shower, evaporation from wet soil, or the existence of rivers or lakes in the vicinity may all be relevant factors in this context.
The second requirement is that the sky must be relatively cloud-free. This deprives the Earth of its protective blanket, so that during the sunless night the temperature of the ground falls below the dew-point of the air above it - below the temperature at which condensation begins to take place.
Cooling of the air begins from the ground up. If there is a gentle movement of the air, turbulent mixing allows the loss of heat to be spread through a depth of, perhaps, 30 feet or so of atmosphere, and the result is fog.
If, on the other hand, there is a substantial breeze, the cold air in contact with the ground is thoroughly mixed with warmer air from above; with this dilution of the cold surface air, the temperature of the mixture becomes too high for any condensation and neither dew nor fog will form.
But if it is very calm, the air remains stratified and significant heat loss is confined to the layer of air in immediate contact with the ground; in these circumstances, the excess moisture is deposited on the surface in the form of dew.
In some desert and semi-arid regions, dew may produce an amount of moisture comparable to the annual rainfall, and may be essential to the well-being of local animals and plants. In cold climates, for example, and in climates that are warm but very dry, dew typically contributes about 12mm of "rainfall" every year.
In warm, humid countries, the equivalent figure is about 70mm, and in the case of Ireland and Britain it is somewhere about halfway between the two.