A. If the air be pretty well saturated with vapour during the day, as soon as its capacity for holding vapour is lessened by the cold night, it deposits some of the superabundant vapour in the form of dew or fog.
Q. Why is there very often a fog over marshes and rivers at night-time?
A. The air of marshes is almost always near saturation; and, therefore, the least depression of temperature, will compel it to relinquish some part of its moisture in dew or fog.
Q. What is the difference between dew and rain?
A. In dew, the condensation is made near the earth’s surface:
In rain, the drops fall from a considerable height; but the cause of both is the same, viz.—cold condensing the vapour of the air, when it is near the point of saturation.
Q. Why does mist and fog vanish at sunrise?
A. Because the condensed particles are again changed into invisible vapour, by the heat of the sun.
Q. What is the difference between a mist and fog?
A. Mist is generally applied to vapours condensed on marshes, rivers, and lakes.