A Guide To The Scientific Knowledge Of Things Familiar Part 50

A. These fogs (which occur generally in the winter time) are occasioned thus:--Some current of air (being suddenly _cooled_) _descends into the warm streets_, preventing the rise of the smoke, and _forcing it back in a ma.s.s_ towards the earth.

Q. _Why are there not ALWAYS FOGS every night?_

A. Because the air will always hold in solution a certain quant.i.ty of vapour, (which varies according to its temperature): and when the air is _not saturated with vapour_, it may be condensed without parting with it.

Q. _Why are there EVER FOGS at night?_

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_.

FOG is generally applied to _vapours condensed on land_, especially if those vapours are _laden with smoke_.

Q. _What is the reason why condensed vapour sometimes forms into CLOUDS, and sometimes into FOG?_

A. If the surface of the EARTH be _hotter than the air_, then the vapour of the earth (_being chilled by the cold air_) becomes FOG: but if the AIR be _hotter than the earth_, the vapour _rises through the air_, and becomes CLOUD.

Q. _If cold air produces FOG, why is it not foggy on a FROSTY MORNING?_

A. 1st--Because _less vapour is formed on a frosty day_; and

2ndly--The vapour _is frozen upon the ground_ before it can rise from the earth, and becomes h.o.a.r-FROST.

Q. _Why are FOGS more general in AUTUMN than in spring?_

A. In spring _the earth is not so hot_ as it is in autumn. In AUTUMN the _earth_ is generally _warmer than the air_; and, therefore, the vapour (issuing from the earth) _is condensed into fog_ by the chill air.

Q. _Why are FOGS more common in VALLEYS than on HILLS?_

A. 1st--Because valleys _contain more moisture than hills_: and

2ndly--They are _not exposed to so much wind_, (which dissipates the vapour).

Q. _How does WIND dissipate FOGS?_

A. Either by _blowing them away_; or else by _dissolving them into vapour again_.

Q. _What is h.o.a.r-FROST?_

A. There are two sorts of h.o.a.r-frost: 1.--FROZEN DEW: and 2.--FROZEN FOG.

Q. _What is the cause of the GROUND h.o.a.r-FROST, or frozen DEW?_

A. Very _rapid radiation of heat from the earth_; in consequence of which, the _surface is so cooled down_, that it _freezes the dew_ condensed upon it.

Q. _Why is h.o.a.r-FROST seen only after a very CLEAR NIGHT?_

A. Unless the night has been very clear indeed, the earth will not have thrown off heat enough by radiation, to _freeze_ the vapour condensed upon its surface.

Q. _Why does h.o.a.r-FROST very often COVER the GROUND and TREES, when the water of rivers is not frozen?_

A. h.o.a.r-frost is not the effect of cold in the _air_, but the cold of the _earth_ (produced by excessive radiation); in consequence of which, _the dew_ (condensed upon it) _is frozen_.

Q. _Why is the h.o.a.r-FROST upon GRa.s.s and VEGETABLES much thicker than that upon lofty TREES?_

A. Because the air (resting on the _surface of the ground_) is much colder after sun-set, than the _air higher up_; in consequence of which, more vapour is condensed and frozen there.

Q. _Why is the AIR (resting on the surface of the EARTH) colder than that in the HIGHER regions?_

A. Because the _earth radiates more heat_ than the _leaves of lofty trees_; and, therefore, _condenses and freezes_ the vapour of the air _more rapidly_.

Q. _Why are EVERGREENS often FROST-BITTEN, when lofty trees are NOT?_