The Dickinson press., 1899-02-11
lsoTHERMs.

Facts About Isothermio Maps That
Are RarelyISLearnsa n schsol:
Here go somedefnitaons Of isotherms
that alp,pearS inAmeffean textbooks Of
Geography:
.Those lines which are drawn through
places with an equal average Of temper
allure are called1sctherms.
A"1sotherms~ are lines connecting
places having fhe;sssxne mean tempera-
ture for particular periods. as the whole
Of the ySar. the winter or summer
months, etc.
IF upon a map all places having
the same mean temperature are con-
'nected by lines, such lines are called
isothermal lines Of simple isotherms."
These definitions are part Of the
truth. but not the whole of it. It may
not be q great calamity, but the fact is
that most boys and girls leave school
with q misconception az to what an
isotherm is, and they rarely find out in
later years +
They all know that two elements,
latitude and altitude, are the main fac
tors in determining the mean temper
allure of q place; that the farther a
place is from the equator and the higher
1t stands above sea level the cooler it's
climate is But they do not know that
tsothermic maps take into account only
n Of these elements, and that is lati-
tude. They eliminate the influence Of
altitude. The isotherm passing over the
top OF Pixies peak does not show the
mean temperature at the summit of the
mountain, but what the mean temper
allure would be in that immediate neigh
borhood il the land. instead of rising
high above the5sea. stood at the level Of
Coney island.
DO you5s see the reason for thisi It
may be easily explained. Most of the
land; does NOT rise so high above the sea
that the temperature is greatly affected
by altitude. To the majority of man
kind latitude is a far more important
climatic element than altitude. Now,
the effects on temperature of both lati-
tude and altitude cannot well be shown
on one man, and isothermic maps were
devised to show the effects of latitude
and some other element. such as posi-
ton near the sea OF in the far interior.
An isothermic line, therefore, does not
show the actual NEAR temperature of a
place on it unless that place is at sea
level But it is easy to deduce from the
isotherm the actual mean temperature
Of a place, if we know its elevation
above the sea. How this is done is very
clearly explained by DR.. II. R. Mill.
the British ~geographer, as follows:
''The air grows cooler by 1 degree F.
for every 270 feet of elevation above
sea level, but isothermic lines show the
sea level temperature. In using isother-
mic maps we must therefore remember
that places 600 feet above the sea level
have a temperature Q degrees lower
than the isotherms Indicate; places
5000 feet above the sea, % degrees
lower; those 10, 900 feet above the sea,
As degrees lower, and the mountain
slopes 10, 900 feet above the sea no less
than Sc degrees lower than the sea level
temperature shown by the .isotherms.
This accounts for the fact that none Of
the important towns in the temperate
zones is situated more than 1900 feet
above the sea, while in the tropics they
are, built at as great elevations al 8. 900
or 10, 900 feet"
Weather charts are an exception to
this rule. They record the actual
thermometrical readings at the points
of observation New York Sun.