New Ulm review., 1899-01-25
lSOTHERMS.

Facts About Isotfcrmic Maps That
Are Rarely Learned ut school,.
Here are some definitions of isotherms
that appear in American textbooks Of
geography:
"Those lines which are drawn through
places with an equal averae e Of temper
ature are called isotherms
1'Isotherms are lines connecting
places having the same mean tempera
ture for particular periods, as the whole
Of the year, the win-~r or summer
months, etc.
eff upon a map all places having
the same mean temperature are con-
nected by lines, such lines are called
isothermal lines or simple isother1ns. >
These definitions are part OF the
truth, but not the whole Of it. It may
not DE a great calamity, but the fact is
that most boys and girls leave school
with a misconception as to what an
isotherm is, and they rarely find out in
later years.
They all know that two clements
latitude and altitude, are the main fac
tors in determining the mean temper
ature of a place; that the farther g
place is from the equator and the higher
it stands above sea level the cooler it's
climate is. But they do not know that
isothermic maps take into account only
one 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
ature would be in that immediate neigh
borhood if the land, instead of rising
high above the sea, stood at the level of
Coney Island.
Do you see the reason for this? 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 far more important
climatic element than altitude. Now,
the effects on temperature of both lati-
tude and altitude cannot we'll be shown
on one man, and isothermic maps were
devised to show the effects of latitude
and some other element, such as posi-
tion near the sea or in the far interior.
An isothermic line, therefore, does not
show the actual mean 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 place, if we know it's elevation
above the sea. How this is done is very
clearly explained by DR.. Fl. R. Mill,
the British geographer, as follows:
''The air grows cooler by 1 degree +.
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 degrees lower
than the isotherms Indicate; places
5000 feet above the sea, % degrees
lower; those $4,000 feet above the sea,
As degrees lower, and the mountain
slopes IS, 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 as 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.