The Sauk Centre herald., 1899-02-16
.lsoTHERMs.

Facts About Isothernifc Maps That
Are Rarely Learned at school.
Here are some definitions of isotherms
that appear in American textbooks of
geography:
Those lines which are drawn through
places with an equal avera. Of temper
ature are called isotherms
sIsotherms are lines connecting
places having the same mean tempera-
ture for particular periods. as the who'll
of the year. the win.4f or summer
months, etc.
IF upon map all places having
the same mean temperature are con-
nected by lines, such lines are called
isoahermal lines or simple isotherms.
These definitions are part of the
truth, but not the whole of it. It may
not be 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 elements,
latitude and altitude, are the main fac
tors in determining the mean temper
ature of a place; that the farther a
place is from the equator and the higher
it stands above sea level the cooler its
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 Theisotherm 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? 1t
may be easily explained. Most of the
land does not rise soshigh 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 well be shown
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 a place, if we know its elevation
above the sea. How this is done is very
clearly explained by DR.. H. R. Mill.
the British geographer, as follows:
'.The air grows cooler by / 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 800 feet above the sea level
have temperature degrees lower
than the isotherms indicate; places
5000 feet above the sea. % degrees
lower; those 10. 900 feet above the sea.
45 degrees lower, and the mountain
slopes 1S, 600 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 1000 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.