Williston graphic. (Williston, Williams County, N.D.) 1895-1919, 1899-03-24
ISOTHERMS.
A Few Facts About Isothermic Maps
That Are Rarely Learned
ut School.

Here are some definitions of iso-
therms that appear in American text
books of geography.
Those lines which are drawn
through places with an equal average
of temperature are called isotherms.
"IsCtherms are lines connecting
places having the same mean tempera-
ture for particular periods, as the
whole of the year, the winter or sum-
mer months, etc.
IF upon a map all places having the
same mean temperature are connected
by lines, such lines are called isother-
NAL lines or simple isotherms."
These definitions are part of the
truth, but not the whole of it. 1t may
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 ele-
ments in determining the mean temper
allure of a place; that the further a
place is from the equator and the
higher it stands above sea level the
cooler the climate is. But they Go not
know that isothermic maps take into
account only one of these elements,
and that is latitude. They eliminate
the influence of altitude. The iso,herm
passing over the top of Pixies Peak
does not show the mean temperature at
the summit of the mountain, but what
the mean temperature would be in that
immediate neighborhood if the land,
instead of rising 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 SO high above the
sea that the temperature is greatly af-
fected by altitude. To the majority Of
mankind, latitude is far more impor-
tant climatic element than altitude.
Now the effects on temperature Of
both latitude and altitude cannot well
be shown on one map; and isothermic
maps were devised to show the effects
of latitude and some other elements,
such as position near the sea or in the
far interior.
An isothermic line, therefore, does
not show the actual mean temperature
of place on it unless that place is at
sea level. But it is easy to deduce from
the isotherm the actual mean tempera-
ture of 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 l degree
Fahrenheit for every $50 feet of eleva-
ton above sea level, but isothermic
lines show the sea level temperature.
In using isothermic maps we must
therefore remember that places GOO
feet above the sea level have temper
ature degrees lower than the iso-
therms indicate; places COCO feet above
the sea, % degrees lower; those $4,000
feet above the sea, As degrees lower,
and the mountain slopes $4,000 feet
above the sea no less than &C 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 stu
ated more than 1000 feet above the sea.
while in the tropics they are built at as
great elevations as 5000 or 10000 feet"
Weather charts are an exception to
this rule. They record the actual Their
mometrical readings at the points of
observation.--N. Y. Sun.