The Worthington advance. [volume] (Worthington, Minn.) 1874-1908, 1899-03-02
1soTHERMs.
A Few Facts About IsOthermic Mapi
That Are Rarely Learned
at School. 80

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 sotherms.
"Isotherms 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. a..
If upon 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. It may
be great calamity, but the fact is that
most boys and girls leave school with
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 do not
know that isothermic maps take into
account only one of these elements
and that is latitude. 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 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? It
may be easily explained. Most Of the
land does not rise so high above the
sea that the temperature is greatly at.
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 it's elevation
above the sea. How this is done is very
clearly explained by DR.. H. R. Mill, the
i British geographer, as follows:
1.-THE air grows cooler by 1 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 600
feet above the sea level have temper
allure 2 degrees lower than the iso.
/ therms indicate; places 5000 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, 1900 feet above the sea
while in the tropics they are built at as
great elevations as 5000 or 10 900 feet
Weather charts are an exception tc
this rule. They record the actual Their.
mometrical readings at the points Of
Internationally. 7. Sun. IN -