Alexandria gazette. [volume] (Alexandria, D.C.) 1834-1974, 1899-02-08
iSOTAERMS.
Facts About Isothernic Maps That
Are Rarely Learned Mt School.

Facts About Isotherinic Maps That
Are Rarely Learned Mt School.
Here are some definitions of isotherms
that appear in American textbooks of
geography;
Those lines which are drawn through
places with an equal average OF temper
azure are called isotherms.
''Isotherms are lines connecting
places having the same mean tempera-
ture for particular periods, as the whole
of the year. the winter or summer
months, etc.
IF upon map all places having
the same mean temperature are con-
nected by lines, such lines are called
isothermal lines or simple isotherms. -
These definitions are part Of the
truth, but not the whole of it. 1t may
not be great calamity, but the fact is
that most boys and girls leave school
with u 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
allure Of a place; that the farther q
place is from the equator and the higher
1t stands above sea level the cooler its
climate is. But they do not know that
isothermic maps take into -ccount 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
allure 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 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 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 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 temperature
of a place, if we know its elevation
above the sea. Hwy 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 600 feet above the sea level
have temperature degrees lower
than the isotherms indicates places
5000 feet above the sea, % degrees
lower; those 10, 900 feet above the sea,
45 degrees lower, and the mountain
slopes is, 900 feet above the sea no less
than 66 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 5000
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