A
line drawn on a
weather map to connect places that have the same
temperature. To make an
isothermic map, you need to first
average all the
temperatures in one
area (
state,
national,
whatever) during the
period of time you want to cover. Once all the
average temperatures are laid out on a
map, you just draw a
line connecting the
places that have the
same average.
In the
Northern Hemisphere,
isotherms are very
irregular because there are more
land masses, all with
varying
altitudes and
temperatures. In the
Southern Hemisphere, which has
fewer land masses, isotherms are more
regular, though they still
curve all over the
map.