Geography Reference
In-Depth Information
But it's a different matter at 9 p.m. Because nearly all of the land's heat is contained within a thin
veneerofsurfacesand,ittendstoradiate backintotheatmosphere ratherfastafterthesungoesdown.
But the lake is different. Only a small portion of its solar inventory is at the surface — exposed to the
atmosphere. After the sun goes down, the lake's heat is radiated back into the atmosphere at a slower
rate. Thus, it retains its absorbed energy longer than land, resulting in a warming evening swim.
Summer versus winter
The fact that land and water heat up and cool down at different rates has significant implications for
climate. Look again at Figure 9-6, but instead of a beach by a lake, assume it portrays a continent
next to an ocean. Also assume that Point A is a city in the middle of that continent and that Point B is
a coastal city a thousand or so miles away. Time-wise, consider summer versus winter as opposed to
mid-afternoon versus 9 p.m.
During the summer, the city in the middle of the continent is likely to be warmer. That is because, just
like in the beach example, solar energy will concentrate at Earth's surface and heat the atmosphere
overhead. In contrast, the coastal city is likely to be less warm during summer. That is because its at-
mosphere will be warmed by heat that radiates off both the land around it and the water offshore. But
because the surface of the water contains so much less heat than the land, the total amount of heat that
is radiated into the atmosphere is far less than occurs in the middle of the continent. In the parlance of
climatology, the water body has a
modifying
or
mitigating effect.
That is, it results in the atmosphere
being less warm than would be the case if there were land all around. The net result, whatever the
vocabulary, is that the city in the middle of the continent will experience a warmer summer than the
one by the sea.
Winter is another matter, however. Regarding the mid-continent city, the heat that was absorbed by
the surrounding land during the summer — being concentrated at the surface and now exposed to
long winter nights — radiates into the atmosphere rather rapidly, contributing to cold temperatures.
These land-related conditions also apply to the coastal city. But something else of significance also
affects the temperature of the atmosphere in the latter locale. Because the heat absorbed by the water
body is distributed over a certain depth — and is therefore not concentrated at the surface and ex-
posed to long winter nights — it is radiated back into the atmosphere at a much slower rate. In effect,
it may serve as a source of atmospheric warmth for a significant portion of the winter, and result in a
warmer winter for the coastal city.
Consider a comparison of Pierre, South Dakota and Portland, Maine. The former is in the middle of
a continent, the latter is on the coast, and their latitudes are nearly the same. In Pierre, the coldest
month of the year averages 17° F and the warmest month of the year averages 75° F. That makes
