Ask A Scientist Environmental Earth Science Archive

name         Herb
status       other
grade        other
location     MI

Question -   If December 21st (or there abouts) is the shortest
day in the Northern Hemisphere, why is it not the coldest?  Why is
it not the middle of winter rather than the beginning?
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Herb,

It takes the Earth a long time to cool off.  Heat has to be released
from a significant storage of energy in the ground, trees, buildings,
etc.  While the cooling is taking place, a lot of energy is still being
released into the atmosphere, slowing the cooling.  So, the coldest
temperatures come later, after this cooling has
occurred (in January or early February).

The reverse situation occurs in the Summer.  June 21 is not the
hottest day of the year in the Northern Hemisphere, as it takes the
a long time for the ground, trees, buildings, etc. to warm up; the warmer
they become as the Summer progresses, the more heat is released into
the atmosphere, resulting in the hottest days being later (July and August).

David R. Cook
Climate Research Section
Environmental Science Division
Argonne National Laboratory
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Herb,

I am assuming that you were thinking that the Northern winter 
solstice should be correlated to temperature since, by definition, 
winter solstice is the day that that hemisphere is most inclined 
away from the sun. If the sun were the only factor in the average 
temperature of that day each year, then you would be correct. 
However, as you might suspect, there are other factors. One of them 
is the thermal inertia stored over the summer (through autumn) 
months. Conversely, one also finds that the summer solstice is not 
the hottest day, but rather the beginning of summer. Again, it is 
because of the relatively low heat from the previous months. It is 
rather like why it goes until August and September that one can 
finally swim in the Northern Atlantic beaches - it takes a while for 
the waters to heat up.

Greg (Roberto Gregorious)
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