MadSci Network: Earth Sciences
Query:

Re: Why are summer days hot when the days are getting shorter?

Date: Thu Aug 14 12:36:33 2003
Posted By: Carolyn Ernst, Grad student, Planetary Geology
Area of science: Earth Sciences
ID: 1055725596.Es
Message:

Why are summer days hot when the days are getting shorter?

This is an excellent question.  Common sense would tell us that the hottest day of 
the year at a given location should be the day on which that location receives the 
most sunlight, or around June 21st for the northern hemisphere.  We know from 
experience that the months of July and August are normally much warmer than the 
month of June – what causes this discrepancy?  

Both the Earth’s surface and atmosphere respond slowly to heat supplied by the 
sun.  This is especially true for the Earth’s oceans, because water has a very high 
heat capacity (it takes a lot of heat to warm water a little bit).  This slow response 
time is why costal regions on Earth stay cooler than slightly inland regions during 
the summer and warmer than slightly inland regions during the winter.  In the 
summer, the water is cooler than the land (it warms up more slowly), so breezes 
coming off of the ocean cool costal regions.  In the winter, the water is warmer than 
the land (cools off more slowly), so breezes off of the ocean warm coastal regions.  

A similar phenomenon causes the time lag between the longest day of the year 
(June) and the warmest day of the year (August) (and likewise the shortest day of 
the year (December) and the coldest month (February) (and on a shorter timescale, 
why the sun is highest in the sky at noon, but the warmest portion of the day is 
during the early afternoon).  The land, ocean and atmosphere of the Earth take time 
to warm up in response to solar heating, thus the longest day of the year occurs 
earlier than the warmest day of the year.



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