These notes are meant
to replace a short section from Chapter 3 in the textbook (for those
students buying chapters individually from the publisher and wanting
save a few dollars).
How would you describe Tucson's climate? Hot and dry?
You're basically relaying information about temperature and
precipitation. Here we'll just concern ourselves with
temperature. It does get pretty hot in Tucson in the
summer. But it doesn't stay hot all year. With just two
numbers, the annual mean or annual average temperature and the annual
range of temperature you can give someone a pretty complete idea of the
temperature in Tucson and how it changes during the year. More
detailed statistics for Tucson is available at this Tucson climate data link.
There are three or four main factors that determine a region's annual
mean and annual temperature range.
Latitude affects both the annual mean and the annual range of
temperature. The polar regions have colder annual average
temperature than any other location on earth. Some of the other
controls of temperature work together with high latitude to make
Antarctica and the South Pole colder than the North Pole. The
hottest regions on earth are found near 30 latitude, not at the Equator.
The annual range of temperature increases with increasing
latitude. There is little or no seasonal change at the Equator.
A region surrounded by land will have a much larger annual range of
temperature than a region surrounded by or near a large body of
water. Oceans are slow to warm during the summer and slow to cool
during the winter. This is partly because water has a higher
specific heat than soil. Some other factors come into play.
The figure below tries to explain why soil and water warm at different
rates during the summer
Water has a higher specific heat. Some of the incoming energy is
used to evaporate rather than warm water. Incoming sunlight
penetrates into a body of water and is used to try to warm a larger
mass of water. These three factors mean that water will warm more
slowly and won't get as hot during the summer as land. If you've
ever been to the beach in the summer you probably remember that
the sand on the beach gets much hotter during the day than the ocean
water.
Here is a link to an Optional Assignment
that you can download and print. This assignment is due at the
beginning of class on Monday Mar. 31.
The table below summarizes the three controls of temperature that we
have covered so far. One of them affects both the annual mean and
annual range, one affects just the mean, and the other just the annual
range.
One final factor:
Cities on the west coast and east coast of the US can have very
different climates even if they are at the same latitude and
altitude. A cold southward flowing ocean current is found along
the West
Coast. The warm Gulf Stream current flows northward along the
East Coast. Winds at middle latitudes generally blow from west to
east. The city on the West Coast will feel the full moderating
effect of the ocean. The city on the East Coast will be affected
by the Atlantic Ocean and also by winds blowing across the interior of
the US.
A graphical summary. You find cold locations over land at high
latitudes (Northern Canada, Siberia). Antarctica is the coldest
region because it is found at high latitude, is a land mass, and much
of Antarctica is high altitude. The hottest regions on earth are
found in the middle of land masses near 30 latitude.
What kind of climate would you expect to find at the X in the middle of
the Pacific Ocean near the Equator. You'll find the answer on p. 81 in
the photocopied class notes. Pohnpei Island in the Federated
States of Micronesia is located at 7 N latitude in the middle of the
Pacific. Monthly average temperatures range from a high of 80.8 F
in February and March to a low of 80.0 in July. The annual range
of temperature on Pohnpei is less than 1 F. The record high
temperature is 96 F and the record low is 66 F. Some parts of
Pohnpei get close to 400 inches of rain per year, it is also one of the
rainiest locations on earth.
Monthly average temperatures in Tucson, by contrast, range from 52 F in
December and January to 86 F in July.