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The state
of Michigan, with our emphasis on the Upper Peninsula in particular,
is greatly affected by the Great Lakes that surround it. In this
section, I will discuss the complex weather that affects the peninsula
and influences the people who choose to call it home.
Climatic
Controls
Latitudinal
Position
Upper Michigan
is located in the mid-latitudes, where the inclination of the
Earth and its revolution around the sun play a major role in the
amount of heat received throughout the year. Long summer days
and the high angle of the sun above the horizon mean that the
peninsula receives much more solar energy during the summer than
during the winter when short days and the very oblique angle of
incoming solar radiation severely reduce the amount of heat received.
December is the month when the U.P. receives the least amount
of solar radiation and June is when it reaches its maximum. Because
of the slow response times of the Earth to incoming radiation,
the seasonal extremes for temperature actually occur a month later.
Airmasses
Located within
the interior of a large continent, the Upper Peninsula is influenced
by a series of airmasses transitioning through the region. The
airmass type is dependent upon the time of year. The three main
airmasses that affect the peninsula are:
Continental
polar (cP): Cold and dry, but not as cold as Arctic (cA) airmasses.
These usually form farther to the south than cA and dominate the
weather picture approximately 15-25% of the time over the area.
These air masses are the ones responsible for bringing clear and
pleasant weather during the summer and bitter cold temperatures
in the winter. Precipitation is usually enhanced along the frontal
boundary as the airmass approaches the area. (How the lakes influence
weather will be discussed more in-depth later.)
Maritime
polar (mP): Cool and moist. Maritime polar air masses form over
the northern Atlantic and the northern Pacific oceans. However,
because of the vast distance from their source region by the time
they reach Michigan, they most often bring cloudy skies with little
precipitation. They are the biggest influence on winter weather,
affecting the area a full 75% of the time! In summer, that figure
drops to about 30-40% of the time. Maritime polar air masses can
form any time of the year and are usually not as cold as cP air
masses.
Maritime
tropical (mT): Warm temperatures with ample moisture. Maritime
tropical air masses are most common in the summer and originate
over the warm waters of the southern Atlantic Ocean and the Gulf
of Mexico. These air masses can form year round, but they are
most prevalent in the Upper Peninsula in summer where they impact
weather around 40% of the time. Maritime tropical air masses are
responsible for the hot, humid days of summer and are also influenced
by the long journey they must make to reach the Great Lakes region.
Precipitation usually comes in the form of airmass rainshowers
and thunderstorms.
Jet
stream Patterns
Associated
with these airmasses are the upper level wind patterns which normally
move from west to east over the Great Lakes. The jetstream (a
core of higher wind speeds located in the westerlies) migrates
with the change in seasons and helps to steer the surface systems
that affect the weather of the region. The genisis of many weather
systems occur under the influence of the jetstream and this very
phenomenon advects the system into the area. The jetstream migrates
north with the summer and creates the most significant changes
to weather during this time. The jetstream also increases the
chances for severe weather to develop as it provides the strong
upper-level divergence necessary for strong storms to develop.
The
Great Lakes
Probably more
so than any other factor, the Great Lakes play a major role in
local weather that affects the various parts of the peninsula.
As weather systems migrate through the region, they may become
amplified depending on the season. In the spring and summer months,
the lake surface is cooler than the air above it, which has the
potential to enhance or strenghten polar airmasses that push through
the area. In winter, the warmer water has the potential to increase
the strength and size of systems that move over the lakes. This
will allow for more precipitation to fall over a longer time frame.
The amount of influence is tied to the storm's track. A longer
residence time over the water will be more significant than a
lesser time.
In summer
and winter, the lakes have a moderating influence on temperature.
This phenomenon is most noticed on the east and southeast (leeside)
shores of the lakes. In winter, the warmer water temperatures
are advected onshore as cold air moving over the water picks up
some of its characteristics. This helps to moderate the colder
temperatures over land. This warm onshore flow also increases
cloud cover, which further aids in moderating temperatures. It
is this very affect that allows the western shores of the lower
peninsula to produce large fruit crops where one wouldn't normally
expect the growing season to be long enough. The reverse affect
occurs in summer. The cooler water temperatures are drawn onshore
by the lake breeze and near shore temperatures can be significantly
cooler than the interior of the peninsula due to evaporative cooling.
The very
phenomenon that affects temperature also affects precipitation.
When low level flow comes in from off the lakes, it brings with
it a moisture influence. The warmer water in winter creates "lake-effect"
snow showers within a 15-40 mile-wide zone starting on the
lee shore of the lake. With a contrast of less than 15° F between
the temperatures of the lake’s surface and the lower atmosphere,
precipitation is unlikely, and with a contrast of less than about
6° F clouds may not even develop. Some snowfalls can be rather
dramatic, but it is usually the cumulative effect of this penomenon
which enhances snowfall totals in the northern parts of the peninsula.
In summer, the lake breeze can
also create localized showers inland from the coasts of the peninsula
while near shore, the skies are clear. The surface of the peninsula
heats up much more rapidly than the surrounding water, which causes
the breeze to set up. These showers, while sometime persistent,
usually do not create signifcant amounts of rainfall. At night,
the reverse can happen and a land
breeze will set up creating isolated rainshowers over the
lakes.
Places along
the shore also have fog more often than further inland. This occurs
mainly during spring and early summer when the Great Lakes are
still cold. The surface layers of warm, moist air moving offshore
are cooled by the water, and the moisture in the air may condense
into fog. A wind shift may cause this fog to be transported inland
for several miles. More importantly, when this fog forms over
the lake; it can create problems for the robust shipping industry
using the Great Lakes.
Temperature
In July, the
average daily maximum temperature for the peninsula reaches 74.5
degrees Fahrenheit (23.5 degrees Celsius) while nighttime minimums
drop to 57.5 degrees Fahrenheit (14.0 degrees Celsius). In January,
the average daily maximum temperature for the peninsula is a frigid
22.5 degrees Fahrenheit (-5.0 degrees Celsius) and nighttime minimums
fall to 10.0 degrees Fahrenheit (-12.0 degrees Celsius). These
temperatures have been averaged for the entire peninsula. As I
touched on earlier, the Great Lakes exert a considerable influence
on locations located nearer to their shores. The interior locations
on the peninsula are more influenced by the airmass and time of
year. The following graphics will give you an idea of the moderating
affect the lakes have on this area. (Mean
Summer Maximums / Mean
Winter Minimums).
Precipitation
Average
annual precipitation in Upper Michigan is about 32 inches.
Notice on the graphic that the windward side of the peninsula
receives more annual total precipitation when compared to the
rest of the area. Also take note that the maximum precipitation
area does not begin right on the northern shore of the peninsula,
but rather starts inland from the beach. This is a reflection
of the combined lee of the lake snow affect and the summer lake
breeze. Summer precipitation is primarily in the form of showers
or thunderstorms, while stratiform, less intense precipitation
dominates the winter. The number of thunderstorms observed annually
averages about 25 in the Upper Peninsula. It receives some of
the largest annual snowfall totals east of the Rocky Mountains.
Annual snowfall ranges from about 30 inches in the extreme southeast
to about 160 inches along the northwestern edge of the peninsula!
This distribution is not uniform, however, because of the affects
produced by the lee of the lake snowfall.
Precipitation
totals are fairly evenly distributed throughout the year, though
periods of no precipitation can last as long as 1 month. Most
of the State receives 1.5-2.0 inches of precipitation per month
from December through March; 2.5-3.0 inches per month during April,
October, and November; 3.0-3.5 inches per month during May, July,
August, and September; and a maximum accumulation of 3.5-4.0 inches
in June.
Now that we
have a little background information on the Upper Peninsula, I
want you to feel free to refer back to the last couple of pages
for an information refresher as needed. Links back to those pages
are provided at the bottom of each day's activity page.
I know you're
excited to get started, so let's head to Day
1 and
begin our trip....
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