T he climate of Michigan
is affected by several types of airmasses. Tropical maritime airmasses,
which originate in the Gulf of Mexico, are the principal source of moisture
(
fig.
1). About 75 percent of Michigan's annual precipitation is associated
with these airmasses. Polar maritime air- masses, which originate in the
north Pacific Ocean and, at times, in the Atlantic Ocean, generally lose
much of their moisture before reaching the Great Lakes. Arctic airmasses
from the Arctic Ocean and polar continental airmasses from northern Canada
deliver little moisture.
I
n addition to the oceans, important moisture sources include local and upwind land surfaces, as well as lakes and reservoirs, from which moisture evaporates into the atmosphere. Typically, as a moisture-laden ocean airmass moves inland, it is modified to include some water that has been recycled one or more times through the land-vegetation-air interface.
A
lthough latitude, which determines the quantity of solar radiation, is the major climatic control, the Great Lakes and differences in land-surface altitude also are important. The combination of the three climatic controls gives most of Michigan a semimarine type of climate despite its midcontinent location. During summer, winds are predominantly from the southwest because of a semipermanent Bermuda high-pressure system centered over the Southeastern United States. During winter, winds are predominantly from the west or northwest, but they change frequently as low- and high-pressure systems move through the area. The eastern Upper Peninsula is an exception because easterly winds prevail during the late fall and early winter. This exception is the result of early winter high-pressure systems that move eastward across Canada and of major storm tracks that push southward (Nurnberger, 1985).
T
he Great Lakes are a secondary or regional source of moisture. Lake-effect precipitation is most prevalent in near-shore areas but also affects areas farther inland. The slow response of the Great Lakes to temperature changes and the dominating westerly winds retard the arrival of both summer and winter. In the spring, the cooler temperatures within a few miles of the shoreline slow the development of vegetation. In the fall, tempering of the cold air by warmer lake water results in additional time required for crops to mature or to reach a stage less vulnerable to frost damage.
A
ir-temperature data from weather stations at similar latitudes in Michigan and Wisconsin illustrate the lake effect on temperature. On the western side of Lake Michigan, the mean temperatures for January at Madison and Milwaukee, Wisc., are 15.6 and 18.7 F (degrees Fahrenheit), respectively (U.S. Weather Bureau, 1951-69; National Oceanic and Atmospheric Administration, 1970-80). On the eastern side of Lake Michigan, the mean temperatures for January at Muskegon and Lansing, Mich., are 23.1 and 21.6 F, respectively, illustrating the warming effect of the lake. The lake effect on temperatures during summer is reversed, and temperatures are slightly cooler closer to the lake. However, the lake effect during summer is less pronounced than during winter.
A
verage annual precipitation in Michigan is about 31 inches, 55-60 percent of which is recorded during the growing season. Summer precipitation is primarily in the form of showers or thunderstorms, whereas steadier, less intense precipitation dominates the winter. The number of thunderstorms observed annually ranges from about 25 in the Upper Peninsula to about 40 in the Lower Peninsula. The Upper Peninsula of Michigan receives among the largest annual snowfall totals east of the Rocky Mountains, except for some isolated areas in the northern New England States. Annual snowfall ranges from about 30 inches in the extreme southeast to about 160 inches along the northwestern edge of the Upper Peninsula. This gradation is not uniform, however, because areas adjacent to the eastern shores of the Great Lakes receive more precipitation than areas just a few miles inland.
S
urface-water supplies are replenished by precipitation, which is fairly evenly distributed throughout the year even 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 3.5-4.0 inches in June. Because of moderate humidity, evaporation is slow.
