CLIMATIC CONTROLS

The climates of Michigan differ because of many "controls", which are here discussed in decreasing order of importance.

Latitudinal Position
The state�s location in the middle latitudes is the primary cause of its seasons. The inclination of the earth on its rotational axis and the revolution of the inclined earth about the sun produce great differences in the amount of heat received in the summer and in the winter. Long summer days, particularly in the Upper Peninsula, and the high angle of the sun above the horizon mean that the state 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. Radiation receipts are least in December and greatest in June, but mean temperatures do not reach their yearly extremes until at least a month later in each season because the earth is slow to respond to seasonal radiation trends.

Continental Location
The location of Michigan within North America affects both temperature and precipitation. Because Michigan is far from any oceanic influence, it has a "continental climate." Land masses retain much less heat than do large bodies of water, so they change temperature more quickly in response to variations in the amount of solar energy received. Consequently, continental climates are characterized by great ranges between summer and winter temperatures. If Michigan were located at the same latitude on the West Coast of North America, it would have less extreme seasons-summers would be cooler, and winters milder.
    The state�s position also helps account for the extremes of its daily weather. Differences in weather from day to day are due largely to the influence of the different air masses that flow across the Midwest. An air mass is large, rather uniform body of air that has acquired some characteristics of the surface over which it was formed; many are so large that they may cover Michigan for two or more days at a time. Any one of three different air masses may invade the state, bringing with them their own characteristic conditions. Polar air (cP) originates over the Arctic reaches of Canada, or even Siberia, and enters the state from the northwest, north, or northeast, bringing cold (for the season) and generally dry weather (except near the Great Lakes in the winter). Pacific air (mP) enters the state from the west or northwest after it has been modified by its passage over the Rocky Mountains. By the time Pacific air reaches Michigan, it is mild for the season but dry. Tropical air (mT) originates over the Gulf of Mexico or the Caribbean and accordingly is warm and moist. It reaches the state from the southwest, south, or southeast and produces the hot, humid days of summer.
    Of the air masses that affect Michigan, only those of tropical origin contain sufficient moisture to generate widespread, heavy precipitation; but of the three air masses, they are the least common in Michigan, in part because the state lies so far from the source region. Therefore, many weather disturbances cross the state without generating significant precipitation. Across Michigan and much of the eastern United States, the amount of moist tropical air that is available accounts for the general patterns in mean annual precipitation. Annual precipitation is higher in states south and east of Michigan where passing disturbances more often interact with tropical air, since they lie closer to the source. Precipitation is less to the west and the north of Michigan because tropical air reaches those areas even less frequently.

Upper Atmospheric Circulation
Air masses and passing disturbances bring precipitation and changes of temperature, but the frequency of these disturbances and their paths depend upon the circulation of the middle and upper atmosphere. Surface weather is "steered" by air flow above 20,000 feet (i.e., the jet stream). Although this flow is generally from west to east (causing it to be termed "westerly" or "the westerlies"), it is changeable and sometimes has a strong northerly or southerly component, and such deviations can continue for weeks or months. The fact that Michigan is located in the westerlies assures that it will have changeable weather, perhaps one of its most prominent climatic characteristics. Slight deviations in the direction taken by these upper-air westerlies lead to repeated invasions of air masses characteristic of the regions in which the weather originates: A northerly component brings Arctic or polar air south out of Canada; a southerly component imports mild Pacific or tropical air masses and excludes polar one.
    The core of strongest west-to-east winds in the upper atmosphere, the jet stream, changes latitudinal position as the seasons progress-shifting north in the summer and south again in the winter. As a result, the Upper Peninsula, which lies nearest the jet stream in summer, normally experiences the most changeable summer weather, and the southern Lower Peninsula may have more periods of prolonged, unchanging weather--particularly of heat and humidity, which are associated with slow-moving tropical air masses. But during the winter, the westerlies and the associated jet stream again shift south and bring changeable weather to all portions of the state.

The Great Lakes
No control on Michigan�s climate probably receives more public attention than do the Great Lakes. Their effect is primarily local and is best developed on the lee-shore (eastern and southeastern shoreline) areas of each of the lakes. There this localized effect of the Great Lakes on the climate is of great importance to the state�s agricultural economy.
    During the season when the lakes are generally colder than the air over them--April to August, but particularly in the spring--they extract heat from the overlying atmosphere. This chilling effect may strengthen the polar air masses entering the Midwest from northern areas and allow them to reach Michigan stronger and colder, and more persistent, than they might otherwise be. The Great Lakes have a reverse, though minor, effect on passing cyclonic storms when the lakes are a source of heat, especially October through December. Certain winter cyclones that cross the Great Lakes probably gain strength or size because of the heat and moisture they acquire from the relatively warm water, but the path these storms take is largely determined by the upper-air circulation. So, we cannot blame the lakes for the winter storms in Michigan, but they may strengthen the storms that do occur, prolonging the associated cloudiness and inclement weather.
    The lakes influence on local weather is more varied and dramatic. During the winter months, when cold air crosses the relatively warm water, the air takes up heat and moisture from the lakes. As a result, the lower layers of the atmosphere are warmed, and upward-moving air currents develop. How much warming there is depends upon the length of time the air spends over the water and the temperature difference between the water surface and the overlying atmosphere. Contrasts in excess of 15� F are common, and differences as great as 50� F or more are possible. This warming leads to the formation of puffy-looking cumulus clouds. In general, the stronger the ascending air currents, the more likely it is to produce precipitation, usually as snow. The snow clouds then drift inland with the prevailing low-level winds, resulting in "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 lake-effect snowfalls can be spectacularly heavy; but such snowfalls require not only a great contrast between the temperatures of the water and of the air but also a comparatively long over-water trek to maximize cloud formation. Because most extremely cold winter air masses in Michigan are accompanied by westerly or northwesterly surface winds, the air crosses the narrow axis of Lake Michigan or, in the case of the Keweenaw Peninsula, passes over only the western portion of Lake Superior. Consequently, the time the air spends over the water is much less than if the flow had been parallel to the lake, and the associated snowfalls seldom exceed 6 to 10 inches. Still, because lake-effect snows are frequent in winter, their cumulative effect greatly augments the average annual snowfall in the lee-shore counties. Lake-effect snow seldom falls in significant amounts further inland than about 40 miles, but it is accentuated where inland-moving snow clouds cross higher terrain, as in Otsego County or on much of the Keweenaw Peninsula/Huron Mountains. Often, the lee-shore counties can be getting significant lake-effect snow while the sun is shining in eastern Michigan. Under exceptional conditions (severe cold and strong low-level westerly circulation), snow clouds from Lake Michigan may cover much of the Lower Peninsula far beyond the usual snow belt, though accumulation in the central and eastern counties is usually small.
    A secondary consequence of lake-induced wintertime cloudiness is a moderation of severe cold immediately downwind from the shoreline. Not only do the lakes add heat to the lower atmosphere and hence prevent exceptionally low nighttime temperatures in lee-shore locations, but the resulting cloudiness helps retain heat. Thus, regions about 20 miles downwind from Lake Michigan enjoy somewhat higher minimum temperatures than do locations farther inland.
    Just as the Great Lakes are a source of heat in winter, they remove heat from the atmosphere in spring and summer, and this influence again is strongest over the peninsulas and downwind areas. Thus, maximum July daily temperatures, maybe 10� - 18� F lower along the Michigan shore of Lake Michigan than along the Wisconsin shore. But this effect seldom extends more than a few miles inland.
    Often, the cooling influence of any of the Great Lakes is developed in a rather sharp zone and coincides with what is called a "lake breeze." The lake breeze develops when there is a strong temperature contrast (more than 15� F) between the cool lake water and the warmer air over the nearby land surface-as is typical on almost any summer afternoon. Under these conditions, a shallow layer of lake-cooled air spreads inland in all directions. Its circulation is weak and local, rarely penetrating more than a few miles inland. The breeze progresses farthest inland where it is not opposed by the regional surface wind, but it usually affects only shoreline areas and may not develop at all when the surface wind blows strongly offshore. The effect of the lake breeze is such that daily summertime maximum temperatures in areas near a shore average several degrees lower than in areas farther inland. When the lake breeze develops, it suppresses cloud formation, and the sky is often clear. For this reason, coastal locations in Michigan are sunnier on the average during the summer than are inland locations.
    Places along the shore do have fog more often, however-chiefly 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 slight shift in the direction of the wind then transports this fog inland. Although usually confined to the immediate shoreline, the fog may extend several miles inland under ideal conditions. The area that is foggy most frequently during the summer is the shoreline of Lake Superior because, of the five Great Lakes, Superior is the deepest, largest, and hence the coldest during this season. Over the open lake, fog is even more prevalent than along the shoreline and has considerable significance for lake shipping.

Terrain and Elevation
In most areas of Michigan, the landforms are not a significant control on the local climate, since neither elevation nor relief is great. But in portions of the central-northern Lower Peninsula and the western Upper Peninsula, they may affect local weather. The elevation of some regions--Kalkaska, Antrim, and Otsego Counties in the Lower Peninsula and Ontonagon, Houghton, Marquette, and Keweenaw Counties in the Upper Peninsula--may cause additional lifting of cold, moisture-laden clouds in winter, and thus more snowfall than would otherwise be the case.
    The hilly terrain in parts of the northern Lower Peninsula and the western Upper Peninsula also encourages the drainage of cold air and the formation of frost pockets in basins throughout these areas. On calm, clear nights, cold air, because it is denser than warm air, collects near the ground and flows down the slope and settles in depressions. Many of the settlements and much of the arable farmland in northern Michigan are located in such basins, which, unfortunately, experience later spring and earlier autumn frosts than the surrounding higher countryside does. Thus, many of the lower, better soils are not as well suited climatically for agriculture as the poorer but warmer sites on the surrounding hills.

Parts of the text on this page have been modified from L.M. Sommers' book entitled, "Michigan: A Geography".