Section 4 HYDROMETEOROLOGY: CLIMATE AND HYDROLOGY OF THE GREAT LAKES Jan A. Derecki 4.1 Introduction (5,473 cu. mi.) stored in the lakes, varying from 484 km3 in Lake Erie to 12,234 km3 in Lake Superior, the Great Lakes have a tremendous 4.1.1 Description and Scope heat storage capacity. Through air-water in­ teraction, the lakes influence the climate over The climate of the Great Lakes Region is them and over adjacent land areas. Because of determined by the general westerly atmos­ the lake effect, air tempt!!ratures are moder­ pheric circulation, the latitude, and the local ated, winds and humidities are increased, and modifying influence of the lakes. Due to the precipitation patterns are modified. Although lake effect, the regional climate alternates be­ these phenomena have been recognized for tween continental and semi-marine. The decades, it is only in recent years that inten­ semi-marine climate is more consistent con­ sive programs have been undertaken to de­ tiguous to the lakes, but with favorable termine the more exact nature and mag­ meterological conditions, it may penetrate nitudes of these processes. deeply inland. The Great Lakes climate and The Great Lakes moderate temperatures of hydrology are closely related. Variations in the overlying air masses and surrounding mean lake levels, and consequently lake out­ land areas by acting as heat sinks or sources. flows, are controlled by the imbalance be­ The process of heat exchange between the tween precipitation and evaporation. lakes and atmosphere is both seasonal and The Great Lakes drainage basin is discussed diurnal. During spring and summer, the lakes in other appendixes and a brief summary of are generally colder than air above and have a the climatic and hydrologic elements of the cooling effect on the atmosphere. During fall drainage basin is given in Section 1 of this and winter, the lakes are generally warmer appendix. The major storm tracks affecting than the atmosphere and serve as a heat the Great Lakes Region are indicated in Fig­ source. However, during the winter months, ure 4-13. Distributions ofmean annual values the ice cover reduces the lake effect. for air temperature, precipitation, runoff, and A daily pattern of heat exchange is water losses over the land areas ofthe Basin, superimposed upon the seasonal pattern. This showinglatitudinal and lake-effect variations, daily pattern is produced by land-water tem­ are indicated in Figures 4-15, 17, 19, and 20, perature differences. Because lakes are more respectively. The average monthly means, efficient than land areas in storing heat, lake highs, and lows of overland air temperature temperatures have a tendency to remain sta­ for the individual lake basins and for the total ble, while land temperatures undergo daily Great Lakes Basin are shown in Figure 4-16. variations that are more in line with the air temperatures. When the land is warmer than water, the relatively warmer airover adjacent 4.1.2 Lake Effect land areas tends to rise and is replaced by colder, heavier air from the lakes. When the With a total water volume of 22,813 km3 land is colder, the process is reversed. This Jan A. Derecki, Lake Survey Center, National Oceanic and Atmospheric Administration, Detroit, Michigan. 71 71 Appendi~ 4 processofheatexchange produces lightwinds, thunderstorms. Duringwinter, the conditions which are known as lake breezes. Theoffshore are reversed, and the cold, inland air passing and onshore lake breezes are illustrated in over relatively warm water becomesles8 sta­ Figure 4-14. The direction of lake breezes is ble and picks up moisture, which encourages governed by the land-water temperature dif­ snow flurries. As the winter air masses move ferences and is independent ofthe general at­ over the lakes, the moisture that accumulates mospheric circulation. However, lake breezes in the air produces heavy snowfalls on the lee occuronly duringrelatively calm weather and sides of the lakes, due to orographic effects of affect a limited air mass along the shoreline, the land mass. The fact is well documented rarely extending more than several kilome­ that heavysnowbelt areas resultfrom thelake ters (2-3 miles) inland. The moderation oftem­ effect. These areas include Houghton on Lake peratures by the lakes affects regional ag­ Superior, Owen Sound on Lake Huron,Buffalo riculture by reducing frost hazards in the on Lake Erie, and Oswego on Lake Ontario. early spring and in the fall, thus lengthening the growing season, especially in coastal areas. Examples of this effect are the cherry 4.1.3 Measurement Networks orchards of the Door Peninsula in Wisconsin and theGrandTraverse Bay area in Michigan, Basic meteorological data in the Great and the vineyards of the western Lake Erie Lakes Basin are available from regularobser­ islands in Ohio. vation networks operated by the Nation­ Because lake breezes have a limited range al Weather Service and the Canadian and require special conditions, the lake effect Meteorological Service. The networks consist on winds is minor. A much more important ofa limited number offirst order stationsthat effect is the considerable increase in geo­ provide hourly observations for air tempera­ strophic wind speed over the lakes. This in­ ture, precipitation (total and snow), wind crease is caused by reduced frictional resis­ speed and direction, humidity, and duration of tance to air movement over the relatively sunshine and cloud cover. More numerous co­ smooth water surface, and by the difference in operative stations provide daily observations atmospheric stability created by air-water for air temperature and/or precipitation. Cer­ temperature differences. Recent studies indi­ tain more specialized stations collect addi­ cate thatthe increase in overwaterwind speed tional data on solar radiation, radiosonde in­ varies from approximately 15 percent in mid­ formation, weather radar, and pan evapora­ summer to as much as 100 percent in late fall tion. Other regularly observed data useful in and early winter. The average annual in­ Great Lakes climatology include water tem­ crease is approximately 60 percent. peratures recorded by municipalities at their The Great Lakes also cause an increase in water intake structures and by Federal agen­ overwater humidity by releasing large quan­ cies at ~elected lake perimeter location& tities of moisture through evaporation. On an In addition to the regular networks, more annual basis the humidity over the lakes av­ sophisticateddataarecollected periodicallyor erages 10 percent to 15 percent higher than seasonally for research on lake climatology. that over the land. Seasonal changes in These include special precipitation networks, humidity over the lake compared to that over established and operated on lake islands and land vary from a decrease ofapproximately 10 adjacent shorelines; synoptic surveys con­ percent due to overwater condensation in the ducted by research vessels that take observa­ late spring, to an increase ofapproximately 10 tions for thewhole rangeofhydrometeorologic percent in the summer, 15 percent in the fall, parameters; lake towers that give meas­ and 30 percent in the winter. urements with vertical profiles for selected The Great Lakes also influence the distribu­ parameters for air-water interaction studies; tion ofcloud cover and precipitation. Modifica­ aerial surveys by conventional aircraft for ice tion ofpreeipitation patternsdue to lake effect reconnaissance and water surface tempera­ is caused by thechanges in atmosphericstabil­ tures, using infrared and airborne radiation ity in combination with prevailing wind direc­ thermometer techniques; and weather satel­ tion and topographic effects. During summer, lites that provide useful information for the the air undergoes overland warming before investigation of doud and ice cover on the passing over the lakes. The warm airover rel­ lakes. atively cold water results in the development Hydrologic data on the Great Lakes Basin of stable atmospheric conditions, which dis­ are compiled and pu~lished by several agen­ courage formation of air-mass showers and cies. Records of tributary streamflow to the HrdrofMteorolon 78 lakes are available from the U.S. Geological from appropriate water level gage ratings Survey and the Canada Centre for Inland Wa­ based on periodic current meter flow meas­ ters, Department of the Environment, urements. Canada. The extent of gaged area increased substantially in the late 1930s, giving cover­ age to approximately 50 percent of the Basin. 4.2 Radiation At present approximately 64 percent of the Basin is gaged; gaged areas for Lakes Supe­ rior, Michigan, Huron, Erie, and Ontario ba­ 4.2.1 Total Radiation Spectrum sinsrepresent approximately 53, 71, 66, 67, and 63 percent of their respective basins. In addi­ Total radiation received atthe surface ofthe tion to surface water data, these agencies and earth consists of shortwave radiation coming the Geological Survey of Canada publish ob­ directly from the sun or scattered downward, servation well records providing information and longwave radiation, emitted from the at­ on ground-water conditions. However, the mosphere. Portions of the incoming radiation network of observation wells useful in deter­ in both short and long wavelengths are re­ mining ground-water flow to the lakes is ex­ flected and additional longwave radiation is tremely limited. emitted to the atmosphere. The main radia­ The Great Lakes levels and outflows are tion exchange processes taking place within available from