heat - oregon state universityoregonstate.edu/instruct/fw456/lecture/pdf/heat.pdf · winter...
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HEAT
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Heat
Heat - energy of interatomic forces and kinetic energy of moving molecules in a substance
Temperature is a measure of the energetic state at a single point
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Heat
Heat is the total energy content
Heat = (mass) (temperature) (specific heat)
Heat = (g) (°C) (Cal/g °C)
Heat = Cal
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Absorption of Solar Radiation
Light is absorbed as it passes through air or water and the amount absorbed increases exponentially with distance
For light at 750 nm, 90% is absorbed within one meter and 99% is absorbed within two metersOver half of the sun's incoming radiation is absorbed within the upper two meters
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Losses of Heat
Heat is lost as thermal radiationHeat is lost as evaporationHeat is lost through outflow from the lakeHeat is lost as water freezes Convection currents mix upper 3 m of water and account for approximately 10% of the distribution of heatWind is the major agent of mixing and accounts for approximately 90% of the distribution of heat
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Development of Thermal Stratification
Ice cover melts in springTotal mixing of the volume of the lake occurs This is called turnover As surface waters heat and become less dense, relative thermal resistance to mixing increasesDifference in temperature between layers of only a few degrees is sufficient to prevent circulation
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Development of Thermal Stratification
Water column is divided into three regions that are resistant to mixing
Epilimnion - upper stratum of uniformly warm, circulating and fairly turbulent waterMetalimnion - stratum of marked thermal discontinuity between epilimnion and hypolimnionHypolimnion - cold, relatively undisturbed stratum below metalimnion
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Thermocline
Thermocline is the plane of maximum rate of decrease of temperature with respect to depth
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Stratification
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Stratification
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Density Difference
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Seasonal Change in Thermocline
Seasonally, the thermocline begins deep after turnover, rises, stabilizes, and then drops
The hypolimnion can be heated by turbulence, density currents, and direct radiation
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Loss of Stratification OccursBecause of Loss of Heat
Surface cools and surface waters are more dense so they sink and mix due to both wind and convection currentsAs a result, the thermocline sinksFinally the thermal resistance to mixing is insufficient to prevent mixing and turnover occurs (fall turnover)Fall turnover may be a very rapid process
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Winter Stratification
As temperature of water approaches 4°C, surface ice can form rapidly with loss of heat at the surfaceMinor density gradient can develop between surface waters at 0°C and lower water at 4°CWeak stratification that occurs is termed inverse stratification Ice cover protects inverse stratification
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Classification of Stratification
Hutchinson (1957) developed the current classification system
Definitions center on patterns of circulation and are restricted to lakes of sufficient depth to allow formation of a hypolimnion
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Dimictic Lakes
Circulate freely twice a year in the spring and fall
Directly stratified in summer
Inversely stratified in winter
Most common in cool temperate regions
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Warm Monomictic Lakes
One regular period of mixing each year in winterStratified in summerIsothermal in winterWater is rarely < 4°COften coastal or high elevation lakes in the subtropics
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Cold Monomictic Lakes
One regular period of mixing each year in summerFrozen over in winter and inversely stratifiedIsothermal in summerWater rarely exceeds 4°CFound in the Arctic or at high elevations
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Oligomictic Lakes
Rare circulation periods at irregular intervals
Water rarely is less than 4°C
In tropical examples, warm waters cool only rarely at which times stratification may be disrupted
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Polymictic Lakes
Lakes with frequent or continuous mixing
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Cold Polymictic Lakes
Water around 4°C
Little seasonal change in temperature
Large area, equatorial, high elevation, high wind
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Warm Polymictic Lakes
Water is well above 4°C
Tropical
Little seasonal change in temperature
Or shallow lakes !!!!!
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Amictic Lakes
Permanently sealed off by ice cover and never circulate
Occur at latitudes of greater than 80°
Occur at elevations of greater than 6000 meters at the equator
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Holomixis and Meromixis
Previous examples were holomicticlakes - mixing involved entire lake volume
Meromictic lakes circulate occasionally but never completely due to the presence of a dense, stagnant, anaerobic lower stratum
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Meromictic Lakes
Meromictic lakes circulate occasionally but never completely due to the presence of a dense, stagnant, anaerobic lower stratum
Monimolimnion - deep stratum of water that is perennially isolatedMixolimnion - upper stratum that mixes periodicallyChemocline - steep salinity gradient that separates the mixolimnion from the monimolimnion
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Meromictic Lakes
Monimolimnion - deep stratum of water that is perennially isolated
Mixolimnion - upper stratum that mixes periodically
Chemocline - steep salinity gradient that separates the mixolimnion from the monimolimnion
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Ectogenic Meromixis
External events (storms, tides, abrupt geologic events) bring saline or fresh water into basins
Silt or irrigation runoff can also cause meromixis
Artificial structures (locks) may also bring saline water into a basin
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Crenogenic Meromixis
Submerged saline springs can bring saline water into lower portions of lakes
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Biogenic Meromixis
Decomposition of organic matter introduces dissolved salts
Meromictic conditions may occur in deep lakes
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Stratification
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Secchi Depth