======Lake Stratification (Overturn) in a Jar======


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====Summary====                   
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====Procedure====  
  - Fill one jar to the brim with warm water and red food coloring; fill the second jar to the brim with cold water and blue food coloring. Place both jars in a large tub to catch spills.  
  - Lay a stiff laminated card over the warm jar’s mouth to make a seal, then invert the warm jar while holding the card against the opening.  
  - Carefully place the inverted warm jar on top of the upright cold jar so their mouths align.  
  - Gently slide the card out so the two water surfaces meet. Observe that the warm, less dense red water stays above the colder, denser blue water with little mixing.  
  - To model overturn, firmly grasp both jars together and flip the stacked pair so the cold jar is now on top. Keep the mouths aligned and remain over the tub.  
  - Watch the cold, denser blue water sink and the warm red water rise, producing visible currents and thorough mixing.  
  - Discuss how this represents seasonal changes when surface water cools and sinks, driving circulation.



====Links==== 
Explore Lake Stratification in a Jar! - Green Mountain Conservation Group: 
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Description or Video title2 - Author2: 
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📄 Lake Overturn (Page 52) - Matt Shade: [[https://www.unco.edu/nhs/science/pdf/demos/2012_CSC_demos.pdf]]\\

====Variations====
  * Replace temperature contrast with salinity: make one jar salty and the other fresh.  
  * Measure temperatures with thermometers before and after to quantify the gradient and mixing.  
  * Use ice water for the cold layer to exaggerate the effect, or try smaller temperature differences to find the threshold where mixing slows.



====Safety Precautions====
  * Use warm, not scalding, water to avoid burns; handle hot water with care.  
  * Perform the entire setup over a tub or in a sink to contain spills.  
  * Jars become heavy when full; use two hands and keep a firm grip when flipping.  
  * Use a thin, stiff laminated card; flimsy sheets can buckle and cause spills.  
  * Check that glass jars are free of cracks and that stacking is stable to prevent breakage.  
  * Supervise children closely during handling and flipping.




====Questions to Consider====
  * Why do the layers remain separate when warm water is on top? (Warm water is less dense, so the arrangement is stable.)  
  * What causes rapid mixing after the flip? (Cold, denser water on top sinks, driving convection that mixes the jars.)  
  * How does this model relate to fall overturn in temperate lakes? (Surface waters cool, become denser than deep water, sink, and mix the lake.)  
  * Would the same thing happen if the temperature difference were small? (Mixing would be slower or minimal because the density contrast is weaker.)  
  * How would adding salt to the bottom jar change the outcome? (Increased density can maintain stratification even if the top is cooler, illustrating salinity-driven layering.)