Two jars of water at different temperatures are stacked mouth to mouth. With warm water on top the layers remain separated, but flipping the pair places cold water above warm and triggers rapid mixing that models seasonal lake overturn.

1. 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.
2. 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.
3. Carefully place the inverted warm jar on top of the upright cold jar so their mouths align.
4. 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.
5. 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.
6. Watch the cold, denser blue water sink and the warm red water rise, producing visible currents and thorough mixing.
7. Discuss how this represents seasonal changes when surface water cools and sinks, driving circulation.

Explore Lake Stratification in a Jar! - Green Mountain Conservation Group:

Description or Video title2 - Author2: demonstrations

📄 Lake Overturn (Page 52) - Matt Shade: https://www.unco.edu/nhs/science/pdf/demos/2012_CSC_demos.pdf

• 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.

• 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.

• 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.)