# Buoyancy

**URL:** <https://boards.straightdope.com/t/buoyancy/394279>\
**Category:** Factual Questions\
**Created:** [March 2, 2007, 5:54am UTC](https://boards.straightdope.com/t/buoyancy/394279 "2007-03-02T05:54:52Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![Baffle](https://avatars.discourse-cdn.com/v4/letter/b/dec6dc/32.png) [@Baffle](https://boards.straightdope.com/u/Baffle)\
**Post date:** [March 2, 2007, 5:54am UTC](https://boards.straightdope.com/t/buoyancy/394279/1 "2007-03-02T05:54:52Z")

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How does the temperature of water affect the buoyancy of objects in it?

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**Author:** ![illoe](https://avatars.discourse-cdn.com/v4/letter/i/a3d4f5/32.png) [@illoe](https://boards.straightdope.com/u/illoe)\
**Post date:** [March 2, 2007, 6:03am UTC](https://boards.straightdope.com/t/buoyancy/394279/2 "2007-03-02T06:03:30Z")

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For a floating object, the upwards buoyant force must equal the downward force of gravity. Archimedes’ principle asserts that the buoyant force is equal to the total weight of the displaced fluid. Since the weight of the fluid is its density times the displaced volume, and the density of water decreases with increasing temperature, you must displace more water to get the same buoyant force with increasing temperature.

So your vessel will sit lower in the water on a hot day.

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**Author:** ![Baffle](https://avatars.discourse-cdn.com/v4/letter/b/dec6dc/32.png) [@Baffle](https://boards.straightdope.com/u/Baffle)\
**Post date:** [March 2, 2007, 6:08am UTC](https://boards.straightdope.com/t/buoyancy/394279/3 "2007-03-02T06:08:12Z")

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Excellent answer! Thank you.

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**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\
**Post date:** [March 2, 2007, 6:39am UTC](https://boards.straightdope.com/t/buoyancy/394279/4 "2007-03-02T06:39:58Z")

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[QUOTE=illoe]  
For a floating object, the upwards buoyant force must equal the downward force of gravity. Archimedes’ principle asserts that the buoyant force is equal to the total weight of the displaced fluid. Since the weight of the fluid is its density times the displaced volume, and the density of water decreases with increasing temperature, you must displace more water to get the same buoyant force with increasing temperature.

So your vessel will sit lower in the water on a hot day.  
[/QUOTE]  
Not so much that you’d notice, though. The difference in density at standard pressure between water at 4 degrees Celsius and 25 degrees Celsius (about 77 degrees F) is less than 0.3%. And it’s a curious property of water (part of its slightly polar nature) that its maximum density occurs at 4 degrees Celsius, then density decreases again as it approaches solid phase at 0 degrees C, causing ice to float, which protects living things below from freezing.

Stranger

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**Author:** ![illoe](https://avatars.discourse-cdn.com/v4/letter/i/a3d4f5/32.png) [@illoe](https://boards.straightdope.com/u/illoe)\
**Post date:** [March 2, 2007, 7:01am UTC](https://boards.straightdope.com/t/buoyancy/394279/5 "2007-03-02T07:01:47Z")

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Yes; I recall reading that if water did not behave in this peculiar way due to the sparse lattice structure of ice, that the Earth’s oceans, once frozen, would never unfreeze. One of those shivering mysteries that makes the anthropic principle so appealing.

And yes, it’s a small effect, but some poor jerks spend years on _ab initio_ simulations to describe and predict similar effects. ::Raises hand::