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There's a tidal effect that we can clearly observe in oceans, which is the effect of gravity from the Sun and the Moon. If gravity affects everything equally, why don't lakes have tides?

TBBT
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1 Answers1

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You probably got voted down cause this can easily be google searched, but the simplest way to explain it is that a tide happens because the lunar tug on one side of the ocean is measurably more than on the other side of the ocean and as the earth rotates the tidal "bump" follows the moon so you get 2 high tides and 2 low tides a day.

A tide is effectively one very large wave. The distance from Peak (High tide) to Trough (low tide) is 1/4 the circumference of the Earth.

Lakes do have tides, but since all lakes are much smaller than 6,000 miles across, it's nothing like ocean tides. The wavelength still applies to all bodies of water, but the lake is so small compared to the wavelength that even a big lake would have a tidal rise and fall of maybe 1/3rd of an inch - too little for most people to notice.

Your bathtub also has tides, but now we're talking about maybe the height of a few atoms. Even the Mediterranean sea isn't large enough for significant tides. It has small ones.

userLTK
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    This is nice, but it is not correct. That wave you described does not and cannot exist. That would be a shallow water wave (a wave whose wavelength is much greater than depth). The depth of the ocean dictates the speed of a shallow water wave, and that speed is much less than the Earth's rotation rate. There is no tidal bulge. – David Hammen Aug 27 '15 at 13:54
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    If I may suggest an edit for this post (in bold): "...most lakes are much smaller than 6,000 miles $\text{across}^{\textbf{[citation needed]}}$..." – Jim Aug 27 '15 at 14:52
  • @Jim Does https://en.wikipedia.org/wiki/List_of_lakes_by_area work for you? It lists the largest (Caspian Sea) @ 745 mi. and Tanganyika (6th by area, but 2nd longest) @ 420 mi. This might be grounds for editing the text to "... all lakes on earth are ..." – tjd Aug 27 '15 at 15:11
  • @DavidHammen You're partly incorrect. There are limits on wave height in lakes and so on, but there most certainly is a tidal bulge and it is measurable in, e.g. the North American Great Lakes. – Carl Witthoft Aug 27 '15 at 15:15
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    @CarlWitthoft That's a great lake example – Jim Aug 27 '15 at 15:17
  • @Jim Sorry for being humor-impaired. – tjd Aug 27 '15 at 15:18
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    @Jim you left out "...try the veal" – Carl Witthoft Aug 27 '15 at 15:18
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    Is across related to North-South or West-East axis? The Atlantic Ocean is less than 4,000 miles-wide but has tides. – A.L Aug 27 '15 at 17:19
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    This question explores the myth of the globe-encircling tidal bulge. –  Aug 27 '15 at 17:44
  • @A.L "across" is nonspecific. It just means from one side to the side immediately opposite it. In context, any line that starts and ends on the shore of the same lake can be said to be across the lake. In that way, the largest crossing is the size referred to – Jim Aug 27 '15 at 20:40
  • The point is that the tidal forcing drives resonance in bodies of water, and that causes there to be high and low spots in the surface which have the right period, but are, in general, out of phase; further their size depends on the $Q$ of the resonance rather than directly on the strength of the tidal interaction. – dmckee --- ex-moderator kitten Aug 27 '15 at 20:48
  • Wow, I didn't expect this, I saw the question was voted down and I gave a quick answer cause it was a little too long for a comment. If I'd known it would be voted up I'd have spent much more time on this and if anyone wants to give more detail, feel free, especially on points I might have not been 100% correct. – userLTK Aug 27 '15 at 21:01