57 votes

Why is the pond in my backyard not frozen when it is -15 °C (5 °F) outside?

Water is a rather strange substance. With most substances, the solid phase is denser than is the liquid phase. This is not the case with water. Ice is less dense than liquid water. A side effect of ...
David Hammen's user avatar
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55 votes
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Does gravity increase the closer to the core you get?

The below figure, taken from Wikipedia shows a model of the free fall acceleration, i.e., 'gravity'. The left-most point corresponds to the center of the Earth; then further right at $6.3\cdot1000$ km ...
Erik's user avatar
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53 votes
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Why is the pond in my backyard not frozen when it is -15 °C (5 °F) outside?

Water melts at 0 °C (32 °F) but freezing is a more complicated affair. It is safe to say water gains the ability to freeze at 0 °C, but it can get much cooler before it actually does so resulting in ...
casey's user avatar
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42 votes
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Where on Earth is the magnetic field intensity stronger?

If we consider only the magnetic field generated by natural sources, and not the ones generated by human activities. The general trend is that higher intensities of the magnetic field happen close to ...
Camilo Rada's user avatar
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37 votes

What is the pressure at the center of the Earth?

It is the pressure gradient that is proportional to the local gravitational force. When that force is integrated over a distance, the pressure gradient is integrated to accumulate a total pressure. ...
Oscar Lanzi's user avatar
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31 votes

What are these undersea lines all around Hawaii?

Short answer: The lines radiating from the island are artifacts from a mapping process that superimposes high-resolution sonar data from standard ship tracks on top of r low- or average sonar data ...
jeffronicus's user avatar
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28 votes
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Where is the calmest place on Earth?

The main resistance that winds have to their movements comes from the topography and surface obstacles. Therefore, as a general rule the closer to the surface the less wind you will find. But I guess ...
Camilo Rada's user avatar
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22 votes

Why is the pond in my backyard not frozen when it is -15 °C (5 °F) outside?

Due to convection (the cold water sinks while the warm water rises), the entire pond needs to be brought to near-freezing temperatures before the surface can freeze. With only the top of the pond in ...
BlueRaja - Danny Pflughoeft's user avatar
22 votes
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Does gravity differ from place to place on Earth

There is an entire field of Geophysics called gravimetry dedicated to measuring the magnitude of the gravitational field. First, we should distinguish between weight (a force) and gravity (an ...
arkaia's user avatar
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22 votes
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Taking into account the decay products of transuranic elements, are the world's uranium reserves growing or shrinking?

Let's start with the easy question: given our current known reserves of transuranic elements and again taking into account their decay products, where will the greatest uranium reserves be in, say, ...
Gimelist's user avatar
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21 votes
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How to quickly or easily prove the world is spherical?

To answer my own question based on the useful comments here, my favorite methods were: The earth's shadow cast on the moon during an eclipse. It is easy to observe and the flat earth counter argument ...
dutch's user avatar
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21 votes

How to quickly or easily prove the world is spherical?

I'm sorry that this answer will be unsatisfactory and, strictly spoken, off topic, but I want to give it anyway. The answer is: You can't. Don't waste your time. Your relative has developed a paranoia....
Peter - Reinstate Monica's user avatar
19 votes
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How bad is geo-engineering?

The bad part about geo-engineering are the unknown unknowns. Our climate models are wrong. All models are wrong, but some are useful.. Our models are useful, but not quite useful enough to trust ...
gerrit's user avatar
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19 votes
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Can ultrasounds be used to map the density of large underground regions?

Sound-like waves are routinely used to image the subsurface, but mostly well below the ultra-sound band. Several methods involve sound-like vibrations: Reflection seismic — the most important ...
Matt Hall's user avatar
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17 votes

Why is the temperature between the earth core and surface not distributed linearly?

The temperature does indeed increase with depth, something that is a problem in deep mines or deep drilling, but a benefit for geotermal heating. The heat originates mostly from radioactive decay, but ...
user2821's user avatar
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17 votes
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How did the Ural mountains form?

The Ural mountains are one of the oldest mountain ranges on Earth. They started forming about 300 Ma ago by the subduction of the oceanic crust once attached to the Kazakhstania plate under the ...
Camilo Rada's user avatar
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15 votes

Why is the pond in my backyard not frozen when it is -15 °C (5 °F) outside?

The water in a pond is in contact with the ground and the ground is not even close to freezing even if the air temperature is 27° below freezing.
BillDOe's user avatar
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15 votes
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Is the global temperature of the earth a physical quantity at all?

There is no unique way to define a global average temperature of the Earth, but that does not mean that calculation of such a quantity is "pointless". If someone tells me that the average ...
David Bailey's user avatar
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14 votes

Are there water molecules in the oceans which will almost never fall down as rain?

Never is a very, very long time. Almost never is a vague term. The age of deep ocean water has been studied with both models and observations, and both indicate a lower limit age in the order of ...
gerrit's user avatar
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13 votes
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Why is the South Pole colder than the North Pole?

The most obvious reason why the South pole is colder than the North pole is altitude. The Arctic is mostly covered by ocean and low lying land whereas the Antarctic continent is really quite high, ...
bon's user avatar
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13 votes
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How is known that the Earth core is solid?

This is a good observation. The reason we can measure S-wave (transverse) propagation in the inner core is because P-waves can set up S-waves and vice versa. When an S-wave hits the mantle-outer core ...
bon's user avatar
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13 votes
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How much water is the atmosphere losing to space?

It is not actual water what is lost to space, because in the high atmosphere water usually dissociate into other molecules or ions. The oxygen ion outflow is frequently assumed to be a proxy for the ...
Camilo Rada's user avatar
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13 votes

Where on Earth is the magnetic field intensity stronger?

Since it is not specified in the question (and to complement the other answer), the strongest magnetic fields on Earth are not naturally generated, but artificially. The current world record holder ...
Graipher's user avatar
  • 331
13 votes

What is the pressure at the center of the Earth?

The previous answers do a fine job already. But I'll try to add a simple thought experiment. Imagine three objects floating in space, clumping together by gravity: ...
stippi's user avatar
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12 votes

How to quickly or easily prove the world is spherical?

Skype. Skype someone at least 10 time zones away. Ask them to point the camera at the sky. Proven. This actually works really well for a psychological reason as well. Its easy to deny abstract ...
George Mauer's user avatar
12 votes

Carbonate reef not horizontal in seismic section?

This phenomenom is called velocity pull-up. The seismic waves that reflected off the top of Layer 2 beneath the carbonate layer have also travelled through the carbonate. Because carbonate (at least ...
stevej's user avatar
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11 votes
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How fast does the Earth's inner-core spin?

Roughly once per day, but a tiny bit faster. As the website for Columbia University's Lamont-Doherty Earth Observatory says: The inner core rotates in the same direction as the Earth and slightly ...
Spencer's user avatar
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11 votes

Are old geophysics textbooks useful?

Mathematics and computer science are exact sciences. If something is discovered and known, it is not wrong. With time, there may be better or new ways of doing something, but the old stuff is still ...
Gimelist's user avatar
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10 votes

Why is the pond in my backyard not frozen when it is -15 °C (5 °F) outside?

This could be because the air cools and heats faster than water. Air has a lower Specific Heat than water. It may take some time for the water to freeze.
Pranav Wadhwa's user avatar

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