<p>The temperature does indeed increase with depth, something that is a problem in <a href="https://en.wikipedia.org/wiki/TauTona_Mine" rel="noreferrer">deep mines</a> or <a href="https://en.wikipedia.org/wiki/Kola_Superdeep_Borehole#Drilling" rel="noreferrer">deep drilling</a>, but a benefit for <a href="https://en.wikipedia.org/wiki/Geothermal_heating" rel="noreferrer">geotermal heating</a>. The heat originates mostly from radioactive decay, but there is also a fraction of primordial heat from the formation of the Earth. </p>

<p>As a rule of thumb, geologists often assume 25°C/km temperature increasements within continental plates, but steeper increasements in oceanic plates or volcanic regions. The temperature curve is not constant, however. Due to the <a href="https://en.wikipedia.org/wiki/Convective_heat_transfer" rel="noreferrer">convection</a>, the temperature increases slower with depth in the mantle and outer core. In that sense, the crust is an isolating cover. The temperature also varies due to subductions, the dynamics of the mantle and radioactivity within the crust. </p>

<p>The shallow soil temperature and the temperature in the air depend mostly on the <a href="https://en.wikipedia.org/wiki/Solar_irradiance" rel="noreferrer">insolation</a>, the radiation heat from the sun. The <a href="http://journals.ametsoc.org/doi/full/10.1175/BAMS-D-14-00265.1" rel="noreferrer">average annual solar isolation</a> is 1361 $W/m^2$ but only 20-500 $mW/m^2$ heat flux comes from Earth's internal heat production. However, this tiny fraction can sometime sbe important, e.g. to understand basal melting of ice sheets and glaciers. </p>

<p><a href="https://i.sstatic.net/KSEzx.jpg" rel="noreferrer"><img src="https://i.sstatic.net/KSEzx.jpg" alt=" Geothermal gradient adapted from Boehler, R. (1996). Melting temperature of the Earth&#39;s mantle and core: Earth&#39;s thermal structure. Annual Review of Earth and Planetary Sciences, 24(1), 15–40. CC"></a>
<em>Temperature gradient from crust to core. Recent research have improved the accuracy, but the principle in the figure remains (Annual Review of Earth and Planetary Sciences, 24(1), 15–40)</em></p>

<p>So why do you experience the temperature colder underground when you are digging? </p>

<p>The temperature at depth is the <a href="http://onlinelibrary.wiley.com/doi/10.1111/j.1745-6584.2005.00052.x/abstract" rel="noreferrer">average temperature</a> for day and night or even the year. The <a href="https://en.wikipedia.org/wiki/Solar_irradiance" rel="noreferrer">energy from the sun</a> reaches only the uppermost soil layer that <a href="https://en.wikipedia.org/wiki/Absorption_(electromagnetic_radiation)" rel="noreferrer">absorbs</a> heat but the <a href="https://en.wikipedia.org/wiki/Thermal_conduction" rel="noreferrer">thermal conductivity</a> is relatively low and it will not transport the heat down. The ground is <a href="https://en.wikipedia.org/wiki/Volumetric_heat_capacity#Thermal_inertia" rel="noreferrer">reluctant to change</a> its temperature regardless of variations on the surface. </p>

<p>How strongly you experience the temperature difference depends on where in the world you are and the local conditions. In some areas, the ground remains frozen, <a href="https://en.wikipedia.org/wiki/Permafrost" rel="noreferrer">known as permafrost</a>, even as the temperatures in the air can be pleasant during the summer and the <a href="https://en.wikipedia.org/wiki/Active_layer" rel="noreferrer">surface layer</a> melts. The <a href="https://en.wikipedia.org/wiki/Permafrost#Base%20depth" rel="noreferrer">depth</a> of the suggests that it's been formed over a long time. It reaches down to the depth where the mean temperature over hundreds or thousands of years is 0°C. At larger depths, the heat from the Earth's interior takes over and melts the frost. </p>

<p><a href="https://i.sstatic.net/V0ARD.jpg" rel="noreferrer"><img src="https://i.sstatic.net/V0ARD.jpg" alt="CC Original contributor, German Wikipedia user, &quot;HylgeriaK&quot;, translation by User:HopsonRoad. Wikimedia"></a>
<em>Temperature in the active layer and permafrost. The thickness of active layer is less than a couple of meters and the thickness of the permafrost depends on its age, the bedrock and the distance to the coast. (By German Wikipedia contributor, "HylgeriaK")</em></p>

<hr>

<p><em>See the plot, linked by @JeopardyTempest in the comments to better understand the variations. Plotted on another scale and depth, annual variations would probably be seen as well.</em> </p>

CC BY-SA 3.0
2017-05-18T11:58:11.913Z
So if I understand this informative answer correct, because the surface temperature used to be much colder and the heat conductivity of soil is low, it will take a very long time until the heat exchange between surface and core will have heated up the layer between them. And could the  speed of this  process be measured by how fast the base of the permafrost is changeing?
HeLi8
CC BY-SA 3.0
2017-05-18T12:02:36.127Z
Heat from the core is lost at the surface, and if more heat is lost than added, the temperature goes down, as we see in the crust of the Earth. For shallow soil, the energy from the sun is much more important than the geothermal energy, but the variations in air temperature are much faster than the soil due to its thermal inertia and limited conductivity.
user2821
CC BY-SA 3.0
2017-05-18T12:06:43.737Z
I've updated the answer, hopefully a bit clearer.
user2821
CC BY-SA 3.0
2017-05-18T12:39:48.707Z
So to wrap up the idea: the thin "skin" of the Earth generally will get hotter nearer the surface because of solar energy, but also cools more quickly near the boundary because of it's exposure to air.  And soil moisture\infiltration also significantly changes temperatures.  So it's often warmer right at the surface compared to below... and it's also often cooler (at night/in cold season/after raining).  Exposure to sun, air, and water really cause a lot of fluctuation right very near the surface.
JeopardyTempest
CC BY-SA 3.0
2017-05-18T12:41:34.237Z
@HeLi8 There's two layers sources of heat, and one sink. The Sun heats from the outside, and the Earth from the inside. The sink is only on the outside (Earth's thermal radiation). The average daily temperature is constant (ignoring seasonal changes, the atmosphere and oceans etc.), which means that on average the amount of heat radiated by the Earth's surface is the same as the amount of incident radiation (from the Sun) and the heat escaping from the Earth source. If you removed the Sun, the surface temperature would get much lower, while the deeper crust would stay the same.
Luaan
CC BY-SA 3.0
2017-05-18T12:42:37.300Z
For a little bit of live data HeLi8, take a look at http://www.mesonet.org/index.php/weather/soil_temperature, where you can see the fluctuations Tbb showed in his image, and how they decrease as you go down (and can see some impacts of rain/weather, and if you change it to the 90 day on the top left, how things change over seasons)
JeopardyTempest
CC BY-SA 3.0
2017-05-18T12:43:30.327Z
@JeopardyTempest I'd expect the air actually has a slight negative impact - it makes the difference between day and night temperatures lower, so the temperature gradient is smaller. If you removed the Earth's atmosphere, the surface would get much colder (for many reasons); if you removed the Sun as well, even more so.
Luaan
CC BY-SA 3.0
2017-05-18T12:46:16.560Z
Nice plot @JeopardyTempest it makes it very clear!
user2821
CC BY-SA 3.0
2017-05-18T12:51:30.013Z
@Luaan Correct. But at the surface the heat from inside is neglectable. At some depth (a few meters) the seasonal are also smoothened out.
user2821
CC BY-SA 3.0
2017-05-18T13:14:58.750Z
Nice figures.  Does it have numbers on its y-axis, perhaps ballpark numbers?
gerrit
CC BY-SA 3.0
2017-05-18T13:23:01.787Z
@gerrit depth of active layer varies in a large range, depending on a lot of factors. Not only temperature but from my experience humidity have been very important. Near glaciers, the active layer can be almost nothing, and in some valleys, with warm summers it can be a couple of meters. The thickness of permafrost varies over an even larger range, depending on its age and the bedrock. It's just a principle sketch, the only CC I could find.
user2821
CC BY-SA 3.0
2017-05-18T13:24:56.927Z
@Tbb Good.  Could you put a source attribution immediately below each figure, just to avoid confusion? I assume it's from one of the linked articles but it's better to be clear.
gerrit
CC BY-SA 3.0
2017-05-18T10:38:42.187Z
2017-05-18T13:32:53.247Z
2017-05-18T13:32:53.247Z
user2821
<p>So I read somewhere that the inner core temperature of Earth might be as high as 7000 °C.</p>

<p>Depending on the composition of the different layers towards the surface, the temperature should decline until the surface temperature of the air, at about average 14 °C, is reached.</p>

<p>This does however not seem to be the case as the temperature at first gets colder if one starts digging?</p>

<p>How can this be explained?</p>

<p>And please excuse any naivety on my behalf as I´m no scientist.</p>

CC BY-SA 3.0
2017-05-18T10:30:38.857Z
Welcome to the site, and thank you for a good question.

I'm sure somebody with more knowledge of this area will be along, but my quick answer would be that the temperature on the very surface is an anomaly, subject to weather, seasons, etc., and that once you go down a short way - into mines or caves - it's fairly constant. So perhaps that shallow depth is what should be considered the "surface" for the purposes of what you are asking.
Semidiurnal Simon
CC BY-SA 3.0
2017-05-23T20:46:48.037Z
The just below the surface effect is because the sun is also a source of heat.
John
<geology><geophysics><geothermal-heat><geodynamics>
Why is the temperature between the earth core and surface not distributed linearly?
HeLi8
# Why is the temperature between the earth core and surface not distributed linearly?

(Asked by: username_0 on 2017-05-18)

So I read somewhere that the inner core temperature of Earth might be as high as 7000 °C.

Depending on the composition of the different layers towards the surface, the temperature should decline until the surface temperature of the air, at about average 14 °C, is reached.

This does however not seem to be the case as the temperature at first gets colder if one starts digging?

How can this be explained?

And please excuse any naivety on my behalf as I´m no scientist.

**Tags:** geology, geophysics, geothermal-heat, geodynamics

> **Comments:**
> **username_1**: Welcome to the site, and thank you for a good question.  I'm sure somebody with more knowledge of this area will be along, but my quick answer would be that the temperature on the very surface is an anomaly, subject to weather, seasons, etc., and that once you go down a short way - into mines or caves - it's fairly constant. So perhaps that shallow depth is what should be considered the "surface" for the purposes of what you are asking.  
> **username_2**: The just below the surface effect is because the sun is also a source of heat.  

---

## 1 answers

### Answer by username_3 on 2017-05-18. Score: 17

The temperature does indeed increase with depth, something that is a problem in [deep mines](https://en.wikipedia.org/wiki/TauTona_Mine) or [deep drilling](https://en.wikipedia.org/wiki/Kola_Superdeep_Borehole#Drilling), but a benefit for [geotermal heating](https://en.wikipedia.org/wiki/Geothermal_heating). The heat originates mostly from radioactive decay, but there is also a fraction of primordial heat from the formation of the Earth.

As a rule of thumb, geologists often assume 25°C/km temperature increasements within continental plates, but steeper increasements in oceanic plates or volcanic regions. The temperature curve is not constant, however. Due to the [convection](https://en.wikipedia.org/wiki/Convective_heat_transfer), the temperature increases slower with depth in the mantle and outer core. In that sense, the crust is an isolating cover. The temperature also varies due to subductions, the dynamics of the mantle and radioactivity within the crust.

The shallow soil temperature and the temperature in the air depend mostly on the [insolation](https://en.wikipedia.org/wiki/Solar_irradiance), the radiation heat from the sun. The [average annual solar isolation](http://journals.ametsoc.org/doi/full/10.1175/BAMS-D-14-00265.1) is 1361 $W/m^2$ but only 20-500 $mW/m^2$ heat flux comes from Earth's internal heat production. However, this tiny fraction can sometime sbe important, e.g. to understand basal melting of ice sheets and glaciers.

[![ Geothermal gradient adapted from Boehler, R. (1996). Melting temperature of the Earth's mantle and core: Earth's thermal structure. Annual Review of Earth and Planetary Sciences, 24(1), 15–40. CC](https://i.sstatic.net/KSEzx.jpg)](https://i.sstatic.net/KSEzx.jpg)
*Temperature gradient from crust to core. Recent research have improved the accuracy, but the principle in the figure remains (Annual Review of Earth and Planetary Sciences, 24(1), 15–40)*

So why do you experience the temperature colder underground when you are digging?

The temperature at depth is the [average temperature](http://onlinelibrary.wiley.com/doi/10.1111/j.1745-6584.2005.00052.x/abstract) for day and night or even the year. The [energy from the sun](https://en.wikipedia.org/wiki/Solar_irradiance) reaches only the uppermost soil layer that [absorbs](https://en.wikipedia.org/wiki/Absorption_(electromagnetic_radiation)) heat but the [thermal conductivity](https://en.wikipedia.org/wiki/Thermal_conduction) is relatively low and it will not transport the heat down. The ground is [reluctant to change](https://en.wikipedia.org/wiki/Volumetric_heat_capacity#Thermal_inertia) its temperature regardless of variations on the surface.

How strongly you experience the temperature difference depends on where in the world you are and the local conditions. In some areas, the ground remains frozen, [known as permafrost](https://en.wikipedia.org/wiki/Permafrost), even as the temperatures in the air can be pleasant during the summer and the [surface layer](https://en.wikipedia.org/wiki/Active_layer) melts. The [depth](https://en.wikipedia.org/wiki/Permafrost#Base%20depth) of the suggests that it's been formed over a long time. It reaches down to the depth where the mean temperature over hundreds or thousands of years is 0°C. At larger depths, the heat from the Earth's interior takes over and melts the frost.

[![CC Original contributor, German Wikipedia user, "HylgeriaK", translation by User:HopsonRoad. Wikimedia](https://i.sstatic.net/V0ARD.jpg)](https://i.sstatic.net/V0ARD.jpg)
*Temperature in the active layer and permafrost. The thickness of active layer is less than a couple of meters and the thickness of the permafrost depends on its age, the bedrock and the distance to the coast. (By German Wikipedia contributor, "HylgeriaK")*

---

*See the plot, linked by @username_4 in the comments to better understand the variations. Plotted on another scale and depth, annual variations would probably be seen as well.*

> **Comments:**
> **username_0**: So if I understand this informative answer correct, because the surface temperature used to be much colder and the heat conductivity of soil is low, it will take a very long time until the heat exchange between surface and core will have heated up the layer between them. And could the  speed of this  process be measured by how fast the base of the permafrost is changeing?  
> **username_3**: Heat from the core is lost at the surface, and if more heat is lost than added, the temperature goes down, as we see in the crust of the Earth. For shallow soil, the energy from the sun is much more important than the geothermal energy, but the variations in air temperature are much faster than the soil due to its thermal inertia and limited conductivity.  
> **username_3**: I've updated the answer, hopefully a bit clearer.  
> **username_4**: So to wrap up the idea: the thin "skin" of the Earth generally will get hotter nearer the surface because of solar energy, but also cools more quickly near the boundary because of it's exposure to air.  And soil moisture\infiltration also significantly changes temperatures.  So it's often warmer right at the surface compared to below... and it's also often cooler (at night/in cold season/after raining).  Exposure to sun, air, and water really cause a lot of fluctuation right very near the surface.  
> **username_5**: @username_0 There's two layers sources of heat, and one sink. The Sun heats from the outside, and the Earth from the inside. The sink is only on the outside (Earth's thermal radiation). The average daily temperature is constant (ignoring seasonal changes, the atmosphere and oceans etc.), which means that on average the amount of heat radiated by the Earth's surface is the same as the amount of incident radiation (from the Sun) and the heat escaping from the Earth source. If you removed the Sun, the surface temperature would get much lower, while the deeper crust would stay the same.  
> **username_4**: For a little bit of live data username_0, take a look at http://www.mesonet.org/index.php/weather/soil_temperature, where you can see the fluctuations Tbb showed in his image, and how they decrease as you go down (and can see some impacts of rain/weather, and if you change it to the 90 day on the top left, how things change over seasons)  
> **username_5**: @username_4 I'd expect the air actually has a slight negative impact - it makes the difference between day and night temperatures lower, so the temperature gradient is smaller. If you removed the Earth's atmosphere, the surface would get much colder (for many reasons); if you removed the Sun as well, even more so.  
> **username_3**: Nice plot @username_4 it makes it very clear!  
> **username_3**: @username_5 Correct. But at the surface the heat from inside is neglectable. At some depth (a few meters) the seasonal are also smoothened out.  
> **username_6**: Nice figures.  Does it have numbers on its y-axis, perhaps ballpark numbers?  
> **username_3**: @username_6 depth of active layer varies in a large range, depending on a lot of factors. Not only temperature but from my experience humidity have been very important. Near glaciers, the active layer can be almost nothing, and in some valleys, with warm summers it can be a couple of meters. The thickness of permafrost varies over an even larger range, depending on its age and the bedrock. It's just a principle sketch, the only CC I could find.  
> **username_6**: @Tbb Good.  Could you put a source attribution immediately below each figure, just to avoid confusion? I assume it's from one of the linked articles but it's better to be clear.  

---

