Pure Distilled Water for Emergencies when Wells are Unusable

• You have noticed that when you have a glass of iced soda, that often droplets of water form on the OUTSIDE of the glass. There are people who think that water is seeping through the glass, but that is not true! What is actually happening is that the cold liquid INSIDE the glass causes the glass itself to get cold (which you know). THEN, when WARM room air happens to pass near that cool glass, the AIR cools down. IF it cools down enough, and if the humidity in the room is high enough, the air cools to below the Dewpoint. What that means is that the cooler air cannot then contain as much water vapor as it normally had, and so droplets of water MUST form on the cool surface as some of the water condenses.
  
  
• This is also why fog occurs (in that case, the water droplets condense around tiny specks of dust in the air) or why basement walls are often damp or why some windows can have water or even ice form on them in winter.
  
  
• Sometimes in the morning, you see a whole yard full of dewdrops on lawn grass. This occurs because the grass had cooled during the night and normally warm and humid air condenses on every blade of cooled grass. This is actually sometimes gallons of very pure (distilled) water as dew, which might seem collectible, but which will simply naturally evaporate as the grass warms in the morning sun. The system described here buries an underground slightly tilted airpath, where the naturally humid air is sent down through a very cool underground passageway so many droplets of water form on the inside walls of the cool tube. The slight tilt, like as a house rain gutter, lets the water flow to one end where a pot or tank can collect it. We maximized the performance of this water collection with modern Engineering techniques.
  
  
• These water droplets that form are PERFECTLY PURE water, also called distilled water. All you would need to do would be to collect all those droplets! THAT is what this system does, but on a far more organized basis and far more efficiently.
  
  
• This is such a simple and inexpensive system that it DOES have a couple of complications. If your climate is such that the air is dusty, you may need to add a simple cloth air filter over the tube inlet, or else the resulting (pure) water can look whitish from suspended dust in the water, which is no safety issue but is easy to eliminate. You could also use a filter, even a carbon filter, when pouring the water from your collection pot into a glass. The other 'complication' is actually not a complication at all! Since this system creates SUCH pure water, it even removes (nearly) all the dissolved minerals that normal water contains which gives it a pleasant taste and sweetness. Again, no safety issues here, but there are easy ways to add appropriate minerals to flavor the water!
  
  
• In the news, you hear of 'water purifiers' which brag about providing safe drinking water. Such devices are astoundingly expensive (often more than a million dollars), huge and heavy (where they cannot easily be transported to remote third world locations), they require continuous supplies of diesel fuel (to power some high pressure pumps they require) and they break down so that a very expensive Technician is nearly constantly required to clean or replace nano-filters or repair equipment. On top of that, those RO (Reverse Osmosis) devices can never make PURE water, but only water which just barely qualifies as potable (drinkable) at 500 parts per million of remaining contaminants. Our NATURAL system of removing humidity from local air is cheap, simple, does not require ANY electricity or diesel, and never breaks down or clogs! And it provides water that is essentially ZERO ppm of contaminants, absolutely pure!
  
  
• If an inexpensive HG 3a heating device is built, this process can be enhanced even more where very large amounts of humidity can be evaporated into the air and this system also can then operate day and night.
  
  
• By sending that warm and humid air through a cool underground tube that is deep enough and long enough, some of the humidity in the air can condense on the tube walls. If that underground tube is slightly sloped downward like a house rain gutter, that water is then collected, and it is essentially distilled water, very pure!
  
  
• This is a very simple and inexpensive system which virtually cannot ever break down!
  
  
• You have certainly seen water dripping from window air conditioners. You probably never realized how extremely PURE that water is! (Although you should not drink it as some air conditioners have lead solder inside them which is dangerous) You may also know that central air conditioners condense as much as 12 quarts (3 gallons) of humidity from the air (per day) in the process of cooling it. Dehumidifiers do the same thing with the INTENTION of removing humidity from the air (by cooling the air briefly!) The water flow through the drain hose of any air conditioner is easily confirmed and is quite impressive!
  
  
• Maybe you think that only an irrelevantly small amount of water exists as humidity in the air. That is not the case! If the entire Earth's atmosphere is considered, we know that about 4,000,000,000,000,000 gallons of water is always in it! There are over 7 billion people now living, but that means that there is enough humidity in the total atmosphere where around 6,000,000 gallons of water could theoretically be provided for every single one of us! And it gets replaced every day as warm ocean waters evaporate over 3/4 of the Earth's surface area!
  
  It is certainly true that over a desert, the humidity is lower, but it is never zero! The water production of this simple system is much less in a desert than in a tropical rain forest, but it ALWAYS works!
  
  
• You have certainly seen water dripping from window air conditioners. You may also know that central air conditioners condense as much as 12 quarts (3 gallons) of humidity from the air in the process of cooling it. The water flow through the drain hose is easily confirmed and is quite impressive!
  
  
• This process of sending air through underground (cool) tubes has long been the process of our "free air conditioning" system, but this case is different in that we intentionally send HUMID outdoor air through the tubes rather than less humid house air.
  
  
• A consistent 2 gallons of water per day is usually produced by this BASIC system, but additional features like the HG 3a can easily increase this to many gallons per day.
  
  
• In fact, we have Engineered a larger-scale Municipal system which might be used in Pure Desalinated Seawater for Port-au-Prince, Haiti to provide 700 gallons or 3,000 gallons of perfectly pure distilled water there every day, and all for a total cost of roughly $10,000, and it is installable in possibly a single day. If they chose to install twenty such systems, they might nearly immediately be able to supply as much as 60,000 gallons of pure water to combat the many severe diseases that are spreading due to people having to use contaminated water. This Municipal system is a Desalinization variant of this humidity condensation system, and of much larger scale than this presentation generally discusses. That system is also suitable for many other places where pure water supplies are limited and ocean water is available, such as Indonesia, Bangladesh, India, Egypt or most islands.

Below, we will give an example of an AVERAGE summer day near Chicago, IL, USA, where an 8-ounce coffee cup full of absolutely pure distilled water is provided every three minutes! This continues constantly for most of the daytime hours, and all straight from the humidity in the atmosphere!

You are probably familiar with the fact that during the summer, concrete basement walls are often damp or even wet. That occurs because the warm outdoor humid air gets into the basement, and when it passes near the colder concrete wall or floor, it cools and loses some of its ability to hold moisture. If the air drops to a temperature low enough, then some of the moisture has to condense into water droplets. That is essentially the concept used here, but this system uses an enclosed chamber to keep the water purer.

You may be familiar with a survival procedure taught to travelers to remote areas, where they spread a small sheet of plastic suspended above the ground. The relatively cool ground beneath it causes the plastic to (often) be cooler than the hot daytime air, and some humidity (moisture) in the air can condense into droplets on that cooler plastic, and then be collected to drink to survive. That very crude method enables capturing a very small amount of the humidity in the air. The system described here is a far more sophisticated and far more effective way of doing that same process.

All atmospheric air contains some moisture, water, which we call humidity. If that air is COOLED, its "RELATIVE" humidity increases, because cooler air cannot contain as much moisture in it. If it is possible to cool it enough, the air gets to 100% relative humidity, and the saturated air starts having tiny droplets of water condense out on cooler surfaces. That water is PERFECTLY PURE water that is called Distilled water.

OK. You are skeptical! How can there be much water in the atmosphere? And, in SOME climates, such as deserts, that concern is valid. But look at this graph of the outdoor relative humidity for a location near Chicago, Illinois, USA. See that the outdoor relative humidity is amazingly high in nearly all months! In the morning, it is nearly always at least 80% and in the afternoon when it is usually lowest, it is still generally over 60%. There is a LOT of water in the atmosphere as humidity!

In the Summer, it works impressively. In the winter, the moisture is still in the air, but the ground is probably not cold enough to cause it to condense there. So, for a climate like Chicago, only about six to eight months of substantial water production is possible with the basic system. However, the (discussed) addition of a $200 accessory, an HG 3a device can produce even larger quantities of water every day of the year, and THAT is true in ANY climate, even a desert!

We have found that for many environments, simply blowing hot daytime air through a COOL underground tube, is able to cool the air enough for the condensation to occur. A Chart and also an automatic Calculator below provides the necessary information to know how many gallons of perfectly pure distilled water can be obtained in this way, directly from the atmosphere! (If any local supply of groundwater happens to be available, no matter how contaminated it might be, the performance of this system can even be greatly increased!)

This new system operates in a way that is also somewhat similar to how a solar still works, except that the Sun is not necessary, no sheets of glass are necessary, and not even any source for contaminated water is necessary! Rather than a solar still HEATING water to increase the humidity inside the chamber, so that it will condense on a relatively cooler glass cover panel, this approach uses the fact that deep underground, the soil is naturally cooler than the daytime air temperature ^{note 3}.

All of these things occur because warm air can hold more water vapor in it than cooler air can, and that the deep soil is always cooler than the air temperature during hot summer days, and usually during winter days as well.

This amazingly simple and inexpensive system can realistically supply 10 gallons (40 liters) of perfectly pure water every day! There is essentially nothing which can break down, so it should reliably provide water for many, many years.

We are showing the warm daytime air as the red arrow at the right of this drawing, where it goes into and down through an inexpensive PVC 4" plastic sewer pipe arrangement. As tiny droplets of water form on the inner walls of the pipe, gravity causes them to flow downward to collect in the bottom of the Tee shown. A small pipe goes downward from there to a storage tank or simply a kitchen pot. The air continues upward and out, shown now as light blue to indicate that the air has actually also been cooled by the cooler ground, as well as the desired dehumidification that provided the pure water.

We show that the pipe must be at least one meter deep to be in cool soil that is not heated by summer heat. Deeper is ALWAYS better! The length of the underground pipe should be as long as is possible. If only a short pipe is used (say 16 meters or 50 feet) it will work fine for a while, but that deep soil will gradually get heated up by the hot air constantly going through the tube, and the effect gets reduced. Thirty meters or 100 feet should give very reliable water supply for most climates. Longer is ALWAYS better!

If this air can be de-humidified so that it becomes around 20% relative humidity, it would then only contain 0.014 pound of water in it. We would capture the difference, around 0.008 pound of water, as actual water droplets. This does not sound like much, but if we send just 1000 pounds of air through our underground tube, this is 8 pounds of water, or around one gallon (4 liters).

The same Psychrometric Chart shows that at that temperature and humidity, one pound of air takes up just over 15.0 cubic feet, so we are talking about 15,000 cubic feet of air. If we hope to produce one gallon (4 liters) of this Distilled Water per hour, we then only need to send around 250 cubic feet of air through the tube every minute (15,000/60), a relatively reasonable airflow.

Depending on the local (daytime) temperature and humidity and the deep ground temperature, two gallons (8 liters) of water produced per hour is generally very realistic! Roughly 10 gallons (40 liters) of absolutely pure water to drink and for washing and bathing every day! All from a very simple underground tube!

This system can be installed WITH an underground tank and hand-pump, or WITHOUT that tank and pump, if a pit is provided so that people would climb down a ladder to get a jug of water. (drawings below).

There are actually a variety of ways that this performance can be easily enhanced. All the water that is produced by this system COMES FROM THE AIR. That means that it is absolutely pure water, called Distilled Water. Those accessories simply increase the relative humidity of the air entering the underground tube by evaporating any source of water that might be available, since when that water evaporates from its source, all the contaminants are left and only the pure water evaporates.

Possible Complications

The only other real complication is that the basic system described here is dependent on the heat from sunlight to evaporate the water into becoming humidity, so the basic system can (usually) only operate during daytime hours. The solution to a larger water production and also 24-hour water production is to include the HG 3a unit as a heat (and water) source.

Technical Information

We will use an example of where the air temperature is 120°F (60°C) and the relative humidity is 30%. (Any other local weather conditions can be similarly analyzed). In the Psychrometric Chart below, this is along the very right edge of this chart, at the bottom right end of the red line. We can see that the air contains about 0.022 pound of water in every pound of air (which the chart also shows takes up a little over 15 cubic feet). THIS is the air that we will have enter the start of the buried tube system. As this air is cooled down by contact with the much cooler (70°F or 21°C) walls of the tube, it first cools in a process that is called reversible adiabatic. This means that the Enthalpy of the dry air, the energy content per pound, stays constant during the process. This is represented by our red line toward the left and upward.

We can see that the Relative Humidity percentage keeps rising as the air gets cooled. This is because cool air cannot hold as much moisture as warm air does. This process can continue until the air becomes saturated, or is at what is called the dew-point. Once our air has cooled to around 88°F (31°C), it has gotten up to 100% Relative Humidity, meaning that it cannot hold any more water in it than that.

At this point, the process necessarily moves along the green line in our example, downward and to the left, as the air continues to be cooled in the underground tube. This process is where moisture can condense out of the air, in our case, on the walls of the cool underground tube. By the time it has gotten to the end of the tube and the air is then at around 70°F or 21°C, the Psychrometric Chart shows us that the air which had initially contained 0.022 pound of water per pound of air at the very start of entering the tube, is now fully saturated air which now contains only 0.016 pound of water in it. The remainder of that initial humidity has necessarily condensed into (absolutely pure, distilled) water droplets on the inside of the underground tube. For every pound of air that entered the tube, (0.022 - 0.016 or) 0.006 pound of water forms inside the tube. If 100 cubic feet of air enters the tube every minute, that is about (100 / 15.3) 6.5 pounds of air every minute or 390 pounds of air every hour. This then means that for this situation, (390 * 0.006) 2.4 pounds of water would condense out every hour, around 0.3 gallon per hour. A realistic two gallons of absolutely pure water every day.

This basic system does not usually work at night, but generally should work well for at least five hours each day, meaning that more than ten gallons (40 liters) of pure safe water would be available from this extremely simple system each day. (One of the possible accessories, the HG 3a unit, enables this system to work 24 hours every day.)

Air needs to be passing through the system. It should NOT be necessary to have to use any blower, because the entrance to the tube could be provided with a wind-vane type of tail to turn the intake into the wind at all times.

However, if the climate is such that a blower is sometimes necessary, the 12-volt blower from a car heater system could be used, powered from a standard 12-volt battery which is charged by a simple windmill, such as a Savonius rotor made of an old 55-gallon drum.

The system as shown only involves maybe $60 of new 4" (10cm) PVC pipe. It could also be created using surplus large diameter pipe that might be locally found. It CANNOT use CORRUGATED pipe, as the water droplets would then be trapped in many puddles inside the tube. It should NOT be made of any pipe or materials that had earlier been used to carry dangerous chemicals. It should also NOT be made of "ceramic drain tiles", because the many joints between the tile sections are likely to leak and lose the precious water being collected, or allow insects or bacteria inside the pipe.

There are countless ways that this system could be created using only locally available materials. For example, if 4" (10 cm) or 6" (15cm) or larger sections of iron pipe are found, they might either be coupled with standard pipe fittings or there are standard rubber couplings and hose clamps that are inexpensive to join two sections of such pipe.

Limited Water Needs

Larger Water Needs

These arrangements really only produce significant amounts of water when the air temperature is high, in other words on sunny days near the middle of the day. Therefore, if a blower is used, there is no sense in it running except during those few hours during the daytime.

For Climates Where the Natural Humidity is too Low

However, even in climates where the natural Relative Humidity is too low for this system to work, there may still be ways to enable it to provide excellent pure water. In the field near the tube's entrance, a large tarp might be spread out on the ground on which saltwater or rainwater other non-potable water might be spread. A second tarp, such as polyethylene, would then be supported several feet above that ^{note 2}. The heat of sunlight would heat the upper tarp and cause the contents of the chamber created to get quite hot, probably even hotter than the 120°F (49°C) we first assumed. More importantly, any water inside that chamber would get heated, with a lot of it evaporating. This would greatly increase the Relative Humidity inside that chamber, possibly even getting up near 100%. In that case, the water there would evaporate (leaving any salt or other contaminants there on the bottom tarp) to raise the Relative Humidity of the air entering the intake tube. As that much higher humidity air has a lot of its moisture condense inside the tube, the system would provide even larger quantities of absolutely pure distilled water. The additional water production is difficult to predict as it is dependent on many variables, but it can be quite significant.

Pure Desalinated Seawater Distilled Water for Off-Grid Residents.

NOTE: The water that is produced by this system CAN appear slightly cloudy! This can occur if the AIR going through the tube has dust particles in it. For any location where the air might contain a lot of dust or even sand particles, it is a good idea to add a filter over the intake to the underground air tube, and/or pour the resulting water through a cloth or better filter. Extremely tiny particles such as cigarette smoke are so tiny that they are harder to filter out, so an even better filter, such as charcoal, might be desirable.

Footnotes

(6) H₂O + (6) CO₂ + sunlight energy gives C₆H₁₂O₆ + (6) O₂.

In words, this says that water from the ground plus carbon dioxide from the air plus the energy from sunlight can produce glucose and free oxygen.

The chemical process of COMPLETE (aerobic) decomposition is exactly the opposite (there are many partial decomposition processes that result in other compounds):

C₆H₁₂O₆ + (6) O₂ gives (6) H₂O + (6) CO₂ + released energy equal to that absorbed from the sunlight.

In words, this says that glucose combined with oxygen from the air can decompose into water (vapor) and carbon dioxide and a lot of energy, primarily due to the activities of certain types of bacteria which are in soil.

More complex organic molecules such as cellulose are first broken down into glucose to permit this process, gaining some extra energy in the process.

The numbers in parentheses are the number of those molecules which are involved in the reaction. They are important.

In Chemistry, we know that those numbers can be used to describe the number of moles of each compound, so in this case, we have one mole of glucose combines with six moles of oxygen from the air to decompose into six moles of water (vapor) and six moles of carbon dioxide. This tells us the quantities of each which are involved. We need to know the molecular weights of each of the compounds, which are 180, 32 and 18 and 44, respectively. It is then really easy to calculate the WEIGHT of each material involved. 1 * 180 + 6 * 32 gives 6 * 18 + 6 * 44. This is true for any unit of weight/mass: grams, kilograms, ounces, pounds, etc.

We confirm that there is the same amount of mass on both sides, 372 units, which confirms Conservation of Mass. If we use grams, then we now know that 180 grams of glucose will combine with 192 grams of oxygen from the air to create 108 grams of water and 264 grams of carbon dioxide. This natural decomposition occurs worldwide every day, every second.

The important point here is that for every 180 units of weight of glucose that decomposes, there are 108 units of water created. So 180 pounds of glucose will give 108 pounds of water, about 15 gallons (60 liters).

The HG 3a device always has rather high humidity inside it, so when additional water is created like this, it gets carried out and away through the exhaust connection, along with the carbon dioxide (gas) that was also created. In this case, once that hot and humid air gets outside, it cools and most of the moisture in that air condenses into water droplets, which we choose to do inside the cooled underground tube. So, without actually using the large amount of heat that the HG 3a unit produces, if just the exhaust gases are sent into the underground tube, 15 gallons (60 liters) of water will be produced for each 180 pounds of organic material that is allowed to decompose. This is in addition to the moisture in the natural air itself, of the underground tube alone.

Where the underground tube alone can only function during the day, due to the energy of sunlight, when the HG 3a is added, the system can then produce water 24 hours a day. Since the HG 3a can reasonably be expected to decompose about 10 pounds per hour, or 240 pounds per day, this source therefore can provide an additional 20 gallons (80 liters) of absolutely pure distilled water every day. This is true even in an extreme desert climate where the atmospheric humidity is very near zero.

Water Evaporation Bag Functioning

Tarps could be used to enclose any source of water, of any level of contamination by any chemicals. If this is done without using an HG 3a unit, then it will be dependent on sunlight to heat up and evaporate the water, which thereby becomes humidity in air that is sent into the underground tube to condense as pure water. A moderate amount of extra water can be provided in this way, but the exact amount is difficult to calculate since there are many variables that can affect performance, especially regarding the sunlight.

But if this sort of evaporation chamber is combined with a HG 3a device, then the 90,000 Btus of heat that the HG 3a system can generate can all be made to come out with the exhaust gases, and therefore into the evaporation chamber. The heat of vaporization of water is around 970 Btu per pound, with another 70 Btu/pound or so used in heating the water up. This means that the 90,000 Btu/hr heat output that the HG 3a unit can continuously create can evaporate roughly 90 pounds of water per hour. However, a simple poly tarp cover can allow considerable heat loss at night, so the water evaporation then will be less. During the daytime, the amount of heat lost outward through the tarp may be greater or less than the amount of sunlight heating which comes in through the poly tarp, so the evaporation rate may be less than or greater than the 90 pounds of water per hour. This is roughly 13 gallons 540 liters) per hour or around 320 gallons (1,300 liters) of water evaporated from the chamber per day. Due to the night heat losses and other reasons, a more practical expected amount is around 250 gallons (1,000 liters) of water per day. Included in this is the 20 gallons (80 liters) of water the HG 3a system naturally creates in a full day of operation, so a total of around 250 gallons (1,000 liters) of perfectly pure distilled water is realistic every day. This quantity is relatively independent of local humidity, since most of the heat used is produced by the HG 3a device, although in extremely cold climates, the water production will be less.

Solar Still Operation

The operation of a solar still has many limitations. The tilted glass cover is usually faced to the south, so early in the morning or late in the afternoon, the sunlight cannot easily get in to heat the water because of the angle of the sunlight. The water is not of a color that is particularly absorbent (technically, high emissivity) to solar energy. A critical part of a solar still is that the high humidity air that is produced by the evaporation of some of the water needs to encounter a COOL or COLD surface, such that the effects described in this presentation can occur, where the (local) relative humidity gets up to 100% and therefore water must condense into droplets. Since the glass cover is the surface in a solar still which must also represent that cooler surface, it would be great if it were as cool as possible. However, as the sunlight passes through the glass on the way in, a little of the heat is absorbed. Also, the location of the glass exposes it to the outdoor air, so the glass can never be cooler than the ambient air temperature. And finally, the glass cover is constantly exposed to the warm or hot air inside the chamber, so it generally becomes quite a bit HOTTER than the current ambient air temperature. Per the Psychrometric Chart, these effects all greatly reduce the amount of water that can be produced in a solar still. A little water is better than no water, true, but our approach of having the cool surface fairly deep UNDERGROUND, that factor alone keeps the condensation surfaces 20°F or 30°F (10°C or 15°C) or COOLER than the glass cover of a solar still. The Chart shows the wonderful advantages of this factor.

Deep Soil Temperatures in the USA


For locations within the United States, this map can give a good idea of the underground tube wall temperature as long as it is buried deep enough to not be too affected by hot summer air temperatures. However, note that the underground tube will gradually warm up due to the warm or hot air that is continuously blown through it. If the flow rate (or equivalently, the water production) is limited, and the tube is sufficiently long, the underground tube will easily lose that heat to the deep soil against the outside of the tube, and remain cool near the temperatures indicated in this map. However, if massive production of water is done with this system, for example, by using the HG 3a unit to be producing 90,000 Btu/hr every hour which is then sent through the underground tube, then either a much longer tube should be used or it should be expected that as the soil warms up, the performance of the system will become reduced. A great feature of this system is that even if that occurs, simply waiting a few hours will allow the underground tube and the soil around it to cool back down to their natural temperature, and full performance can again be had.

An example is here near Chicago, where the deep underground temperature is around 52°F. If we have a common summer day where the air temperature is 85°F and the relative humidity is 50%, the Chart shows us that each pound of air contains around 0.0135 pound of water vapor in it. If that "normal air" is sent into the underground tube, then as soon as the air has cooled to around 71°F (parallel to the red line), water will start to condense. Since the underground tube walls are around 52°F, this would assure that virtually all the air passing through the tube would get near enough to some part of the 52°F walls to have its moisture condense out. As the air is cooled down to near that 52°F, we can see that the MAJORITY of the water in the air condenses out (like the green line in the chart). Finally, after that air has exited the tube and again warmed up to the 85°F ambient air temperature (parallel with the red line but now to the right and down), we can see that it will then have only around 5% relative humidity! Less than 0.002 pound of water will then remain in the pound of air. This means that this simple device will have removed around (0.0135 - 0.002) 0.0115 pound of water from every pound of normal atmospheric air passing through the tube. The chart shows that the air density is generally around one pound per 14 cubic feet. So if we send just 200 cubic feet of air per minute (cfm) through the tube, (which is around 14 pounds of air), we would have (200/14) * 0.0115 or about 1/6 pound of water condense in the tube every minute. An 8-ounce coffee cup full of absolutely pure distilled water every three minutes! This is 10 pounds of water per hour, or about 1.5 gallons per hour. An easy ten gallons of absolutely pure water would be removed from atmospheric humidity on a very normal summer day, without even using the accessories that can so greatly improve this system's performance! (Keep in mind that the basic system relies on the warmth of the air, so it generally only produces water during the daytime. The HG 3a accessory unit would be needed if 24 hour water production was considered important.)

For locations OUTSIDE the USA, there are two easy ways of getting a good estimate of the deep soil temperature. The simpler is to simply obtain local weather records, and locate the average HIGH and LOW temperatures for December 21 (or just December) and the average HIGH and LOW temperatures for June 21 (or just June). Add those four numbers together and divide by four. The result is nearly always within one degree of the deep soil temperature. The reason is very logical. By averaging those four temperatures, you are learning the year average AIR temperature for that location. The deep soil gets and loses nearly all of its heat energy from the air, so its temperature, for ANY location on Earth, is necessarily extremely close to the average air temperature.

The second method is to get a standard thermometer, and place it in the water flow from a local well, for at least one minute of constant flow. That water temperature should become very close to the deep soil temperature, as the rapid water flow does not allow the water to significantly heat up in just the brief time of it coming up to the surface.

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C Johnson, Theoretical Physicist, Physics Degree from Univ of Chicago