Brighton Webs Ltd
statistics for energy and the environment


Brighton Webs is an independent research company located in the south east of England interested in the economics, design and performance of sustainable energy systems. We are currently developing software to link weather data to the output of wind and solar devices.

This stuff is work in progress and should be treated with caution.

Soil Temperature

I started collecting soil temperatures at a randomly selected location in my backyard in the south east of England in the Autumn of 2012.  The measurements are usually made around sunset on a Sunday afternoon.  This page shows the results together with some observations.  It is periodically updatd as more data becomes available.

Measurements are taken at four depths, 0.1m 0.3m, 0.5m and 1.0m.  At 1.0m (the sub soil). the temperature follows the average air temperature and changes slowly whilst at 0.1m (the top soil), the soil is in a state of thermal turbulence, in direct summer sun it becomes much hotter than the air above it and the sub soil below it.  On a clear winter's night, radiative coolng makes it much cooler than it's surroundings.  Around sunset in both winter and summer, the top soil is generally cooling.  The rolling time series shows the greater temperature fluctuations of the top soil relative to the sub soil below it.

Soil Temp - Rolling Time Series

The temperature at 0.1m is subject to a lot of random fluctuations and a better comparision of year-on-year data can be made using measurements from 0.3m and 1.0m.  When I started this exercise, I was not aware that there are "standard" depths for soil temperature measurements, thus I did not star taking measurements at 0.3m until late in 2013.

Soil Temp - Temperature at 0.3m

Even at 0.3m, there are significant week-on-week varations, but these are much less at 1.0m

Soil Temp - Temperature at 1.0m

The winter of 2012/13 was not especially cold, but it dragged on into April.  In contrast that of 203/14 was mild as it has been this year.  In general, the top soil does not reach 10 deg. C until April each year.

The relationship between the top soil and sub soil temperatures is interesting.  The plot shows the 2014 data for 0.3m and 1.0m.

Soil Temp - Rolling Time Series

At the start of the year, the top soil and sub soil temperatures are similar and remain that way until the equinox in March, then the top soil temperature starts to rise fater than the soil beolow it.  It takes until the end of July for the memory of winter to be erased.  Cooling starts at the end of Auguest, during which a reverse process takes place, the heat stored in the sub soil slows the rate of cooling of the soil above it.

Variations during the Day

Whilst collecting data once a week around sunset allows like-for-like comparisons throughout the year, it does not reveal the variations that take place during a given day.  On a spring day in 2014, I took measurements are more or less hourly intevals from sunrise to sunset.

Soil Temp - Variaiton during the day

At sunrise, the sky was more or less free of low cloud and the stars were visible, by noon there was scattered and broken cumulus and a few drops of rain fell but by late evening it was cool and clear again.  The most striking feature of the graph is the variation in the topsoil temperature (0.1m), maybe I missed it's low point, but this could have been 5 or 6 deg. C. by noon it was 16 deg. C, had the clouds been thinner, this might have reached 20 deg. C.  It is this part of the soil where vegetables grow in a thermally turbulent environment.  I am far from being an expert gardener, but I have learnt from the experience of losing seedlings to frost and failure to grow vegetables for winter harvesting that that the choice of crop and the timing of sowing is important.  As the depth of the measurement increases, the variation in temperature with time decreases, at 1.0m the observed range was less than 2 deg. C.

Also on graph is the air temperature from a weather station located approximately 10 km to the west.  I have attempted to collect air temperature data, but shadows from trees, the proximity of walls and our location on the side of an suburban valley make it hard to figure out what is being measured (much the same applies to the soil temperature measurements).  Comparing the soil and air temperature suggests two things.  The first is that whilst there is a correlation between air and soil temperature, the variation of the latter is much greater.  The second is more subtle and this is the complexity of the relationship.  During the morning the ground  warmed and the air followed making it reasonable to assume that there was a relationship between the two events.   However, by late evening, the air temperature started rising even though the ground was subject to radiative cooling.  At this time, the wind which had been blowing gently from the SW all day, veered to the NE, at a guess this slight rise might have been due to advection.

Soil Temperature and Air Temperature

The graph below shows the temperature at 0.1m and 1.0m together with the average air temperature for a nearby weather station.

Soil Temp - Soil and Air Temperatures

The graph clearly shows the wide variations in temperature experienced by the topsoil. In winter radiative cooling on a clear night can take the topsoil temperature several degrees below the air temperature whilst a hot sunny day with a clear sky can raise it several degrees above it. The thermal environment of plants is much more turbulent than one would suppose from simply looking at the air temperature from a weather report.

In contrast to the topsoil, the subsoil temperature closely follows the average air temperature. The relationship between the topsoil and subsoil temperatures is interesting, when the ground is cooling after the Summer Solstice, the subsoil is warmer than the topsoil. After the winter solstice, the subsoil takes a few weeks to start warming. It is as though the subsoil is acting like a storage heater, keeping the soil warm as Autumn and Winter approach.


In April 2012, a row of beetroot took a month to germinate, however, a second row sown towards the end of August pushed their leaves into daylight in the space of a few days.  A possible explanation for this was the warmer soil of late summer.  Spring of that year having been cold and wet.  This prompted me to collect measurements of soil temperature. Usually, these are taken an hour or two before sunset on a Sunday afternoon.

Measurement Site

There is nothing special about the spot selected to take measurements, other than that I had no knowledge of pipes or cables passing under that part of the garden.  The measurement point consists of a length of copper pipe sunk into the ground to a depth of 1 metre.  Temperature measurements are made by lowering a 4.7k  thermistor down the hole and recording the resistance at several levels.  The thermistor is actually measuring the air temperature at a given depth, it is hoped that the presence of a baffle on the plastic pipe carrying the wires to the sensor limits air flow in the hole and that the temperature of the air is close to that of the pipe wall

Soil Temp - Cross section through sampling point

Cross section of the area around the pipe

Soil Temp - Thermistor on the end of a length of tube

Sensor on the end of plastic tubing

Air Temperature and Solar Irradiance

I was intrigued by the lag between solar irradiance and air temperature and by inference that of the soil.  The graph below shows a simulation of the solar irradiance received by the earth's surface and the average air temperature. Solar irradiance peaks at the Summer Solstice in June and is at a minimum at the Winter Solstice in December, yet the warmest month is July and the coldest one is January

Soil Temp - Air Temperature and Solar Irradiance


It is not clear what magnitude of error associated with the measurements.  Thermistors are very sensitive devices.  Experience with this and other projects suggests that to get reliable measurements it is desirable to give the device time to stabilise after a change of position.  One source of error is the conditions when the maeasurements are taken, around sunset the ground is cooling.  If the sky is overcast the rate of cooling is quite slow and there will not be much change in the surface temperature during the 15 - 20 minutes required to collect the data, however, under a clear winter sky, cooling can be rapid and the surface temperature will be noticibly lower at the end of the process than at the start.

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