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.
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
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.
Even at 0.3m, there are significant week-on-week varations, but these are much
less 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.
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.
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.
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.
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
Cross section of the area around the pipe
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
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.