Soil Temperature (A day in the life of)

I've been collecting soil temperatures from my back yard for the better part of two years, normally I do this around sunset on Sunday evening for no better reason than this is usually a pleasant time to sit in the garden and the routine provides some consistency in the data.  The link at the bottom of the page describes the process.

Recently, I found myself in the garden before sunrise (see note below).  This was an opportunity to study the variation in soil temperature during the day.  It was clear  that this could be large, on a spring the morning the ground can have a thin layer of frost and by noon it can be warm to the touch.  The graph below is for a randomly selected day at an arbitrary chosen spot, whilst the absolute values maybe suspect, it does give an indication of their variation.

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.

Whilst messing with the soil temperature data, I took the opportunity to update the graphs of Sunday sunset data (it maybe a few days before the website is updated).  The graph for the 1m is shown below:

This graph also contains significant variation.  The winter of 2012/13 was long and cold (maybe not extreme cold, but cold for a long time) which was due to a cold air mass from continental Europe.  Last winter (2013/14) was mild, but marked by wet and windy storms blowing in from the Atlantic.  During January and February the mainstream media was full of pictures of flooding and wind damage, but by April, they were free to return to party political gossip.  This contrast between the two winters is apparent in the soil temperatures, this year they are up to 5 deg. C higher than last year.

Description of the data collection process

This link describes the data collection process and the page is periodically updated with data as it becomes available:

Soil Temperature

Bloggers Note

The drinking habits of my dog are not good.  Whilst he has an adequate supply of clean water he is drawn to ponds, puddles and worse (much, much worse). Normally his digestive system copes with this abuse, but at 04:00 on Tuesday, it failed to do so dramatically.  By 05:00 it was clear that frequent trips to the garden lay ahead, so to offset my irritation and boredom, I set up the soil temperature measuring equipment and produced the graph above.  Should anyone be interested, the dog has made a full recovery and the next rain shower should restore the lawn to its former state..  I am still making up the lost sleep.