Temperature comparison inside and out

As expected the temperature inside the polytunnel heats up really quickly when the sun is out. I believe this is due to infrared radiation being unable to pass through the polythene (or glass) so heat builds up. (but with thick cloud cover the temperatures inside and out are very similar in the day time).

In the evening I've noticed that the temperature inside falls quickly and by early morning it is a degree or two colder than the outside air temperature. I wonder why this is so as I'd expect it to stay at least as warm as the outside temperature if heat is being lost by conduction alone from a hotter body (the PT) to a colder body (air outside). As a consequence, the tatties I was growing for Xmas have mostly died off even though the frosts outside have been very light. Could it be that other radiation wavelengths continue to escape all night but are not being topped up by the sun, especially as there's no IR effect?

If someone could explain the physics of it, I'd appreciate it!

 

Hi,

Cannot explain the exact physics of it, but clearly its down to the polythenes insulation properties holding the cold in as the morning sun comes up.

Simple ways to help retain the heat is to fill large tubs etc with water and /or stack old house bricks etc as both will act as storage heater , absorbing the daytime heat and releasing it at night.

If its only a small tunnel and you have electricity, a fan heater with a frost setting might be cost effective... ?

 

Greenhouses and polytunnels heat up by allowing near infrared light to pass through and blocking slightly longer wavelengths of infra red so they heat up in daylight. At night greenhouses and polytunnels lose heat mainly by conduction and convection. The longest wavelengths of infra red can pass through both glass and plastic so some heat is lost this way at night. Conduction and convection depend very much on the material, surroundings and airflow inside and outside the greenhouse/polytunnel.
As Flanders and Swann put it
The Second Law of Thermodymamics:
Heat cannot of itself pass from one body to a hotter body
At night the worst case scenario is that the temperature inside the polytunnel is equal to the external temperature. When/if the sun shines the greenhouse/polytunnel warms up and as a consequence lose heat to the outside. If a warm front comes through bringing warmer air then the greenhouse/polytunnel will start to warm up through convection/conduction and it will be warmer outside than inside for a while.
@Pedro61 it would be interesting to see some actual numbers and times.

 

One thing I have noticed is that the temperature on a frosty night is still falling at dawn, infact the coldest part of a frosty night is usually as the sun comes up.

From that I can only guess you are seeing a lag, for a time, between the night temperature in the tunnel catching up with the rising outside temperature.

 

Thermodynamics. That word has fascinated me since a kid, however I have to admit I have never really studied it or the natural laws related to it.

From practical experience and experimenting. Firstly John. You won't catch me in my greenhouse at such an ungodly hour. Comparing a polytunnel to a timber or metal framed greenhouse. The polytunnel covering is different from what ever kind of polythene/bubble plastic as might be used for insulation purposes. Using the latter, polythene sheeting or bubble plastic on the interior of a greenhouse provides various results. For instance. Results will differ if for instance sheet polythene is used compared to bubble plastic. A difference in temperature change of either material is placed directly to either glass or modern day polycarbonate. Using polycarbonate instead of glass, the temperature variation can be slightly different, depending upon whetherot not it's twin walled or triple walled.

Likewise, of either type of insulation is applied by fixing it to a framework, leaving a space between the insulation and the (inside) of the outer material, then once again a different temperature reading will result.

I suspect that allowances have to be made for temperature fluctuations inside, perhaps due to temp/condensation changes.

In the case of the greenhouses of many gardeners. I imagine this principle can be applied also to polytunnels. Sectioning off parts of the interior can often help conserve heat. Finally I remember once reading that. When using any insulation, always allow the material to acclimatize to th local temperature. Especially when perhaps using polystyrene. Hope this helps.

 

I did have this picture of you sitting in your greenhouse all night . Marking the hourly temperature on a piece of graph paper

 

All,

hanks for your responses! I guess it's a combination of lag, warmer air associated with a front, some outgoing radiation etc etc. One thing I didn't consider was that the thermometers may not be accurate, especially as they cost only £1 each.

My days of recording data and plotting it by hand are long gone so don't have any data to back up my comments above. I am considering going to the other extreme and installing a weather station, including a temperature sensor in the PT, such as a Davis station.

 

Those who get up before sunrise every day, and have enough spare time to hang around their garden in ghe darkness watching external thermometers will know it's normally coldest around dawn, but it can be during the early hours if clear then and cloudy later.

I can recall checking thermometers around dawn and finding the reading for inside was colder than outside the tunnel, but at the time put it down siting and/or callibration error.

I've 2 USB temperature data loggers that can be programmed to record temps at short intervals and be left inside and out overnight, then download the info onto a laptop and overlaw the 2 graphs. The big advantage of this is eliminating the need to get up before dawn and run around outside with a torch comparing the readings on 2 thermometers.

Will set them up one night when a clear, frosty but sunny morning is forecast.

 

I had a weather station setup last winter in my polytunnel as I wanted to know what was going on temperature wise. It has two remote sensors and a third wired direct sensor I did not log it scientifically but I know my tunnel was always a couple of degrees warmer than outside and that my "hotbox" was 1.5 degrees warmer still. My hotbox below...







As to the cold at dawn, I have a theory, I once asked a question on a fishing forum about why the water on a lake would be still all night, resembling a mirror. Then at dawn that mirror would break and ripples would start to appear. A glider pilot replied, I hope I can do the explanation justice here.

As the sun rises it starts to heat the water, the heat rises from the water upwards, this is a thermal, the rising action then draws air into the area and this causes the breeze that causes the ripple on the water so breaking that mirror like quality the lake had all night.

My theory is that the wind the thermal created moves the cold around and so drops the temperature. Whether that is correct or not is for others to determine.

Back to my tunnel, I have a three foot wide paving path, I think this and the bigger size of the tunnel stop it from getting to cold at night, but all bets are off when the sun comes out and it instantly gets too hot. My tunnel size 28 x 18 x 8ft tall.

Hope this helps someone/anyone.

Steve...

 

I've just viewed this video by Charles Dowding - see from approximately 11 minutes onwards.

His observations seem to match my own. Looks like it could be due to lack of air movement inside the PT.