In order to know you are providing the right thermal environment for you plants through ventilation, oscillating fans etc. you need to understand the physical nature of the 'heat' in grow spaces, how it should be measured, and the mistakes that can easily be made.
Essentially there are two aspects to the thermal environment that you need to be aware of:
1. The AIR temperature
2. The RADIANT temperature
Although obviously related, these are 2 distinctly separate phenomena and your plants have different tolerances for each.
1. Air temperature
Your standard mercury/alcohol-bulb or digital thermometer is designed to measure the temperature of the air (But not radiant heat). This is what is being referred to in the usual growers 'rules of thumb' such as maintaining your grow above 16degC and below 30degC.
Most thermometers are only designed to measure air temperature growers often mistakenly place their thermometer in direct light; radiant energy will warm your thermometer and give a higher than actual reading.
*Assuming your air is well mixed up by oscillating fans, it doesnt really matter where you locate your thermometer. The best spot is probably about half the way up a wall, with a piece of cardboard over it (To shade it from radiant heat), exposed to the mixed air in the grow room.
2. Radiant temperature
Radiant temperature is the result of heat transfer bewteen objects at different temperatures without whats inbetween i.e. the air, being effected. In our case the plants are naturally at a lower temperature that the light and reflector so heat is transferred to the plants foliage from the light/refecltor through radiation. This causes the leaves to heat up. The plants can withstand much higher radiant temperatures (around 40-50degC) than they can air temperatures but if this is too high the plants can 'burn'.
The plants can withstand much higher radiant temperatures than they can air temperatures, but if this is too high the plants can burn.
'The hand test': Place your hand in a comfortable position for some time at the tips of your plants should be. If your hand feels hot, then the lights are too close, the radiant energy is too intense, and light burn may result. Do this test and raise your HID lights/reflectors periodically to keep light burn to a minimum.
Finally, monitor the conditions in your grow regularly as things are changing all the time, and you will soon get to understand the thermal dynamics of your particular grow space so you can get the most from it.
[Cannabis loves high light levels/high radiant energy levels, but exceeding their tolerance for light can result in light burn and chlorophyll leaching out of the leaves. 600's & 1000s put out a lot of light, and light burn can easily damage clones and young plants.
Light burn damage: initially the leaves look pale green (the edges may turn upwards, mimicking a Mg deficiency), then they turn yellow and finally yellow/brown as the leaves scorch.
Often you can see a circular pattern of intense light where the reflector has focused the light onto the plants. If plants outside of this intense light zone look greener and healthier, consider raising your lights.]
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