Temperature and relative humidity are perhaps the most common measures of atmospheric conditions, especially on radio and television forecasts. The reason is that they are probably the easiest to understand at first glance.
Relative humidity, describes how close the air is to being saturated with water. When the relative humidity is 100%, the air is completely saturated (and it is usually raining outside). The lower the percentage is below 100%, the drier the air is.
To date, there has been little positive evidence to correlate the relative humidity with marijuana's potency. However, a slightly lower humidity, in the 50 to 70 percent region, does appear to produce plants with slightly more potent buds than those grown with a higher relative humidity.
A dry atmosphere seems to produce more potent plants because, when the humidity is about 50% or less, plant development is more compact, and the leaves tend to have thinner blades. Conversely, when the air is humid, plant growth is faster, and the leaves develop luxuriously, with wider blades. The advantage to the plant is that the wider blades have more surface area and can therefore transpire more water. So, plants grown with a lower relative humidity have thinner leaves and use less water. The higher potency may be due to the less leaf tissue for a given amount of cannabinoids and resin glands.
The temperature also influences the shape and size of the plant and leaves. At higher temperatures, the leaves tend to grow closer together and under cooler temperatures, the leaves are larger, have wider blades, and are spaced further apart. Warmer temperatures seems to yield slightly more potent plants for the same reason as a drier atmosphere does.
However, differences in potency caused by any of the growth factors (light, nutrients, water, temperature, humidity, etc) are small compared to differences caused by the variety (hereditary genetic structure) and full maturation. For example, the humidity in Jamaica, Colombia, Thailand, and many other countries associated with fine marijuana is relatively high and averages at about 80 percent.
So, you should try to keep the atmosphere as dry as possible. The atmosphere in heated or air-conditioned homes is already dry (usually about 15 to 40 percent). For this reason, many growers sow their seed so that the plants mature during the winter if the home is heated or in mid-summer if it is air-conditioned.
With normal levels of CO2, in a well vented space, 90 degrees would seem to be the absolute max, while 85 may be closer to optimum, even with a great deal of light available. Do not let the room temperature get over 35 C (95 F) as this hurts growth. Optimal temperature is 27-30 C (80-86 F) if you have a strong light with no CO2 enrichment. Less than 21 C (70 F) is too cold for good growth.
Low temperatures at night are OK down to about 60 degrees outdoors, then start to effect the growth in a big way. Mid 50s will cause mild shock and 40s will kill your plants with repeated exposure. Keep your plants warm, especially the roots. Elevate pots if you think the ground is sucking the heat out of the roots. This is an issue if you have a slab or other type of cold floor.
Plants of some varieties turn purple/red when temperatures dip down to the low 50's F (12-13 C)
http://www.highgrow.us/temperature.html
