Marijuana Horticulture: The Indoor/Outdoor Medical Grower's Bible

humidity. That is four times more moisture! Where does this water go when the temperature drops? It condenses, onto the surface of plants and grow-room walls, just as dew condenses outdoors.
    Relative humidity increases when the temperature drops at night. The greater the temperature variation, the greater the relative humidity variation will be. Supplemental heat or extra ventilation is often necessary at night if temperatures fluctuate more than 15°F (8°C).
    Seedlings and vegetative plants grow best when the relative humidity is from 60 to 70 percent. Flowering plants grow best in a relative humidity range from 40 to 60 percent. The lower humidity discourages diseases and pests. As with temperature, consistent humidity promotes healthy, even growth. Relative humidity level affects the transpiration rate of the stomata. When humidity is high, water evaporates slowly. The stomata close, transpiration slows, and so does plant growth. Water evaporates quickly into drier air causing stomata to open, increasing transpiration, fluid flow, and growth. Transpiration in arid conditions will be rapid only if there is enough water available for roots to draw in. If water is inadequate, stomata will close to protect the plant from dehydration, causing growth to slow.
    When the relative humidity climbs beyond 70 percent, the pressure outside the leaf is too high and inside too low. This causes the stomata to close, which slows growth. For example, a 40-inch (1 m) tall plant can easily transpire a gallon (4 L) per day when the humidity is below 50 percent. However, the same plant will transpire about a half-pint (0.5 L) on a cool humid day.
     
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A 10 × 10 × 8 foot (800 cubic feet) (21.5 m 3 ) grow room can hold:
    4 oz (118 ml) of water at 32°F (0°C).
    7 oz (207 ml) of water at 50°F (10°C).
    14 oz (414 ml) of water at 70°F (21°C).
    18 oz (532 ml) of water at 80°F (26°C).
    28 oz (828 ml) of water at 90°F (32°C).
    56 oz (1.65 L) of water at 100°F (38°C).
    The moisture holding capacity of air doubles with every 20°F (10°C) increase in temperature.
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A maximum/minimum digital hygrometer registers the high and low humidity as well as current humidity.

This is the best relative humidity range for clones, as well as vegetative and flowering plants.

Air conditioners cool and dehumidify grow rooms. Growers must weigh the benefits against the extra electricity they use.
Measuring Relative Humidity
    Relative humidity control is an integral part of insect and fungus prevention and control. Humidity above 80 percent discourages spider mites but promotes fungus as well as root and stem rot. Humidity levels below 60 percent reduce the chances of fungus and rot.
    Measure relative humidity with a hygrometer. This extremely important instrument will save you and your garden much frustration and fungi. By knowing the exact moisture content in the air, humidity may be adjusted to a safe 40 to 60 percent level that encourages transpiration and discourages fungus growth.
    There are two common types of hygrometers. The spring type is accurate within 5 to 10 percent. This hygrometer is inexpensive and adequate for most hobby growers whose main concern is to keep the humidity near 55 to 60 percent. The second type, a psychrometer, is more expensive but very accurate. A psychrometer that measures relative humidity with a wet and dry bulb is an excellent way to keep an accurate vigil on relative humidity. Today there are many exceptionally accurate high-tech gadgets, plus they are equipped with memory!
    A humidistat is similar to a thermostat, but regulates humidity instead of temperature. Humidistats are wonderful and make controlling the environment very easy. Humidistats cost less than $100 and are worth their weight in resin glands. A humidistat and thermostat can be wired “in line” to control a vent fan. Each can operate the fan independently. As soon as the humidity (or temperature) exceeds the acceptable range, the fan turns on to vent the humid (or hot) air outdoors.
    The HID lamp and ballast radiate heat, which lowers humidity. Heat from the HID system and a vent fan on a thermostat/humidistat are all the humidity control necessary for most grow rooms. Other dry heat sources, such as hot air vented from a furnace or wood stove, dry the air and lower the humidity. Be careful. Do not let piped-in, warm, dry air blow directly on foliage. It will rapidly dehydrate plants.
    Increase humidity by misting the air with