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   Humidity is moisture that is suspended in the air. The temperature of the air determines how much moisture it can hold. Warm air holds more moisture than cool air, so as the temperature rises, the percentage of moisture in the air decreases. Because of this, more moisture needs to be added to maintain the desired level of humidity. When the air temperature cools, the air gives up its moisture in the form of rain (or snow if the air is cold enough).

   Here is a bit of trivia for you: the driest air on the planet can be found not in the Sahara Desert, but at the South Pole. Humidity can be measured using a simple instrument called a hygrometer. You should be able to purchase a hygrometer at any large garden center.

   Orchids lose water through pores (stomata) found on their leaves. The drier the air, the faster the orchid will loose its moisture. Keeping the humidity high reduces the speed of water loss (dehydration). Orchids prefer a location where there is a range of humidity between 40% and 70%.

   You should not try and keep your humidity at the same percentage all the time. A fluctuation of humidity is desirable and gives the plants a chance to "dry out." A 40% drop during the day for short periods can be beneficial to your orchids.   
Controlling Humidity
Raising humidity
    In a greenhouse, you can wet down the walks and floors to increase the humidity.

   In your home, a humidity tray works very well. A humidity tray is any kind of shallow container that will hold water. Just add some pebbles and water. Place the orchid above the tray making sure the pot does not touch the gravel. You can also increase the water evaporation from the tray by adding a heating cable (sold for germination of seeds) under the gravel.
   Group your plants together as much as you can, but not so close that it prevents free air movement around each plant. The grouped plants will assist each other in forming their own beneficial micro--climate cause by their own evaporation.
   Humidifiers work very well. There are many types and many prices. You will have to see what your budget can afford and then check out the various types available in your area. Before investing in a humidifier, however, you should first invest in a hygrometer to see if you really need one.
   If possible, grow your orchids in the kitchen by the sink, in the bathroom, the wash room, or anywhere hot water is used.
   The evaporation of the water adds moisture to the air. If the air is extra dry, you can enclose the orchid in a plastic "tent". The plastic cover acts like a mini greenhouse.
   Don't make your enclosure airtight, and do not expose your orchid to direct sunlight when it is enclosed in a tent.
   Make sure your furnace and air conditioning units are not blowing hot or cold air directly on your orchids. Water in the air quickly evaporates. If you have to mist be sure and do it early in the day so that any moisture on the plants leaves will be dry by evening.
Lowering Humidity
   Excessive humidity is rarely a problem for orchids grown in the home. High humidity is usually more prominent in a greenhouse because of the greenhouse's sealed environment. This is especially true if you live in an area where the climate is damp or in and area where winters are dull and cloudy most of the time.
    Signs of excessive humidity or damp stagnant air are the formation of brown water spots on the leaves, small pink or brown spots on the blooms and flower sheaths turning black.
   To correct the problem, you need to increase the air movement with fans to dry out the area. Vent the area to the outside if the humidity outside is lower than inside.
   Be careful not to introduce hot or cold drafts on your orchids while venting. Decrease watering if possible and don't mist.