Humidity
Most palms come from tropical and sub-tropical climates throughout the world and rely on an extensive wet season and seasonally high humidity.  With the exception of Jubaea, Brahea, Sabal, Nannorrhops, Butia, Medemia, Washingtonia, Trachycarpus, Livistona, Chamaerops, and a few other species, the majority of palms thrive in a climate that has a range of 25-80% relative humidity.  Humidity makes water vapor in the air available for the leaves and is crucial in transpiration.  Higher humidity typically results in a higher growth rate, greener leaves, and overall better health. 
 
With regard to cold hardiness, a palm tree can usually resist frost more effectively if the relative humidity is higher.  Cold air will reduce a palm’s immunity to disease and fungus, especially if water is left sitting in the crown from rain or irrigation.  Hot, dry air is also detrimental in that it causes the leaves to rapidly lose their H2O reserves and can lead to wilting.  When a palm is fighting off water loss, its growth rate is usually slowed and its resistance to disease is weakened. 
 
Keeping tropical and sub-tropical palms in a greenhouse or conservatory during cold weather is beneficial.  Even if the temperature in the greenhouse is the same as the outside temperature, the higher humidity in the greenhouse provides a sort of “cushion” that protects the leaves from frost burn. 
 
This cushion can also be achieved by planting a palm and surrounding it with other trees and bushes creating a “thicket effect” much like a dense jungle or rainforest floor. 
 
Overhead Protection
Cold sinks and heat rises.  During a cold snap, the frigid air originates from the clouds and sinks down to the earth.  This cold air settles onto tropical palms and can lead to leaf burn or death if left unchecked.  One effective way to prevent this is to plant your cold-sensitive palms underneath a larger leafy tree that is frost hardy.  The larger tree will take the brunt of the settling cold air and protect the more sensitive palm below- sometimes keeping it up to five degrees warmer.
 
This same concept can be applied to palm trees and their root systems.  Many growers who live in climates that are slightly too cold for their favorite palms to thrive in keep the palms in containers and bring them inside during cold snaps or freezes.  While this is necessary if you live in New York, it is not all that necessary if you live in a Mediterranean or desert climate.  Planting marginal palms in the ground is beneficial in that the palms’ root systems can quickly develop and form a larger underground mass (as opposed to a limited mass in a container).  The ground will then provide protection from outside cold air and also dissipate the earth’s natural heat.  This will typically keep the root system a few to several degrees warmer than the outside air.  As a general rule, if a palm’s root system is kept warm and active, the rest of the tree will benefit as well.