How is the cold hardiness of a palm species ascertained?  It’s simple, people grow the palm in climates that are slightly colder or harsher than the palm’s natural habitat.  For example, Flame Thrower Palms (Chambeyronia macrocarpa) come from New Caledonia, which is a sub-tropical island in the South Pacific.  Collectors began growing them in Mediterranean climates and desert climates to see if they were versatile enough to survive.  After much trial and error, these growers discovered that they can grow in desert climates if properly watered and protected from sun and wind (there is a nice specimen growing in Rancho Cucamonga, CA).  They can also be grown with minimal maintenance (irrigation and fertilizer) in Mediterranean climates such as coastal Southern California, France, and Italy. 
The cold hardiness of a certain palm species is much more involved than just a number.  For instance, if you are told that a Bismark Palm (Bismarkia nobilis) is “cold hardy to 18 degrees” it does not mean that the palm will instantly fall over dead the moment the air temperature falls to 17 degrees.  More accurately, the Bismark will begin to lose its ability to photosynthesize sunlight once the temperature falls below this point. The key word to remember is duration and there are two factors involved:
Duration of Cold
If the temperature hits 18 degrees for just 30 minutes, the Bismark will sustain minimal leaf burn (probably just at the tips) and likely will not die.  However, if the temperature drops to 18 degrees and remains there overnight, the palm will probably sustain complete leaf burn (causing all leaves to die) followed by freezing in the growth point, or heart.  Once the heart is damaged beyond recovery, the palm will likely die a slow death over the next few days.  Sometimes, the freeze damage to the heart is minimal enough to cause slow decline and death over the course of weeks or months.  Once a palm is defoliated (loses all of its leaves) it loses its ability to photosynthesize light and push new growth.  Even if the heart is not killed by the cold, the loss of leaves basically cuts off the palm’s food source and the tree will starve to death.  Some species are resilient enough to force new growth even after all leaves have been killed.  The chances of this happening become better if the weather quickly warms up, the growth point is treated with a fungicide to prevent further disease, and the palm is already mature (able to utilize reserves of sugar and water from its stem).
Duration of Warmth
More important than duration of cold is the duration of warmth.  The duration of warmth has to do with the climate in a certain area.  (Don’t confuse weather with climate.  Weather refers to the current conditions in a particular area.  Climate refers to the meteorological conditions, including temperature, precipitation, and wind, that characteristically prevail in a particular region).  We will use the Coconut Palm (Cocos nucifera) as an example in this case.  The Coconut Palm is generally known as a Zone 11 plant (cold hardy to 40 degrees F).  If the temperature falls below 40 degrees, the palm begins to halt photosynthesis (as explained above).  Coconut Palms grow well in Southwest Florida, which is considered a sub-tropical climate.  The average annual  temperature in SW Florida is 72.5 degrees F. 
Now lets take a look at coastal Southern California, which is considered a Mediterranean climate, where a few collectors are successfully growing Coconut Palms outdoors.  Learn about climates.  The average annual temperature in coastal Southern California is 63 degrees F.  Thus, on any given day, the temperature in SW Florida is an average of 9.5 degrees warmer than it is in coastal Southern California.  This translates to 15% more warmth on average throughout the year.  Now lets make a connection between the 15% higher average warmth and the cold hardiness of the Coconut Palm.  Two equally-sized Coconut Palms, one in SW Florida and one in coastal Southern California, are both subjected to 35 degrees from a northerly storm  for a period of three hours.  Both trees are growing in beach sand and both have received the same amount of water to the roots.  The three hours of cold has caused both trees lose 25% of their fronds.  The difference comes when the weather returns back to average in both locations.  Since the Coconut Palm in Florida receives 15% more warmth annually, it has an advantage over the Coconut Palm in California by producing 15% more new growth, 15% larger leaves, and a stem that is 15% thicker in diameter (roughly). 
Is it beginning to become clearer why duration of warmth is important?  Even if both locations drop down to 35 degrees for the same amount of time and the same number of days in a given year (both are Zone 10b), the Coconut has a better chance at survival following cold snaps because it is warmer, on average, throughout the year.  The Florida Coconut will also grow faster, larger, and with less care than the California Coconut.  The other major factor that affects cold hardiness is relative humidity.