The Disadvantages of Atomic Clocksby John Lister
The "atomic" clocks on the consumer market are in fact radio-controlled clocks that take a time signal from an actual atomic clock, usually operated by a government agency. A consumer atomic clock will always be at least as good as an ordinary clock, because it uses a traditional quartz clock as a backup. However, any potential extra benefits from the atomic aspect can be limited by geography and atmospheric conditions. These limitations can in turn limit the financial value of paying extra money for a clock with an atomic time feature.
Only a limited number of atomic clock facilities are set up as "time signal stations" that broadcast to consumer units. As of 2013, the International Bureau of Weights and Measures reported around three dozen such sites. Some regions of the world, most notably Africa and Australia, have no time signal stations. Any consumer atomic clocks there won't work well or at all -- except by virtue of their quartz backups -- as any atomic clock signals are too far away.
Consumer atomic clocks rely on picking up the radio waves from time signal stations. The ability to pick up a strong enough signal can depend on the device pointing towards the station and being away from sources of interference. It may also depend on favorable atmospheric conditions, with high-pressure or low-lying fog potentially affecting reception. The extent on which clock placement and atmosphere causes problems will usually depend on how far you are from the signal station: the farther away, the bigger the risk of suffering these effects.
Distance And Accuracy
The way consumer atomic clocks work has an inherent limitation: the time it takes for the radio signal to reach the consumer's clock. The signal gives the time at the point the signal left the station, but the actual time at which it reaches your home clock will be slightly later. The National Institute of Standards & Technology notes this discrepancy can mean a notable inaccuracy in those areas at the farthest reaches of the signal from a particular station, such as the east and west coasts of the United States. How much of a disadvantage this factor is will depend on the level of precision with which your clock displays the time and your expectations of accuracy.
Consumer atomic clocks use a quartz crystal oscillator of the type found in many ordinary analog wristwatches to keep time in between checking and synchronizing with a time signal station. The performance of this quartz may vary, and this factor will become more significant if reception problems prevent synchronization for an extended time signal. Your clock could potentially gain or lose time. Again, how much of a disadvantage this discrepancy is will depend on the level of precision with which your clock displays the time and your expectations of accuracy.