Atmospheric sciences are—much like the sister science of astronomy—primarily observational sciences and not primarily experimental sciences. The atmospheric systems that we study are not well-defined systems under controlled conditions, but ill-defined systems under completely uncontrolled conditions. Let me elaborate on this concept: 1. Atmospheric systems are open systems. They are neither isolated nor closed. There are continuous interchanges of both matter and energy between free atmospheric systems and their environments. 2. Both system masses (M) and system volumes (V) are indeterminate as a consequence of these interchanges. Both the spatial boundaries and the temporal boundaries of atmospheric systems are vague and ill-defined. Atmospheric systems are not separate from their environments, but are vigorously mingling and interacting with them. Expressions that require a definite value for either system mass or system volume have limited applicability to the study of the free atmosphere. 3. Within any atmospheric system, the values for pressure, temperature, and molecular number density are continuously changing. This change is from place to place within the system at any given instant, and from time to time at any given location. This is why these variable are shown as means (bar over parameter designation) in most atmospheric studies rather than uniform values (no bar) as in many laboratory studies. We prefer to speak of system temperatures and pressures (plural) rather than system temperature and pressure (singular). This usage recognizes the dynamic nature of atmospheric systems. 4. Atmospheric systems all contain water vapor. Most weather systems contain liquid water and/or solid water (ice) as well. Since the triple-point for water (273.16K) is well within the normal range of atmospheric temperatures, changes of phase are common occurrences in atmospheric systems. Hence, observations based on the behavior of dry air under laboratory conditions are of limited value in understanding the operation of real free atmospheric systems. 5. Atmospheric systems are non-equilibrium systems. All atmospheric systems in the real free atmosphere are dynamic systems. Change is the essence of weather systems. Weather systems are never under conditions of equilibrium. If you are looking at a real atmospheric system, you may safely assume that it is not functioning under conditions of equilibrium. A tiny little “dust devil” at the corner of a building is a good example of a simple atmospheric system. A cumulonimbus cloud is a somewhat more complex atmospheric system. Neither is ever operating under conditions of equilibrium. [I define a "real" system as one whose parameters were obtained by observation and measurement--not through postulates or hypotheses. I define a "free" system as one that is not contained or restricted in any way.] Summary. Very few—if any—gas laws are universal. Many of them apply only to ideal gases, and many others require conditions of equilibrium to be valid. Some require that an atmospheric parameter be held constant while one or more others are varied. This is simply not going to happen with free atmospheric systems. Generally speaking, when considering the application of a particular gas law to any atmospheric system, you should also consider the many prerequisites that must exist for that law to be valid.