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A meteorologist studies the atmosphere and weather using observations, physical theory and numerical models. Fluid mechanics, thermodynamics and radiative processes connect measured pressure, temperature and moisture with changing winds and clouds.

Forecasting requires an estimated initial state as well as evolution equations. Uncertainty in observations and model approximations can grow, so an ensemble of forecasts is useful for expressing a range of plausible outcomes.

A physical constant is a quantity whose value is independent of the particular experiment within a stated physical theory and regime. Examples include the speed of light in vacuum and Planck constant. A numerical value depends on the units of measurement; a dimensionless constant avoids that dependence.

Some constants have exact numerical values by the definition of the International System of Units. Others, such as the fine-structure constant, remain measured quantities with experimental uncertainty. Exact unit-defining values do not imply that every realization of those units is exact.^[ref]

A physical theory is an organized set of principles and mathematical relations that predicts measurable behavior within a stated domain. It connects idealized quantities to experimental procedures, allowing observations to test more than an isolated fitted formula.

For example, Newtonian mechanics relates force to acceleration and predicts orbital motion, while general relativity changes the description of gravity when spacetime curvature becomes relevant. A theory can remain useful within its approximation regime even after a broader theory is established. Its usefulness depends on explicit assumptions, reproducible predictions and known accuracy, rather than on treating every successful approximation as exact at all scales.

A physicist investigates physical phenomena through experiments, observations, mathematical models and computation. Quantitative predictions connect a physical theory's assumptions with measurements, while discrepancies can expose missing effects or a failed mathematical model.

Experimental and theoretical work often interact: a new measurement can motivate a new description, and a physical theory can suggest which quantities to measure or which background effects must be excluded. The specialized lists organize individual contributions by their main subject.