Seasonality is the tendency for a market variable to show a recurring pattern at roughly the same part of the calendar year because underlying economic, biological, operational or behavioral forces also recur. In commodity markets it can appear in production, demand, inventories, spreads, volatility and prices: harvests arrive on a schedule, heating and cooling demand shifts with weather, storage is built and drawn, and marketing activity changes through the crop year. The key word is tendency. Seasonality describes a historically recurring pattern, not a deterministic forecast of the next move.
That calendar link separates seasonality from broader cycles. A commodity or business cycle may expand and contract without returning at the same time each year; a seasonal pattern is anchored, however imperfectly, to recurring dates, months or parts of the year. The economic mechanism matters more than the shape of the historical average. Two markets can display similar monthly price patterns for different reasons, while the same market can lose an old seasonal pattern when production geography, imports, storage capacity, technology, regulation or demand behavior changes.
A seasonal average can outlive its mechanism
Seasonal studies therefore need more than a multi-year average. The sample period, calendar alignment, treatment of outliers and choice of mean versus median can change the apparent pattern, especially when a few unusual years dominate the history. Stability should be tested across subperiods and regimes, and the current economic mechanism should still be plausible. A pattern that was persistent before a structural change may remain visible in a long sample even after it has weakened in recent years.
In futures, construction is part of the question
Researchers must also define what is being made seasonal. Outright price levels, returns, calendar spreads, inventories and volatility measure different phenomena. Futures add another complication because a long history is usually assembled from contracts that expire and are replaced. Contract-selection and roll rules can therefore affect the timing or magnitude of an apparent seasonal pattern; an unadjusted jump created when one contract is replaced by another should not be mistaken for a recurring market move.
Seasonality is most useful as a conditional research framework: it identifies periods when recurring forces have historically mattered and focuses attention on whether those forces are present now. It becomes misleading when the historical pattern is treated as a stand-alone signal, when the sample is unstable, or when data construction is allowed to masquerade as economics.