Definition
The dominant wind pattern in the mid-latitudes (roughly 30° to 60° north and south) that flows generally from west to east. These winds result from the combination of large-scale atmospheric circulation and the deflection caused by Coriolis force acting on air moving between the subtropical high-pressure belts and the polar regions.
Plain English
Across most of the United States and other mid-latitude regions, the wind tends to blow from west to east most of the time. This is the normal large-scale flow of air in those latitudes.
Context Anchor
Seen in weather theory, flight planning, and discussions of why many weather systems in the United States tend to move from west to east.
Derivation
‘Prevailing’ comes from the Latin praevalere, meaning ‘to be stronger’ — the wind direction that wins out most of the time. ‘Westerly’ describes a wind coming from the west (winds are always named for the direction they come from, not where they’re going).
Why Pilots Care
These winds affect route planning, fuel use, and flight times, especially on long east-west trips where they can create strong tailwinds or headwinds.
Grounding Statement
If you are flying eastbound in an area with prevailing westerly winds, the wind is often helping push the airplane along; westbound, it may slow the airplane down.
Intuition Check
Do not read westerly as “going west.” A westerly wind comes from the west and blows toward the east. Prevailing also does not mean constant; it means most common or dominant.
Example Sentence 1
Because of the prevailing westerly winds, the flight from Los Angeles to New York was about an hour shorter than the return trip.
Example Sentence 2
When reviewing winds aloft forecasts, remember that prevailing westerly winds often strengthen with altitude in the mid-latitudes.