Definition
The dominant west-to-east airflow found at high altitudes in the middle latitudes of both hemispheres, produced by the combination of solar heating differences between the equator and poles and the deflecting effect of Coriolis force on a rotating Earth. These winds are strongest in the upper troposphere and lower stratosphere and form the broad current within which the jet stream is embedded.
Plain English
High up in the atmosphere, the wind in the middle of each hemisphere generally blows from west to east most of the time. That steady west-to-east flow is what 'prevailing upper level westerly winds' refers to.
Context Anchor
Seen in weather theory and flight planning when explaining why many weather systems and winds aloft tend to move from west to east across much of the United States.
Derivation
Prevailing' means 'most common' or 'dominant.' 'Westerly' in meteorology means a wind coming from the west (winds are named for where they come from, not where they go). So a westerly wind blows from west to east. Put together, the phrase simply means 'the winds that usually blow from the west at high altitudes.'
Why Pilots Care
These winds strongly affect groundspeed, fuel burn, and routing decisions, especially when flying with or against the jet stream.
Grounding Statement
At higher altitudes, a pilot flying east across the United States often expects help from the general west-to-east wind flow, while a pilot flying west may often expect a headwind.
Intuition Check
Do not read “westerly” as “going west.” A westerly wind comes from the west and blows toward the east. “Prevailing” does not mean constant; it means the usual or dominant pattern.
Example Sentence 1
When planning a long cross-country from California to New York, pilots often benefit from the prevailing upper level westerly winds, which provide a tailwind at cruise altitude.
Example Sentence 2
When planning the return trip, the crew accounted for headwinds from the prevailing upper level westerly winds over the northern route.