The Portland Airport is located within the City of Portland, and subject to the usual city smoke. Prevailing winds are from the northwest and southeast due to its location on the Willamette River. Ocean air may reach Portland from the northwest, through the lower Columbia River gorge, and from the 3outh southwest through a gap in the Coast Range. It is also exposed to the interior climate of eastern Washington and Oregon, through the upper Columbia River gorge. At times the interplay of continental and ocean air may become delicately balanced for hours, with the continuation of pre-warm-frontal fog. It has been found that formation and duration of pre-warm frontal fog may be forecast within reasonable limits. When the precipitation at Portland is warmer than the surface air into which it is falling, formation of prewarm frontal fog is favored. Gradient winds of 20–30 mph are most favorable for light pre-warm-frontal precipitation. Duration of the fog depends on a continued supply of cold surface air, warm precipitation falling into the cold surface air, and light surface winds.

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*This study was undertaken as a research project under Prof. P. E. Church, University of Washington. Read at the Seattle meeting, June, 1940.