At Goroka, mean humidity varies from 87% at 09:00 hours to 57% at 15:00 hours. Associated with this typical cycling of cloud movement found in the Highlands in the early morning valley slopes and bottom are submerged in fog with high hills and ranges protruding into clear sunlight. As the day progresses the fog lifts from the valleys and clouds begin to form on the ranges frequently with associated local convectional storms. Towards the evening the cloud extends from the ranges to the valleys and with cooling sinks to valley bottoms. This cycle of afternoon cloud and storm on the ranges and morning fog in the valley bottoms, where airstrips are located, makes the maintenance of flight schedules difficult.