Winter Solstice

The winter solstice is upon us. On the 20th-21st June the shortest day occurs in the southern hemisphere. At noon at Geelong the sun reaches only 28.5 degrees above the horizon and daylight only lasts 9.5 hours. The average daily maximum temperature is 14.3 degrees C.

After this date the days lengthen, but temperatures continue to drop until around the middle of July, owing largely to an increased cloud coverage associated with frontal activity and low pressure systems.

The tilt of the earth’s axis in relation to the sun is what causes the seasons. In winter we receive only weak incoming solar radiation because the sun is so low in the sky. In summer the situation is reversed. The situation is shown in the diagram below.  Please click to enlarge.

Last week the highest barometric pressure recorded in Geelong for three years occurred due to the slow overhead passage of a large high pressure system which crossed Victoria. At Geelong on the 13th and 14th a reading of 1042 hectopascals (30.77 inches of mercury) was reached.

This resulted in overnight frosts and fog, clear blue skies, little wind and high relative humidity levels. The overnight temperature fell to zero degrees at Mt Duneed airport at around 8 am on the 14th. In a typical suburban backyard the temperature was 1 or 2 degrees.